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在Android全部经常使用的原生控件其中。使用方法最复杂的应该就是ListView了，它专门用于处理那种内容元素许多。手机屏幕无法展示出全部内容的情况。

ListView能够使用列表的形式来展示内容，超出屏幕部分的内容仅仅须要通过手指滑动就能够移动到屏幕内了。

另外ListView另一个很奇妙的功能，我相信大家应该都体验过，即使在ListView中载入很许多的数据。比方达到成百上千条甚至许多其它，ListView都不会发生OOM或者崩溃，而且随着我们手指滑动来浏览许多其它数据时，程序所占用的内存居然都不会跟着增长。

那么ListView是怎么实现这么奇妙的功能的呢？当初我就抱着学习的心态花了很长时间把ListView的源代码通读了一遍，基本了解了它的工作原理，在感叹Google大神能够写出如此精妙代码的同一时候我也有所敬畏，由于ListView的代码量比較大，复杂度也很高，很难用文字表达清晰。于是我就放弃了把它写成一篇博客的想法。那么如今回忆起来这件事我已经肠子都悔青了。由于没过几个月时间我就把当初梳理清晰的源代码又忘的一干二净。于是如今我又又一次定下心来再次把ListView的源代码重读了一遍，那么这次我一定要把它写成一篇博客，分享给大家的同一时候也当成我自己的笔记吧。

首先我们先来看一下ListView的继承结构，例如以下图所看到的：

能够看到，ListView的继承结构还是相当复杂的，它是直接继承自的AbsListView。而AbsListView有两个子实现类，一个是ListView。另一个就是GridView。因此我们从这一点就能够猜出来。ListView和GridView在工作原理和实现上都是有许多共同点的。然后AbsListView又继承自AdapterView。AdapterView继承自ViewGroup，后面就是我们所熟知的了。先把ListView的继承结构了解一下。待会儿有助于我们更加清晰地分析代码。

Adapter的作用

Adapter相信大家都不会陌生。我们平时使用ListView的时候一定都会用到它。

那么话说回来大家有没有细致想过。为什么须要Adapter这个东西呢？总感觉正由于有了Adapter，ListView的使用变得要比其它控件复杂得多。那么这里我们就先来学习一下Adapter究竟起到了什么样的一个作用。

事实上说究竟，控件就是为了交互和展示数据用的。仅仅只是ListView更加特殊。它是为了展示许多许多数据用的，可是ListView仅仅承担交互和展示工作而已，至于这些数据来自哪里，ListView是不关心的。因此，我们能设想到的最基本的ListView工作模式就是要有一个ListView控件和一个数据源。

只是假设真的让ListView和数据源直接打交道的话。那ListView所要做的适配工作就很繁杂了。

由于数据源这个概念太模糊了，我们仅仅知道它包括了许多数据而已。至于这个数据源究竟是什么样类型。并没有严格的定义，有可能是数组，也有可能是集合，甚至有可能是数据库表中查询出来的游标。

所以说假设ListView真的去为每一种数据源都进行适配操作的话，一是扩展性会比較差，内置了几种适配就仅仅有几种适配，不能动态进行加入。二是超出了它本身应该负责的工作范围，不再是仅仅承担交互和展示工作就能够了，这样ListView就会变得比較臃肿。

那么显然Android开发团队是不会同意这样的事情发生的。于是就有了Adapter这样一个机制的出现。

顾名思义，Adapter是适配器的意思。它在ListView和数据源之间起到了一个桥梁的作用，ListView并不会直接和数据源打交道，而是会借助Adapter这个桥梁来去訪问真正的数据源。与之前不同的是，Adapter的接口都是统一的。因此ListView不用再去操心不论什么适配方面的问题。

而Adapter又是一个接口(interface)，它能够去实现各种各样的子类，每一个子类都能通过自己的逻辑来去完毕特定的功能，以及与特定数据源的适配操作，比方说ArrayAdapter能够用于数组和List类型的数据源适配。SimpleCursorAdapter能够用于游标类型的数据源适配，这样就很巧妙地把数据源适配困难的问题解决掉了。而且还拥有相当不错的扩展性。简单的原理示意图例如以下所看到的：

当然Adapter的作用不仅仅仅仅有数据源适配这一点，另一个很很重要的方法也须要我们在Adapter其中去重写。就是getView()方法，这个在以下的文章中还会详细讲到。

RecycleBin机制

那么在開始分析ListView的源代码之前。另一个东西是我们提前须要了解的。就是RecycleBin机制，这个机制也是ListView能够实现成百上千条数据都不会OOM最重要的一个原因。事实上RecycleBin的代码并不多。仅仅有300行左右，它是写在AbsListView中的一个内部类。所以全部继承自AbsListView的子类。也就是ListView和GridView，都能够使用这个机制。那我们来看一下RecycleBin中的主要代码，例如以下所看到的：

  1. /**
  2.  * The RecycleBin facilitates reuse of views across layouts. The RecycleBin
  3.  * has two levels of storage: ActiveViews and ScrapViews. ActiveViews are
  4.  * those views which were onscreen at the start of a layout. By
  5.  * construction, they are displaying current information. At the end of
  6.  * layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews
  7.  * are old views that could potentially be used by the adapter to avoid
  8.  * allocating views unnecessarily.
  9.  *
  10.  * @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
  11.  * @see android.widget.AbsListView.RecyclerListener
  12.  */
  13. class RecycleBin {
  14. 	private RecyclerListener mRecyclerListener;
  15.  
  16. 	/**
  17. 	 * The position of the first view stored in mActiveViews.
  18. 	 */
  19. 	private int mFirstActivePosition;
  20.  
  21. 	/**
  22. 	 * Views that were on screen at the start of layout. This array is
  23. 	 * populated at the start of layout, and at the end of layout all view
  24. 	 * in mActiveViews are moved to mScrapViews. Views in mActiveViews
  25. 	 * represent a contiguous range of Views, with position of the first
  26. 	 * view store in mFirstActivePosition.
  27. 	 */
  28. 	private View[] mActiveViews = new View[0];
  29.  
  30. 	/**
  31. 	 * Unsorted views that can be used by the adapter as a convert view.
  32. 	 */
  33. 	private ArrayList<View>[] mScrapViews;
  34.  
  35. 	private int mViewTypeCount;
  36.  
  37. 	private ArrayList<View> mCurrentScrap;
  38.  
  39. 	/**
  40. 	 * Fill ActiveViews with all of the children of the AbsListView.
  41. 	 *
  42. 	 * @param childCount
  43. 	 *            The minimum number of views mActiveViews should hold
  44. 	 * @param firstActivePosition
  45. 	 *            The position of the first view that will be stored in
  46. 	 *            mActiveViews
  47. 	 */
  48. 	void fillActiveViews(int childCount, int firstActivePosition) {
  49. 		if (mActiveViews.length < childCount) {
  50. 			mActiveViews = new View[childCount];
  51. 		}
  52. 		mFirstActivePosition = firstActivePosition;
  53. 		final View[] activeViews = mActiveViews;
  54. 		for (int i = 0; i < childCount; i++) {
  55. 			View child = getChildAt(i);
  56. 			AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
  57. 			// Don't put header or footer views into the scrap heap
  58. 			if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
  59. 				// Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in
  60. 				// active views.
  61. 				// However, we will NOT place them into scrap views.
  62. 				activeViews[i] = child;
  63. 			}
  64. 		}
  65. 	}
  66.  
  67. 	/**
  68. 	 * Get the view corresponding to the specified position. The view will
  69. 	 * be removed from mActiveViews if it is found.
  70. 	 *
  71. 	 * @param position
  72. 	 *            The position to look up in mActiveViews
  73. 	 * @return The view if it is found, null otherwise
  74. 	 */
  75. 	View getActiveView(int position) {
  76. 		int index = position - mFirstActivePosition;
  77. 		final View[] activeViews = mActiveViews;
  78. 		if (index >= 0 && index < activeViews.length) {
  79. 			final View match = activeViews[index];
  80. 			activeViews[index] = null;
  81. 			return match;
  82. 		}
  83. 		return null;
  84. 	}
  85.  
  86. 	/**
  87. 	 * Put a view into the ScapViews list. These views are unordered.
  88. 	 *
  89. 	 * @param scrap
  90. 	 *            The view to add
  91. 	 */
  92. 	void addScrapView(View scrap) {
  93. 		AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
  94. 		if (lp == null) {
  95. 			return;
  96. 		}
  97. 		// Don't put header or footer views or views that should be ignored
  98. 		// into the scrap heap
  99. 		int viewType = lp.viewType;
  100. 		if (!shouldRecycleViewType(viewType)) {
  101. 			if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
  102. 				removeDetachedView(scrap, false);
  103. 			}
  104. 			return;
  105. 		}
  106. 		if (mViewTypeCount == 1) {
  107. 			dispatchFinishTemporaryDetach(scrap);
  108. 			mCurrentScrap.add(scrap);
  109. 		} else {
  110. 			dispatchFinishTemporaryDetach(scrap);
  111. 			mScrapViews[viewType].add(scrap);
  112. 		}
  113.  
  114. 		if (mRecyclerListener != null) {
  115. 			mRecyclerListener.onMovedToScrapHeap(scrap);
  116. 		}
  117. 	}
  118.  
  119. 	/**
  120. 	 * @return A view from the ScrapViews collection. These are unordered.
  121. 	 */
  122. 	View getScrapView(int position) {
  123. 		ArrayList<View> scrapViews;
  124. 		if (mViewTypeCount == 1) {
  125. 			scrapViews = mCurrentScrap;
  126. 			int size = scrapViews.size();
  127. 			if (size > 0) {
  128. 				return scrapViews.remove(size - 1);
  129. 			} else {
  130. 				return null;
  131. 			}
  132. 		} else {
  133. 			int whichScrap = mAdapter.getItemViewType(position);
  134. 			if (whichScrap >= 0 && whichScrap < mScrapViews.length) {
  135. 				scrapViews = mScrapViews[whichScrap];
  136. 				int size = scrapViews.size();
  137. 				if (size > 0) {
  138. 					return scrapViews.remove(size - 1);
  139. 				}
  140. 			}
  141. 		}
  142. 		return null;
  143. 	}
  144.  
  145. 	public void setViewTypeCount(int viewTypeCount) {
  146. 		if (viewTypeCount < 1) {
  147. 			throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
  148. 		}
  149. 		// noinspection unchecked
  150. 		ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
  151. 		for (int i = 0; i < viewTypeCount; i++) {
  152. 			scrapViews[i] = new ArrayList<View>();
  153. 		}
  154. 		mViewTypeCount = viewTypeCount;
  155. 		mCurrentScrap = scrapViews[0];
  156. 		mScrapViews = scrapViews;
  157. 	}
  158.  
  159. }

这里的RecycleBin代码并不全，我仅仅是把最基本的几个方法提了出来。

那么我们先来对这几个方法进行简单解读，这对后面分析ListView的工作原理将会有很大的帮助。

  • fillActiveViews() 这种方法接收两个參数，第一个參数表示要存储的view的数量，第二个參数表示ListView中第一个可见元素的position值。

    RecycleBin其中使用mActiveViews这个数组来存储View，调用这种方法后就会依据传入的參数来将ListView中的指定元素存储到mActiveViews数组其中。

  • getActiveView() 这种方法和fillActiveViews()是相应的，用于从mActiveViews数组其中获取数据。该方法接收一个position參数。表示元素在ListView其中的位置，方法内部会自己主动将position值转换成mActiveViews数组相应的下标值。

    须要注意的是，mActiveViews其中所存储的View，一旦被获取了之后就会从mActiveViews其中移除，下次获取相同位置的View将会返回null，也就是说mActiveViews不能被反复利用。

  • addScrapView() 用于将一个废弃的View进行缓存，该方法接收一个View參数，当有某个View确定要废弃掉的时候(比方滚动出了屏幕)，就应该调用这种方法来对View进行缓存，RecycleBin其中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。
  • getScrapView 用于从废弃缓存中取出一个View，这些废弃缓存中的View是没有顺序可言的。因此getScrapView()方法中的算法也很easy，就是直接从mCurrentScrap其中获取尾部的一个scrap view进行返回。

  • setViewTypeCount() 我们都知道Adapter其中能够重写一个getViewTypeCount()来表示ListView中有几种类型的数据项，而setViewTypeCount()方法的作用就是为每种类型的数据项都单独启用一个RecycleBin缓存机制。实际上，getViewTypeCount()方法通常情况下使用的并非许多。所以我们仅仅要知道RecycleBin其中有这样一个功能即可了。

了解了RecycleBin中的主要方法以及它们的用处之后。以下就能够開始来分析ListView的工作原理了。这里我将还是依照曾经分析源代码的方式来进行，即跟着主线运行流程来逐步阅读并点到即止，不然的话要是把ListView全部的代码都贴出来，那么本篇文章将会很长很长了。

第一次Layout

无论怎么说，ListView即使再特殊终于还是继承自View的。因此它的运行流程还将会依照View的规则来运行，对于这方面不太熟悉的朋友能够參考我之前写的 Android视图绘制流程全然解析。带你一步步深入了解View(二) 。

View的运行流程无非就分为三步，onMeasure()用于測量View的大小，onLayout()用于确定View的布局，onDraw()用于将View绘制到界面上。而在ListView其中，onMeasure()并没有什么特殊的地方。由于它终归是一个View，占用的空间最多而且通常也就是整个屏幕。onDraw()在ListView其中也没有什么意义。由于ListView本身并不负责绘制，而是由ListView其中的子元素来进行绘制的。那么ListView大部分的奇妙功能事实上都是在onLayout()方法中进行的了。因此我们本篇文章也是主要分析的这种方法里的内容。

假设你到ListView源代码中去找一找。你会发现ListView中是没有onLayout()这种方法的，这是由于这种方法是在ListView的父类AbsListView中实现的，代码例如以下所看到的：

  1. /**
  2.  * Subclasses should NOT override this method but {@link #layoutChildren()}
  3.  * instead.
  4.  */
  5. @Override
  6. protected void onLayout(boolean changed, int l, int t, int r, int b) {
  7. 	super.onLayout(changed, l, t, r, b);
  8. 	mInLayout = true;
  9. 	if (changed) {
  10. 		int childCount = getChildCount();
  11. 		for (int i = 0; i < childCount; i++) {
  12. 			getChildAt(i).forceLayout();
  13. 		}
  14. 		mRecycler.markChildrenDirty();
  15. 	}
  16. 	layoutChildren();
  17. 	mInLayout = false;
  18. }

能够看到，onLayout()方法中并没有做什么复杂的逻辑操作，主要就是一个推断，假设ListView的大小或者位置发生了变化，那么changed变量就会变成true。此时会要求全部的子布局都强制进行重绘。除此之外倒没有什么难理解的地方了，只是我们注意到。在第16行调用了layoutChildren()这种方法，从方法名上我们就能够猜出这种方法是用来进行子元素布局的。只是进入到这种方法其中你会发现这是个空方法。没有一行代码。这当然是能够理解的了，由于子元素的布局应该是由详细的实现类来负责完毕的。而不是由父类完毕。那么进入ListView的layoutChildren()方法。代码例如以下所看到的： 

  1. @Override
  2. protected void layoutChildren() {
  3.     final boolean blockLayoutRequests = mBlockLayoutRequests;
  4.     if (!blockLayoutRequests) {
  5.         mBlockLayoutRequests = true;
  6.     } else {
  7.         return;
  8.     }
  9.     try {
  10.         super.layoutChildren();
  11.         invalidate();
  12.         if (mAdapter == null) {
  13.             resetList();
  14.             invokeOnItemScrollListener();
  15.             return;
  16.         }
  17.         int childrenTop = mListPadding.top;
  18.         int childrenBottom = getBottom() - getTop() - mListPadding.bottom;
  19.         int childCount = getChildCount();
  20.         int index = 0;
  21.         int delta = 0;
  22.         View sel;
  23.         View oldSel = null;
  24.         View oldFirst = null;
  25.         View newSel = null;
  26.         View focusLayoutRestoreView = null;
  27.         // Remember stuff we will need down below
  28.         switch (mLayoutMode) {
  29.         case LAYOUT_SET_SELECTION:
  30.             index = mNextSelectedPosition - mFirstPosition;
  31.             if (index >= 0 && index < childCount) {
  32.                 newSel = getChildAt(index);
  33.             }
  34.             break;
  35.         case LAYOUT_FORCE_TOP:
  36.         case LAYOUT_FORCE_BOTTOM:
  37.         case LAYOUT_SPECIFIC:
  38.         case LAYOUT_SYNC:
  39.             break;
  40.         case LAYOUT_MOVE_SELECTION:
  41.         default:
  42.             // Remember the previously selected view
  43.             index = mSelectedPosition - mFirstPosition;
  44.             if (index >= 0 && index < childCount) {
  45.                 oldSel = getChildAt(index);
  46.             }
  47.             // Remember the previous first child
  48.             oldFirst = getChildAt(0);
  49.             if (mNextSelectedPosition >= 0) {
  50.                 delta = mNextSelectedPosition - mSelectedPosition;
  51.             }
  52.             // Caution: newSel might be null
  53.             newSel = getChildAt(index + delta);
  54.         }
  55.         boolean dataChanged = mDataChanged;
  56.         if (dataChanged) {
  57.             handleDataChanged();
  58.         }
  59.         // Handle the empty set by removing all views that are visible
  60.         // and calling it a day
  61.         if (mItemCount == 0) {
  62.             resetList();
  63.             invokeOnItemScrollListener();
  64.             return;
  65.         } else if (mItemCount != mAdapter.getCount()) {
  66.             throw new IllegalStateException("The content of the adapter has changed but "
  67.                     + "ListView did not receive a notification. Make sure the content of "
  68.                     + "your adapter is not modified from a background thread, but only "
  69.                     + "from the UI thread. [in ListView(" + getId() + ", " + getClass()
  70.                     + ") with Adapter(" + mAdapter.getClass() + ")]");
  71.         }
  72.         setSelectedPositionInt(mNextSelectedPosition);
  73.         // Pull all children into the RecycleBin.
  74.         // These views will be reused if possible
  75.         final int firstPosition = mFirstPosition;
  76.         final RecycleBin recycleBin = mRecycler;
  77.         // reset the focus restoration
  78.         View focusLayoutRestoreDirectChild = null;
  79.         // Don't put header or footer views into the Recycler. Those are
  80.         // already cached in mHeaderViews;
  81.         if (dataChanged) {
  82.             for (int i = 0; i < childCount; i++) {
  83.                 recycleBin.addScrapView(getChildAt(i));
  84.                 if (ViewDebug.TRACE_RECYCLER) {
  85.                     ViewDebug.trace(getChildAt(i),
  86.                             ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);
  87.                 }
  88.             }
  89.         } else {
  90.             recycleBin.fillActiveViews(childCount, firstPosition);
  91.         }
  92.         // take focus back to us temporarily to avoid the eventual
  93.         // call to clear focus when removing the focused child below
  94.         // from messing things up when ViewRoot assigns focus back
  95.         // to someone else
  96.         final View focusedChild = getFocusedChild();
  97.         if (focusedChild != null) {
  98.             // TODO: in some cases focusedChild.getParent() == null
  99.             // we can remember the focused view to restore after relayout if the
  100.             // data hasn't changed, or if the focused position is a header or footer
  101.             if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {
  102.                 focusLayoutRestoreDirectChild = focusedChild;
  103.                 // remember the specific view that had focus
  104.                 focusLayoutRestoreView = findFocus();
  105.                 if (focusLayoutRestoreView != null) {
  106.                     // tell it we are going to mess with it
  107.                     focusLayoutRestoreView.onStartTemporaryDetach();
  108.                 }
  109.             }
  110.             requestFocus();
  111.         }
  112.         // Clear out old views
  113.         detachAllViewsFromParent();
  114.         switch (mLayoutMode) {
  115.         case LAYOUT_SET_SELECTION:
  116.             if (newSel != null) {
  117.                 sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
  118.             } else {
  119.                 sel = fillFromMiddle(childrenTop, childrenBottom);
  120.             }
  121.             break;
  122.         case LAYOUT_SYNC:
  123.             sel = fillSpecific(mSyncPosition, mSpecificTop);
  124.             break;
  125.         case LAYOUT_FORCE_BOTTOM:
  126.             sel = fillUp(mItemCount - 1, childrenBottom);
  127.             adjustViewsUpOrDown();
  128.             break;
  129.         case LAYOUT_FORCE_TOP:
  130.             mFirstPosition = 0;
  131.             sel = fillFromTop(childrenTop);
  132.             adjustViewsUpOrDown();
  133.             break;
  134.         case LAYOUT_SPECIFIC:
  135.             sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
  136.             break;
  137.         case LAYOUT_MOVE_SELECTION:
  138.             sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
  139.             break;
  140.         default:
  141.             if (childCount == 0) {
  142.                 if (!mStackFromBottom) {
  143.                     final int position = lookForSelectablePosition(0, true);
  144.                     setSelectedPositionInt(position);
  145.                     sel = fillFromTop(childrenTop);
  146.                 } else {
  147.                     final int position = lookForSelectablePosition(mItemCount - 1, false);
  148.                     setSelectedPositionInt(position);
  149.                     sel = fillUp(mItemCount - 1, childrenBottom);
  150.                 }
  151.             } else {
  152.                 if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
  153.                     sel = fillSpecific(mSelectedPosition,
  154.                             oldSel == null ? childrenTop : oldSel.getTop());
  155.                 } else if (mFirstPosition < mItemCount) {
  156.                     sel = fillSpecific(mFirstPosition,
  157.                             oldFirst == null ? childrenTop : oldFirst.getTop());
  158.                 } else {
  159.                     sel = fillSpecific(0, childrenTop);
  160.                 }
  161.             }
  162.             break;
  163.         }
  164.         // Flush any cached views that did not get reused above
  165.         recycleBin.scrapActiveViews();
  166.         if (sel != null) {
  167.             // the current selected item should get focus if items
  168.             // are focusable
  169.             if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
  170.                 final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
  171.                         focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
  172.                 if (!focusWasTaken) {
  173.                     // selected item didn't take focus, fine, but still want
  174.                     // to make sure something else outside of the selected view
  175.                     // has focus
  176.                     final View focused = getFocusedChild();
  177.                     if (focused != null) {
  178.                         focused.clearFocus();
  179.                     }
  180.                     positionSelector(sel);
  181.                 } else {
  182.                     sel.setSelected(false);
  183.                     mSelectorRect.setEmpty();
  184.                 }
  185.             } else {
  186.                 positionSelector(sel);
  187.             }
  188.             mSelectedTop = sel.getTop();
  189.         } else {
  190.             if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {
  191.                 View child = getChildAt(mMotionPosition - mFirstPosition);
  192.                 if (child != null) positionSelector(child);
  193.             } else {
  194.                 mSelectedTop = 0;
  195.                 mSelectorRect.setEmpty();
  196.             }
  197.             // even if there is not selected position, we may need to restore
  198.             // focus (i.e. something focusable in touch mode)
  199.             if (hasFocus() && focusLayoutRestoreView != null) {
  200.                 focusLayoutRestoreView.requestFocus();
  201.             }
  202.         }
  203.         // tell focus view we are done mucking with it, if it is still in
  204.         // our view hierarchy.
  205.         if (focusLayoutRestoreView != null
  206.                 && focusLayoutRestoreView.getWindowToken() != null) {
  207.             focusLayoutRestoreView.onFinishTemporaryDetach();
  208.         }
  209.         mLayoutMode = LAYOUT_NORMAL;
  210.         mDataChanged = false;
  211.         mNeedSync = false;
  212.         setNextSelectedPositionInt(mSelectedPosition);
  213.         updateScrollIndicators();
  214.         if (mItemCount > 0) {
  215.             checkSelectionChanged();
  216.         }
  217.         invokeOnItemScrollListener();
  218.     } finally {
  219.         if (!blockLayoutRequests) {
  220.             mBlockLayoutRequests = false;
  221.         }
  222.     }
  223. }

这段代码比較长。我们挑重点的看。

首先能够确定的是。ListView其中眼下还没有不论什么子View，数据都还是由Adapter管理的，并没有展示到界面上，因此第19行getChildCount()方法得到的值肯定是0。接着在第81行会依据dataChanged这个布尔型的值来推断运行逻辑。dataChanged仅仅有在数据源发生改变的情况下才会变成true，其它情况都是false，因此这里会进入到第90行的运行逻辑。调用RecycleBin的fillActiveViews()方法。按理来说，调用fillActiveViews()方法是为了将ListView的子View进行缓存的。可是眼下ListView中还没有不论什么的子View。因此这一行临时还起不了不论什么作用。

接下来在第114行会依据mLayoutMode的值来决定布局模式，默认情况下都是普通模式LAYOUT_NORMAL，因此会进入到第140行的default语句其中。而以下又会紧接着进行两次if推断，childCount眼下是等于0的，而且默认的布局顺序是从上往下，因此会进入到第145行的fillFromTop()方法，我们跟进去瞧一瞧：

  1. /**
  2.  * Fills the list from top to bottom, starting with mFirstPosition
  3.  *
  4.  * @param nextTop The location where the top of the first item should be
  5.  *        drawn
  6.  *
  7.  * @return The view that is currently selected
  8.  */
  9. private View fillFromTop(int nextTop) {
  10.     mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
  11.     mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
  12.     if (mFirstPosition < 0) {
  13.         mFirstPosition = 0;
  14.     }
  15.     return fillDown(mFirstPosition, nextTop);
  16. }

从这种方法的凝视中能够看出。它所负责的主要任务就是从mFirstPosition開始，自顶至底去填充ListView。而这种方法本身并没有什么逻辑。就是推断了一下mFirstPosition值的合法性，然后调用fillDown()方法。那么我们就有理由能够推測，填充ListView的操作是在fillDown()方法中完毕的。进入fillDown()方法。代码例如以下所看到的： 

  1. /**
  2.  * Fills the list from pos down to the end of the list view.
  3.  *
  4.  * @param pos The first position to put in the list
  5.  *
  6.  * @param nextTop The location where the top of the item associated with pos
  7.  *        should be drawn
  8.  *
  9.  * @return The view that is currently selected, if it happens to be in the
  10.  *         range that we draw.
  11.  */
  12. private View fillDown(int pos, int nextTop) {
  13.     View selectedView = null;
  14.     int end = (getBottom() - getTop()) - mListPadding.bottom;
  15.     while (nextTop < end && pos < mItemCount) {
  16.         // is this the selected item?
  17.         boolean selected = pos == mSelectedPosition;
  18.         View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
  19.         nextTop = child.getBottom() + mDividerHeight;
  20.         if (selected) {
  21.             selectedView = child;
  22.         }
  23.         pos++;
  24.     }
  25.     return selectedView;
  26. }

能够看到，这里使用了一个while循环来运行反复逻辑，一開始nextTop的值是第一个子元素顶部距离整个ListView顶部的像素值，pos则是刚刚传入的mFirstPosition的值。而end是ListView底部减去顶部所得的像素值，mItemCount则是Adapter中的元素数量。因此一開始的情况下nextTop必然是小于end值的，而且pos也是小于mItemCount值的。那么每运行一次while循环，pos的值都会加1，而且nextTop也会添加。当nextTop大于等于end时，也就是子元素已经超出当前屏幕了，或者pos大于等于mItemCount时。也就是全部Adapter中的元素都被遍历结束了，就会跳出while循环。

那么while循环其中又做了什么事情呢？值得让人留意的就是第18行调用的makeAndAddView()方法，进入到这种方法其中，代码例如以下所看到的：

  1. /**
  2.  * Obtain the view and add it to our list of children. The view can be made
  3.  * fresh, converted from an unused view, or used as is if it was in the
  4.  * recycle bin.
  5.  *
  6.  * @param position Logical position in the list
  7.  * @param y Top or bottom edge of the view to add
  8.  * @param flow If flow is true, align top edge to y. If false, align bottom
  9.  *        edge to y.
  10.  * @param childrenLeft Left edge where children should be positioned
  11.  * @param selected Is this position selected?
  12.  
  13. * @return View that was added
  14.  */
  15. private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
  16.         boolean selected) {
  17.     View child;
  18.     if (!mDataChanged) {
  19.         // Try to use an exsiting view for this position
  20.         child = mRecycler.getActiveView(position);
  21.         if (child != null) {
  22.             // Found it -- we're using an existing child
  23.             // This just needs to be positioned
  24.             setupChild(child, position, y, flow, childrenLeft, selected, true);
  25.             return child;
  26.         }
  27.     }
  28.     // Make a new view for this position, or convert an unused view if possible
  29.     child = obtainView(position, mIsScrap);
  30.     // This needs to be positioned and measured
  31.     setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
  32.     return child;
  33. }

这里在第19行尝试从RecycleBin其中高速获取一个active view，只是很遗憾的是眼下RecycleBin其中还没有缓存不论什么的View，所以这里得到的值肯定是null。

那么取得了null之后就会继续向下运行，到第28行会调用obtainView()方法来再次尝试获取一个View。这次的obtainView()方法是能够保证一定返回一个View的，于是以下立马将获取到的View传入到了setupChild()方法其中。那么obtainView()内部究竟是怎么工作的呢？我们先进入到这种方法里面看一下： 

  1. /**
  2.  * Get a view and have it show the data associated with the specified
  3.  * position. This is called when we have already discovered that the view is
  4.  * not available for reuse in the recycle bin. The only choices left are
  5.  * converting an old view or making a new one.
  6.  *
  7.  * @param position
  8.  *            The position to display
  9.  * @param isScrap
  10.  *            Array of at least 1 boolean, the first entry will become true
  11.  *            if the returned view was taken from the scrap heap, false if
  12.  *            otherwise.
  13.  *
  14.  * @return A view displaying the data associated with the specified position
  15.  */
  16. View obtainView(int position, boolean[] isScrap) {
  17. 	isScrap[0] = false;
  18. 	View scrapView;
  19. 	scrapView = mRecycler.getScrapView(position);
  20. 	View child;
  21. 	if (scrapView != null) {
  22. 		child = mAdapter.getView(position, scrapView, this);
  23. 		if (child != scrapView) {
  24. 			mRecycler.addScrapView(scrapView);
  25. 			if (mCacheColorHint != 0) {
  26. 				child.setDrawingCacheBackgroundColor(mCacheColorHint);
  27. 			}
  28. 		} else {
  29. 			isScrap[0] = true;
  30. 			dispatchFinishTemporaryDetach(child);
  31. 		}
  32. 	} else {
  33. 		child = mAdapter.getView(position, null, this);
  34. 		if (mCacheColorHint != 0) {
  35. 			child.setDrawingCacheBackgroundColor(mCacheColorHint);
  36. 		}
  37. 	}
  38. 	return child;
  39. }

obtainView()方法中的代码并不多。但却包括了很很重要的逻辑。不夸张的说。整个ListView中最重要的内容可能就在这种方法里了。那么我们还是依照运行流程来看，在第19行代码中调用了RecycleBin的getScrapView()方法来尝试获取一个废弃缓存中的View，相同的道理，这里肯定是获取不到的，getScrapView()方法会返回一个null。这时该怎么办呢？没有关系，代码会运行到第33行，调用mAdapter的getView()方法来去获取一个View。那么mAdapter是什么呢？当然就是当前ListView关联的适配器了。而getView()方法又是什么呢？还用说吗，这个就是我们平时使用ListView时最最经常重写的一个方法了，这里getView()方法中传入了三个參数，各自是position，null和this。

那么我们平时写ListView的Adapter时，getView()方法一般会怎么写呢？这里我举个简单的样例：

  1. @Override
  2. public View getView(int position, View convertView, ViewGroup parent) {
  3. 	Fruit fruit = getItem(position);
  4. 	View view;
  5. 	if (convertView == null) {
  6. 		view = LayoutInflater.from(getContext()).inflate(resourceId, null);
  7. 	} else {
  8. 		view = convertView;
  9. 	}
  10. 	ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
  11. 	TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
  12. 	fruitImage.setImageResource(fruit.getImageId());
  13. 	fruitName.setText(fruit.getName());
  14. 	return view;
  15. }

getView()方法接受的三个參数，第一个參数position代表当前子元素的的位置。我们能够通过详细的位置来获取与其相关的数据。第二个參数convertView，刚才传入的是null。说明没有convertView能够利用，因此我们会调用LayoutInflater的inflate()方法来去载入一个布局。

接下来会对这个view进行一些属性和值的设定。最后将view返回。

那么这个View也会作为obtainView()的结果进行返回，并终于传入到setupChild()方法其中。事实上也就是说，第一次layout过程其中。全部的子View都是调用LayoutInflater的inflate()方法载入出来的，这样就会相对照较耗时。可是不用操心。后面就不会再有这样的情况了，那么我们继续往下看：

  1. /**
  2.  * Add a view as a child and make sure it is measured (if necessary) and
  3.  * positioned properly.
  4.  *
  5.  * @param child The view to add
  6.  * @param position The position of this child
  7.  * @param y The y position relative to which this view will be positioned
  8.  * @param flowDown If true, align top edge to y. If false, align bottom
  9.  *        edge to y.
  10.  * @param childrenLeft Left edge where children should be positioned
  11.  * @param selected Is this position selected?
  12.  
  13. * @param recycled Has this view been pulled from the recycle bin?
  14.  
  15. If so it
  16.  *        does not need to be remeasured.
  17.  */
  18. private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
  19.         boolean selected, boolean recycled) {
  20.     final boolean isSelected = selected && shouldShowSelector();
  21.     final boolean updateChildSelected = isSelected != child.isSelected();
  22.     final int mode = mTouchMode;
  23.     final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
  24.             mMotionPosition == position;
  25.     final boolean updateChildPressed = isPressed != child.isPressed();
  26.     final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
  27.     // Respect layout params that are already in the view. Otherwise make some up...
  28.     // noinspection unchecked
  29.     AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
  30.     if (p == null) {
  31.         p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
  32.                 ViewGroup.LayoutParams.WRAP_CONTENT, 0);
  33.     }
  34.     p.viewType = mAdapter.getItemViewType(position);
  35.     if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
  36.             p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
  37.         attachViewToParent(child, flowDown ? -1 : 0, p);
  38.     } else {
  39.         p.forceAdd = false;
  40.         if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
  41.             p.recycledHeaderFooter = true;
  42.         }
  43.         addViewInLayout(child, flowDown ?
  44.  
  45. -1 : 0, p, true);
  46.     }
  47.     if (updateChildSelected) {
  48.         child.setSelected(isSelected);
  49.     }
  50.     if (updateChildPressed) {
  51.         child.setPressed(isPressed);
  52.     }
  53.     if (needToMeasure) {
  54.         int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
  55.                 mListPadding.left + mListPadding.right, p.width);
  56.         int lpHeight = p.height;
  57.         int childHeightSpec;
  58.         if (lpHeight > 0) {
  59.             childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
  60.         } else {
  61.             childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
  62.         }
  63.         child.measure(childWidthSpec, childHeightSpec);
  64.     } else {
  65.         cleanupLayoutState(child);
  66.     }
  67.     final int w = child.getMeasuredWidth();
  68.     final int h = child.getMeasuredHeight();
  69.     final int childTop = flowDown ? y : y - h;
  70.     if (needToMeasure) {
  71.         final int childRight = childrenLeft + w;
  72.         final int childBottom = childTop + h;
  73.         child.layout(childrenLeft, childTop, childRight, childBottom);
  74.     } else {
  75.         child.offsetLeftAndRight(childrenLeft - child.getLeft());
  76.         child.offsetTopAndBottom(childTop - child.getTop());
  77.     }
  78.     if (mCachingStarted && !child.isDrawingCacheEnabled()) {
  79.         child.setDrawingCacheEnabled(true);
  80.     }
  81. }

setupChild()方法其中的代码尽管比較多，可是我们仅仅看核心代码的话就很easy了。刚才调用obtainView()方法获取到的子元素View，这里在第40行调用了addViewInLayout()方法将它加入到了ListView其中。那么依据fillDown()方法中的while循环，会让子元素View将整个ListView控件填满然后就跳出。也就是说即使我们的Adapter中有一千条数据，ListView也仅仅会载入第一屏的数据，剩下的数据反正眼下在屏幕上也看不到，所以不会去做多余的载入工作。这样就能够保证ListView中的内容能够迅速展示到屏幕上。

那么到此为止，第一次Layout过程结束。

第二次Layout

尽管我在源代码中并没有找出详细的原因。但假设你自己做一下实验的话就会发现，即使是一个再简单的View，在展示到界面上之前都会经历至少两次onMeasure()和两次onLayout()的过程。事实上这仅仅是一个很小的细节。平时对我们影响并不大。由于无论是onMeasure()或者onLayout()几次。反正都是运行的相同的逻辑，我们并不须要进行过多关心。可是在ListView中情况就不一样了。由于这就意味着layoutChildren()过程会运行两次，而这个过程其中涉及到向ListView中加入子元素，假设相同的逻辑运行两遍的话，那么ListView中就会存在一份反复的数据了。因此ListView在layoutChildren()过程其中做了第二次Layout的逻辑处理。很巧妙地攻克了这个问题。以下我们就来分析一下第二次Layout的过程。

事实上第二次Layout和第一次Layout的基本流程是差点儿相同的，那么我们还是从layoutChildren()方法開始看起：

  1. @Override
  2. protected void layoutChildren() {
  3.     final boolean blockLayoutRequests = mBlockLayoutRequests;
  4.     if (!blockLayoutRequests) {
  5.         mBlockLayoutRequests = true;
  6.     } else {
  7.         return;
  8.     }
  9.     try {
  10.         super.layoutChildren();
  11.         invalidate();
  12.         if (mAdapter == null) {
  13.             resetList();
  14.             invokeOnItemScrollListener();
  15.             return;
  16.         }
  17.         int childrenTop = mListPadding.top;
  18.         int childrenBottom = getBottom() - getTop() - mListPadding.bottom;
  19.         int childCount = getChildCount();
  20.         int index = 0;
  21.         int delta = 0;
  22.         View sel;
  23.         View oldSel = null;
  24.         View oldFirst = null;
  25.         View newSel = null;
  26.         View focusLayoutRestoreView = null;
  27.         // Remember stuff we will need down below
  28.         switch (mLayoutMode) {
  29.         case LAYOUT_SET_SELECTION:
  30.             index = mNextSelectedPosition - mFirstPosition;
  31.             if (index >= 0 && index < childCount) {
  32.                 newSel = getChildAt(index);
  33.             }
  34.             break;
  35.         case LAYOUT_FORCE_TOP:
  36.         case LAYOUT_FORCE_BOTTOM:
  37.         case LAYOUT_SPECIFIC:
  38.         case LAYOUT_SYNC:
  39.             break;
  40.         case LAYOUT_MOVE_SELECTION:
  41.         default:
  42.             // Remember the previously selected view
  43.             index = mSelectedPosition - mFirstPosition;
  44.             if (index >= 0 && index < childCount) {
  45.                 oldSel = getChildAt(index);
  46.             }
  47.             // Remember the previous first child
  48.             oldFirst = getChildAt(0);
  49.             if (mNextSelectedPosition >= 0) {
  50.                 delta = mNextSelectedPosition - mSelectedPosition;
  51.             }
  52.             // Caution: newSel might be null
  53.             newSel = getChildAt(index + delta);
  54.         }
  55.         boolean dataChanged = mDataChanged;
  56.         if (dataChanged) {
  57.             handleDataChanged();
  58.         }
  59.         // Handle the empty set by removing all views that are visible
  60.         // and calling it a day
  61.         if (mItemCount == 0) {
  62.             resetList();
  63.             invokeOnItemScrollListener();
  64.             return;
  65.         } else if (mItemCount != mAdapter.getCount()) {
  66.             throw new IllegalStateException("The content of the adapter has changed but "
  67.                     + "ListView did not receive a notification. Make sure the content of "
  68.                     + "your adapter is not modified from a background thread, but only "
  69.                     + "from the UI thread. [in ListView(" + getId() + ", " + getClass()
  70.                     + ") with Adapter(" + mAdapter.getClass() + ")]");
  71.         }
  72.         setSelectedPositionInt(mNextSelectedPosition);
  73.         // Pull all children into the RecycleBin.
  74.         // These views will be reused if possible
  75.         final int firstPosition = mFirstPosition;
  76.         final RecycleBin recycleBin = mRecycler;
  77.         // reset the focus restoration
  78.         View focusLayoutRestoreDirectChild = null;
  79.         // Don't put header or footer views into the Recycler. Those are
  80.         // already cached in mHeaderViews;
  81.         if (dataChanged) {
  82.             for (int i = 0; i < childCount; i++) {
  83.                 recycleBin.addScrapView(getChildAt(i));
  84.                 if (ViewDebug.TRACE_RECYCLER) {
  85.                     ViewDebug.trace(getChildAt(i),
  86.                             ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);
  87.                 }
  88.             }
  89.         } else {
  90.             recycleBin.fillActiveViews(childCount, firstPosition);
  91.         }
  92.         // take focus back to us temporarily to avoid the eventual
  93.         // call to clear focus when removing the focused child below
  94.         // from messing things up when ViewRoot assigns focus back
  95.         // to someone else
  96.         final View focusedChild = getFocusedChild();
  97.         if (focusedChild != null) {
  98.             // TODO: in some cases focusedChild.getParent() == null
  99.             // we can remember the focused view to restore after relayout if the
  100.             // data hasn't changed, or if the focused position is a header or footer
  101.             if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {
  102.                 focusLayoutRestoreDirectChild = focusedChild;
  103.                 // remember the specific view that had focus
  104.                 focusLayoutRestoreView = findFocus();
  105.                 if (focusLayoutRestoreView != null) {
  106.                     // tell it we are going to mess with it
  107.                     focusLayoutRestoreView.onStartTemporaryDetach();
  108.                 }
  109.             }
  110.             requestFocus();
  111.         }
  112.         // Clear out old views
  113.         detachAllViewsFromParent();
  114.         switch (mLayoutMode) {
  115.         case LAYOUT_SET_SELECTION:
  116.             if (newSel != null) {
  117.                 sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
  118.             } else {
  119.                 sel = fillFromMiddle(childrenTop, childrenBottom);
  120.             }
  121.             break;
  122.         case LAYOUT_SYNC:
  123.             sel = fillSpecific(mSyncPosition, mSpecificTop);
  124.             break;
  125.         case LAYOUT_FORCE_BOTTOM:
  126.             sel = fillUp(mItemCount - 1, childrenBottom);
  127.             adjustViewsUpOrDown();
  128.             break;
  129.         case LAYOUT_FORCE_TOP:
  130.             mFirstPosition = 0;
  131.             sel = fillFromTop(childrenTop);
  132.             adjustViewsUpOrDown();
  133.             break;
  134.         case LAYOUT_SPECIFIC:
  135.             sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
  136.             break;
  137.         case LAYOUT_MOVE_SELECTION:
  138.             sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
  139.             break;
  140.         default:
  141.             if (childCount == 0) {
  142.                 if (!mStackFromBottom) {
  143.                     final int position = lookForSelectablePosition(0, true);
  144.                     setSelectedPositionInt(position);
  145.                     sel = fillFromTop(childrenTop);
  146.                 } else {
  147.                     final int position = lookForSelectablePosition(mItemCount - 1, false);
  148.                     setSelectedPositionInt(position);
  149.                     sel = fillUp(mItemCount - 1, childrenBottom);
  150.                 }
  151.             } else {
  152.                 if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
  153.                     sel = fillSpecific(mSelectedPosition,
  154.                             oldSel == null ? childrenTop : oldSel.getTop());
  155.                 } else if (mFirstPosition < mItemCount) {
  156.                     sel = fillSpecific(mFirstPosition,
  157.                             oldFirst == null ?
  158.  
  159. childrenTop : oldFirst.getTop());
  160.                 } else {
  161.                     sel = fillSpecific(0, childrenTop);
  162.                 }
  163.             }
  164.             break;
  165.         }
  166.         // Flush any cached views that did not get reused above
  167.         recycleBin.scrapActiveViews();
  168.         if (sel != null) {
  169.             // the current selected item should get focus if items
  170.             // are focusable
  171.             if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
  172.                 final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
  173.                         focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
  174.                 if (!focusWasTaken) {
  175.                     // selected item didn't take focus, fine, but still want
  176.                     // to make sure something else outside of the selected view
  177.                     // has focus
  178.                     final View focused = getFocusedChild();
  179.                     if (focused != null) {
  180.                         focused.clearFocus();
  181.                     }
  182.                     positionSelector(sel);
  183.                 } else {
  184.                     sel.setSelected(false);
  185.                     mSelectorRect.setEmpty();
  186.                 }
  187.             } else {
  188.                 positionSelector(sel);
  189.             }
  190.             mSelectedTop = sel.getTop();
  191.         } else {
  192.             if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {
  193.                 View child = getChildAt(mMotionPosition - mFirstPosition);
  194.                 if (child != null) positionSelector(child);
  195.             } else {
  196.                 mSelectedTop = 0;
  197.                 mSelectorRect.setEmpty();
  198.             }
  199.             // even if there is not selected position, we may need to restore
  200.             // focus (i.e. something focusable in touch mode)
  201.             if (hasFocus() && focusLayoutRestoreView != null) {
  202.                 focusLayoutRestoreView.requestFocus();
  203.             }
  204.         }
  205.         // tell focus view we are done mucking with it, if it is still in
  206.         // our view hierarchy.
  207.         if (focusLayoutRestoreView != null
  208.                 && focusLayoutRestoreView.getWindowToken() != null) {
  209.             focusLayoutRestoreView.onFinishTemporaryDetach();
  210.         }
  211.         mLayoutMode = LAYOUT_NORMAL;
  212.         mDataChanged = false;
  213.         mNeedSync = false;
  214.         setNextSelectedPositionInt(mSelectedPosition);
  215.         updateScrollIndicators();
  216.         if (mItemCount > 0) {
  217.             checkSelectionChanged();
  218.         }
  219.         invokeOnItemScrollListener();
  220.     } finally {
  221.         if (!blockLayoutRequests) {
  222.             mBlockLayoutRequests = false;
  223.         }
  224.     }
  225. }

相同还是在第19行，调用getChildCount()方法来获取子View的数量。仅仅只是如今得到的值不会再是0了。而是ListView中一屏能够显示的子View数量，由于我们刚刚在第一次Layout过程其中向ListView加入了这么多的子View。以下在第90行调用了RecycleBin的fillActiveViews()方法，这次效果可就不一样了，由于眼下ListView中已经有子View了，这样全部的子View都会被缓存到RecycleBin的mActiveViews数组其中，后面将会用到它们。

接下来将会是很很重要的一个操作，在第113行调用了detachAllViewsFromParent()方法。这种方法会将全部ListView其中的子View全部清除掉，从而保证第二次Layout过程不会产生一份反复的数据。那有的朋友可能会问了。这样把已经载入好的View又清除掉，待会还要再又一次载入一遍，这不是严重影响效率吗？不用操心。还记得我们刚刚调用了RecycleBin的fillActiveViews()方法来缓存子View吗。待会儿将会直接使用这些缓存好的View来进行载入，而并不会又一次运行一遍inflate过程。因此效率方面并不会有什么明显的影响。

那么我们接着看。在第141行的推断逻辑其中，由于不再等于0了。因此会进入到else语句其中。

而else语句中又有三个逻辑推断。第一个逻辑推断不成立，由于默认情况下我们没有选中不论什么子元素，mSelectedPosition应该等于-1。第二个逻辑推断一般是成立的。由于mFirstPosition的值一開始是等于0的。仅仅要adapter中的数据大于0条件就成立。那么进入到fillSpecific()方法其中，代码例如以下所看到的：

  1. /**
  2.  * Put a specific item at a specific location on the screen and then build
  3.  * up and down from there.
  4.  *
  5.  * @param position The reference view to use as the starting point
  6.  * @param top Pixel offset from the top of this view to the top of the
  7.  *        reference view.
  8.  *
  9.  * @return The selected view, or null if the selected view is outside the
  10.  *         visible area.
  11.  */
  12. private View fillSpecific(int position, int top) {
  13.     boolean tempIsSelected = position == mSelectedPosition;
  14.     View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
  15.     // Possibly changed again in fillUp if we add rows above this one.
  16.     mFirstPosition = position;
  17.     View above;
  18.     View below;
  19.     final int dividerHeight = mDividerHeight;
  20.     if (!mStackFromBottom) {
  21.         above = fillUp(position - 1, temp.getTop() - dividerHeight);
  22.         // This will correct for the top of the first view not touching the top of the list
  23.         adjustViewsUpOrDown();
  24.         below = fillDown(position + 1, temp.getBottom() + dividerHeight);
  25.         int childCount = getChildCount();
  26.         if (childCount > 0) {
  27.             correctTooHigh(childCount);
  28.         }
  29.     } else {
  30.         below = fillDown(position + 1, temp.getBottom() + dividerHeight);
  31.         // This will correct for the bottom of the last view not touching the bottom of the list
  32.         adjustViewsUpOrDown();
  33.         above = fillUp(position - 1, temp.getTop() - dividerHeight);
  34.         int childCount = getChildCount();
  35.         if (childCount > 0) {
  36.              correctTooLow(childCount);
  37.         }
  38.     }
  39.     if (tempIsSelected) {
  40.         return temp;
  41.     } else if (above != null) {
  42.         return above;
  43.     } else {
  44.         return below;
  45.     }
  46. }

fillSpecific()这算是一个新方法了。只是事实上它和fillUp()、fillDown()方法功能也是差点儿相同的。基本的差别在于，fillSpecific()方法会优先将指定位置的子View先载入到屏幕上，然后再载入该子View往上以及往下的其它子View。

那么由于这里我们传入的position就是第一个子View的位置。于是fillSpecific()方法的作用就基本上和fillDown()方法是差点儿相同的了，这里我们就不去关注太多它的细节。而是将精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法，代码例如以下所看到的：

  1. /**
  2.  * Obtain the view and add it to our list of children. The view can be made
  3.  * fresh, converted from an unused view, or used as is if it was in the
  4.  * recycle bin.
  5.  *
  6.  * @param position Logical position in the list
  7.  * @param y Top or bottom edge of the view to add
  8.  * @param flow If flow is true, align top edge to y. If false, align bottom
  9.  *        edge to y.
  10.  * @param childrenLeft Left edge where children should be positioned
  11.  * @param selected Is this position selected?
  12.  * @return View that was added
  13.  */
  14. private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
  15.         boolean selected) {
  16.     View child;
  17.     if (!mDataChanged) {
  18.         // Try to use an exsiting view for this position
  19.         child = mRecycler.getActiveView(position);
  20.         if (child != null) {
  21.             // Found it -- we're using an existing child
  22.             // This just needs to be positioned
  23.             setupChild(child, position, y, flow, childrenLeft, selected, true);
  24.             return child;
  25.         }
  26.     }
  27.     // Make a new view for this position, or convert an unused view if possible
  28.     child = obtainView(position, mIsScrap);
  29.     // This needs to be positioned and measured
  30.     setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
  31.     return child;
  32. }

仍然还是在第19行尝试从RecycleBin其中获取Active View，然而这次就一定能够获取到了，由于前面我们调用了RecycleBin的fillActiveViews()方法来缓存子View。那么既然如此，就不会再进入到第28行的obtainView()方法，而是会直接进入setupChild()方法其中。这样也省去了许多时间，由于假设在obtainView()方法中又要去infalte布局的话，那么ListView的初始载入效率就大大减少了。

注意在第23行。setupChild()方法的最后一个參数传入的是true，这个參数表明当前的View是之前被回收过的，那么我们再次回到setupChild()方法其中：

  1. /**
  2.  * Add a view as a child and make sure it is measured (if necessary) and
  3.  * positioned properly.
  4.  *
  5.  * @param child The view to add
  6.  * @param position The position of this child
  7.  * @param y The y position relative to which this view will be positioned
  8.  * @param flowDown If true, align top edge to y. If false, align bottom
  9.  *        edge to y.
  10.  * @param childrenLeft Left edge where children should be positioned
  11.  * @param selected Is this position selected?
  12.  * @param recycled Has this view been pulled from the recycle bin? If so it
  13.  *        does not need to be remeasured.
  14.  */
  15. private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
  16.         boolean selected, boolean recycled) {
  17.     final boolean isSelected = selected && shouldShowSelector();
  18.     final boolean updateChildSelected = isSelected != child.isSelected();
  19.     final int mode = mTouchMode;
  20.     final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
  21.             mMotionPosition == position;
  22.     final boolean updateChildPressed = isPressed != child.isPressed();
  23.     final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
  24.     // Respect layout params that are already in the view. Otherwise make some up...
  25.     // noinspection unchecked
  26.     AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
  27.     if (p == null) {
  28.         p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
  29.                 ViewGroup.LayoutParams.WRAP_CONTENT, 0);
  30.     }
  31.     p.viewType = mAdapter.getItemViewType(position);
  32.     if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
  33.             p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
  34.         attachViewToParent(child, flowDown ? -1 : 0, p);
  35.     } else {
  36.         p.forceAdd = false;
  37.         if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
  38.             p.recycledHeaderFooter = true;
  39.         }
  40.         addViewInLayout(child, flowDown ?
  41.  
  42. -1 : 0, p, true);
  43.     }
  44.     if (updateChildSelected) {
  45.         child.setSelected(isSelected);
  46.     }
  47.     if (updateChildPressed) {
  48.         child.setPressed(isPressed);
  49.     }
  50.     if (needToMeasure) {
  51.         int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
  52.                 mListPadding.left + mListPadding.right, p.width);
  53.         int lpHeight = p.height;
  54.         int childHeightSpec;
  55.         if (lpHeight > 0) {
  56.             childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
  57.         } else {
  58.             childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
  59.         }
  60.         child.measure(childWidthSpec, childHeightSpec);
  61.     } else {
  62.         cleanupLayoutState(child);
  63.     }
  64.     final int w = child.getMeasuredWidth();
  65.     final int h = child.getMeasuredHeight();
  66.     final int childTop = flowDown ? y : y - h;
  67.     if (needToMeasure) {
  68.         final int childRight = childrenLeft + w;
  69.         final int childBottom = childTop + h;
  70.         child.layout(childrenLeft, childTop, childRight, childBottom);
  71.     } else {
  72.         child.offsetLeftAndRight(childrenLeft - child.getLeft());
  73.         child.offsetTopAndBottom(childTop - child.getTop());
  74.     }
  75.     if (mCachingStarted && !child.isDrawingCacheEnabled()) {
  76.         child.setDrawingCacheEnabled(true);
  77.     }
  78. }

能够看到，setupChild()方法的最后一个參数是recycled。然后在第32行会对这个变量进行推断，由于recycled如今是true。所以会运行attachViewToParent()方法。而第一次Layout过程则是运行的else语句中的addViewInLayout()方法。这两个方法最大的差别在于。假设我们须要向ViewGroup中加入一个新的子View，应该调用addViewInLayout()方法。而假设是想要将一个之前detach的View又一次attach到ViewGroup上，就应该调用attachViewToParent()方法。那么由于前面在layoutChildren()方法其中调用了detachAllViewsFromParent()方法，这样ListView中全部的子View都是处于detach状态的，所以这里attachViewToParent()方法是正确的选择。

经历了这样一个detach又attach的过程，ListView中全部的子View又都能够正常显示出来了，那么第二次Layout过程结束。

滑动载入许多其它数据

经历了两次Layout过程，虽说我们已经能够在ListView中看到内容了，然而关于ListView最奇妙的部分我们却还没有接触到，由于眼下ListView中仅仅是载入并显示了第一屏的数据而已。比方说我们的Adapter其中有1000条数据，可是第一屏仅仅显示了10条，ListView中也仅仅有10个子View而已，那么剩下的990是如何工作并显示到界面上的呢？这就要看一下ListView滑动部分的源代码了，由于我们是通过手指滑动来显示许多其它数据的。

由于滑动部分的机制是属于通用型的，即ListView和GridView都会使用相同的机制。因此这部分代码就肯定是写在AbsListView其中的了。那么监听触控事件是在onTouchEvent()方法其中进行的。我们就来看一下AbsListView中的这种方法：

  1. @Override
  2. public boolean onTouchEvent(MotionEvent ev) {
  3. 	if (!isEnabled()) {
  4. 		// A disabled view that is clickable still consumes the touch
  5. 		// events, it just doesn't respond to them.
  6. 		return isClickable() || isLongClickable();
  7. 	}
  8. 	final int action = ev.getAction();
  9. 	View v;
  10. 	int deltaY;
  11. 	if (mVelocityTracker == null) {
  12. 		mVelocityTracker = VelocityTracker.obtain();
  13. 	}
  14. 	mVelocityTracker.addMovement(ev);
  15. 	switch (action & MotionEvent.ACTION_MASK) {
  16. 	case MotionEvent.ACTION_DOWN: {
  17. 		mActivePointerId = ev.getPointerId(0);
  18. 		final int x = (int) ev.getX();
  19. 		final int y = (int) ev.getY();
  20. 		int motionPosition = pointToPosition(x, y);
  21. 		if (!mDataChanged) {
  22. 			if ((mTouchMode != TOUCH_MODE_FLING) && (motionPosition >= 0)
  23. 					&& (getAdapter().isEnabled(motionPosition))) {
  24. 				// User clicked on an actual view (and was not stopping a
  25. 				// fling). It might be a
  26. 				// click or a scroll. Assume it is a click until proven
  27. 				// otherwise
  28. 				mTouchMode = TOUCH_MODE_DOWN;
  29. 				// FIXME Debounce
  30. 				if (mPendingCheckForTap == null) {
  31. 					mPendingCheckForTap = new CheckForTap();
  32. 				}
  33. 				postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
  34. 			} else {
  35. 				if (ev.getEdgeFlags() != 0 && motionPosition < 0) {
  36. 					// If we couldn't find a view to click on, but the down
  37. 					// event was touching
  38. 					// the edge, we will bail out and try again. This allows
  39. 					// the edge correcting
  40. 					// code in ViewRoot to try to find a nearby view to
  41. 					// select
  42. 					return false;
  43. 				}
  44.  
  45. 				if (mTouchMode == TOUCH_MODE_FLING) {
  46. 					// Stopped a fling. It is a scroll.
  47. 					createScrollingCache();
  48. 					mTouchMode = TOUCH_MODE_SCROLL;
  49. 					mMotionCorrection = 0;
  50. 					motionPosition = findMotionRow(y);
  51. 					reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
  52. 				}
  53. 			}
  54. 		}
  55. 		if (motionPosition >= 0) {
  56. 			// Remember where the motion event started
  57. 			v = getChildAt(motionPosition - mFirstPosition);
  58. 			mMotionViewOriginalTop = v.getTop();
  59. 		}
  60. 		mMotionX = x;
  61. 		mMotionY = y;
  62. 		mMotionPosition = motionPosition;
  63. 		mLastY = Integer.MIN_VALUE;
  64. 		break;
  65. 	}
  66. 	case MotionEvent.ACTION_MOVE: {
  67. 		final int pointerIndex = ev.findPointerIndex(mActivePointerId);
  68. 		final int y = (int) ev.getY(pointerIndex);
  69. 		deltaY = y - mMotionY;
  70. 		switch (mTouchMode) {
  71. 		case TOUCH_MODE_DOWN:
  72. 		case TOUCH_MODE_TAP:
  73. 		case TOUCH_MODE_DONE_WAITING:
  74. 			// Check if we have moved far enough that it looks more like a
  75. 			// scroll than a tap
  76. 			startScrollIfNeeded(deltaY);
  77. 			break;
  78. 		case TOUCH_MODE_SCROLL:
  79. 			if (PROFILE_SCROLLING) {
  80. 				if (!mScrollProfilingStarted) {
  81. 					Debug.startMethodTracing("AbsListViewScroll");
  82. 					mScrollProfilingStarted = true;
  83. 				}
  84. 			}
  85. 			if (y != mLastY) {
  86. 				deltaY -= mMotionCorrection;
  87. 				int incrementalDeltaY = mLastY != Integer.MIN_VALUE ?
  88.  
  89. y - mLastY : deltaY;
  90. 				// No need to do all this work if we're not going to move
  91. 				// anyway
  92. 				boolean atEdge = false;
  93. 				if (incrementalDeltaY != 0) {
  94. 					atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
  95. 				}
  96. 				// Check to see if we have bumped into the scroll limit
  97. 				if (atEdge && getChildCount() > 0) {
  98. 					// Treat this like we're starting a new scroll from the
  99. 					// current
  100. 					// position. This will let the user start scrolling back
  101. 					// into
  102. 					// content immediately rather than needing to scroll
  103. 					// back to the
  104. 					// point where they hit the limit first.
  105. 					int motionPosition = findMotionRow(y);
  106. 					if (motionPosition >= 0) {
  107. 						final View motionView = getChildAt(motionPosition - mFirstPosition);
  108. 						mMotionViewOriginalTop = motionView.getTop();
  109. 					}
  110. 					mMotionY = y;
  111. 					mMotionPosition = motionPosition;
  112. 					invalidate();
  113. 				}
  114. 				mLastY = y;
  115. 			}
  116. 			break;
  117. 		}
  118. 		break;
  119. 	}
  120. 	case MotionEvent.ACTION_UP: {
  121. 		switch (mTouchMode) {
  122. 		case TOUCH_MODE_DOWN:
  123. 		case TOUCH_MODE_TAP:
  124. 		case TOUCH_MODE_DONE_WAITING:
  125. 			final int motionPosition = mMotionPosition;
  126. 			final View child = getChildAt(motionPosition - mFirstPosition);
  127. 			if (child != null && !child.hasFocusable()) {
  128. 				if (mTouchMode != TOUCH_MODE_DOWN) {
  129. 					child.setPressed(false);
  130. 				}
  131. 				if (mPerformClick == null) {
  132. 					mPerformClick = new PerformClick();
  133. 				}
  134. 				final AbsListView.PerformClick performClick = mPerformClick;
  135. 				performClick.mChild = child;
  136. 				performClick.mClickMotionPosition = motionPosition;
  137. 				performClick.rememberWindowAttachCount();
  138. 				mResurrectToPosition = motionPosition;
  139. 				if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {
  140. 					final Handler handler = getHandler();
  141. 					if (handler != null) {
  142. 						handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap
  143. 								: mPendingCheckForLongPress);
  144. 					}
  145. 					mLayoutMode = LAYOUT_NORMAL;
  146. 					if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
  147. 						mTouchMode = TOUCH_MODE_TAP;
  148. 						setSelectedPositionInt(mMotionPosition);
  149. 						layoutChildren();
  150. 						child.setPressed(true);
  151. 						positionSelector(child);
  152. 						setPressed(true);
  153. 						if (mSelector != null) {
  154. 							Drawable d = mSelector.getCurrent();
  155. 							if (d != null && d instanceof TransitionDrawable) {
  156. 								((TransitionDrawable) d).resetTransition();
  157. 							}
  158. 						}
  159. 						postDelayed(new Runnable() {
  160. 							public void run() {
  161. 								child.setPressed(false);
  162. 								setPressed(false);
  163. 								if (!mDataChanged) {
  164. 									post(performClick);
  165. 								}
  166. 								mTouchMode = TOUCH_MODE_REST;
  167. 							}
  168. 						}, ViewConfiguration.getPressedStateDuration());
  169. 					} else {
  170. 						mTouchMode = TOUCH_MODE_REST;
  171. 					}
  172. 					return true;
  173. 				} else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
  174. 					post(performClick);
  175. 				}
  176. 			}
  177. 			mTouchMode = TOUCH_MODE_REST;
  178. 			break;
  179. 		case TOUCH_MODE_SCROLL:
  180. 			final int childCount = getChildCount();
  181. 			if (childCount > 0) {
  182. 				if (mFirstPosition == 0
  183. 						&& getChildAt(0).getTop() >= mListPadding.top
  184. 						&& mFirstPosition + childCount < mItemCount
  185. 						&& getChildAt(childCount - 1).getBottom() <= getHeight()
  186. 								- mListPadding.bottom) {
  187. 					mTouchMode = TOUCH_MODE_REST;
  188. 					reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
  189. 				} else {
  190. 					final VelocityTracker velocityTracker = mVelocityTracker;
  191. 					velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
  192. 					final int initialVelocity = (int) velocityTracker
  193. 							.getYVelocity(mActivePointerId);
  194. 					if (Math.abs(initialVelocity) > mMinimumVelocity) {
  195. 						if (mFlingRunnable == null) {
  196. 							mFlingRunnable = new FlingRunnable();
  197. 						}
  198. 						reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);
  199. 						mFlingRunnable.start(-initialVelocity);
  200. 					} else {
  201. 						mTouchMode = TOUCH_MODE_REST;
  202. 						reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
  203. 					}
  204. 				}
  205. 			} else {
  206. 				mTouchMode = TOUCH_MODE_REST;
  207. 				reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
  208. 			}
  209. 			break;
  210. 		}
  211. 		setPressed(false);
  212. 		// Need to redraw since we probably aren't drawing the selector
  213. 		// anymore
  214. 		invalidate();
  215. 		final Handler handler = getHandler();
  216. 		if (handler != null) {
  217. 			handler.removeCallbacks(mPendingCheckForLongPress);
  218. 		}
  219. 		if (mVelocityTracker != null) {
  220. 			mVelocityTracker.recycle();
  221. 			mVelocityTracker = null;
  222. 		}
  223. 		mActivePointerId = INVALID_POINTER;
  224. 		if (PROFILE_SCROLLING) {
  225. 			if (mScrollProfilingStarted) {
  226. 				Debug.stopMethodTracing();
  227. 				mScrollProfilingStarted = false;
  228. 			}
  229. 		}
  230. 		break;
  231. 	}
  232. 	case MotionEvent.ACTION_CANCEL: {
  233. 		mTouchMode = TOUCH_MODE_REST;
  234. 		setPressed(false);
  235. 		View motionView = this.getChildAt(mMotionPosition - mFirstPosition);
  236. 		if (motionView != null) {
  237. 			motionView.setPressed(false);
  238. 		}
  239. 		clearScrollingCache();
  240. 		final Handler handler = getHandler();
  241. 		if (handler != null) {
  242. 			handler.removeCallbacks(mPendingCheckForLongPress);
  243. 		}
  244. 		if (mVelocityTracker != null) {
  245. 			mVelocityTracker.recycle();
  246. 			mVelocityTracker = null;
  247. 		}
  248. 		mActivePointerId = INVALID_POINTER;
  249. 		break;
  250. 	}
  251. 	case MotionEvent.ACTION_POINTER_UP: {
  252. 		onSecondaryPointerUp(ev);
  253. 		final int x = mMotionX;
  254. 		final int y = mMotionY;
  255. 		final int motionPosition = pointToPosition(x, y);
  256. 		if (motionPosition >= 0) {
  257. 			// Remember where the motion event started
  258. 			v = getChildAt(motionPosition - mFirstPosition);
  259. 			mMotionViewOriginalTop = v.getTop();
  260. 			mMotionPosition = motionPosition;
  261. 		}
  262. 		mLastY = y;
  263. 		break;
  264. 	}
  265. 	}
  266. 	return true;
  267. }

这种方法中的代码就许多了，由于它所处理的逻辑或许多，要监听各种各样的触屏事件。

可是我们眼下所关心的就仅仅有手指在屏幕上滑动这一个事件而已。相应的是ACTION_MOVE这个动作，那么我们就仅仅看这部分代码就能够了。

能够看到。ACTION_MOVE这个case里面又嵌套了一个switch语句，是依据当前的TouchMode来选择的。那这里我能够直接告诉大家，当手指在屏幕上滑动时，TouchMode是等于TOUCH_MODE_SCROLL这个值的，至于为什么那又要牵扯到另外的好几个方法。这里限于篇幅原因就不再展开解说了，喜欢寻根究底的朋友们能够自己去源代码里找一找原因。

这样的话。代码就应该会走到第78行的这个case里面去了，在这个case其中并没有什么太多须要注意的东西。唯一一点很重要的就是第92行调用的trackMotionScroll()方法。相当于我们手指仅仅要在屏幕上略微有一点点移动。这种方法就会被调用，而假设是正常在屏幕上滑动的话，那么这种方法就会被调用许多次。

那么我们进入到这种方法中瞧一瞧，代码例如以下所看到的：

  1. boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
  2. 	final int childCount = getChildCount();
  3. 	if (childCount == 0) {
  4. 		return true;
  5. 	}
  6. 	final int firstTop = getChildAt(0).getTop();
  7. 	final int lastBottom = getChildAt(childCount - 1).getBottom();
  8. 	final Rect listPadding = mListPadding;
  9. 	final int spaceAbove = listPadding.top - firstTop;
  10. 	final int end = getHeight() - listPadding.bottom;
  11. 	final int spaceBelow = lastBottom - end;
  12. 	final int height = getHeight() - getPaddingBottom() - getPaddingTop();
  13. 	if (deltaY < 0) {
  14. 		deltaY = Math.max(-(height - 1), deltaY);
  15. 	} else {
  16. 		deltaY = Math.min(height - 1, deltaY);
  17. 	}
  18. 	if (incrementalDeltaY < 0) {
  19. 		incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
  20. 	} else {
  21. 		incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
  22. 	}
  23. 	final int firstPosition = mFirstPosition;
  24. 	if (firstPosition == 0 && firstTop >= listPadding.top && deltaY >= 0) {
  25. 		// Don't need to move views down if the top of the first position
  26. 		// is already visible
  27. 		return true;
  28. 	}
  29. 	if (firstPosition + childCount == mItemCount && lastBottom <= end && deltaY <= 0) {
  30. 		// Don't need to move views up if the bottom of the last position
  31. 		// is already visible
  32. 		return true;
  33. 	}
  34. 	final boolean down = incrementalDeltaY < 0;
  35. 	final boolean inTouchMode = isInTouchMode();
  36. 	if (inTouchMode) {
  37. 		hideSelector();
  38. 	}
  39. 	final int headerViewsCount = getHeaderViewsCount();
  40. 	final int footerViewsStart = mItemCount - getFooterViewsCount();
  41. 	int start = 0;
  42. 	int count = 0;
  43. 	if (down) {
  44. 		final int top = listPadding.top - incrementalDeltaY;
  45. 		for (int i = 0; i < childCount; i++) {
  46. 			final View child = getChildAt(i);
  47. 			if (child.getBottom() >= top) {
  48. 				break;
  49. 			} else {
  50. 				count++;
  51. 				int position = firstPosition + i;
  52. 				if (position >= headerViewsCount && position < footerViewsStart) {
  53. 					mRecycler.addScrapView(child);
  54. 				}
  55. 			}
  56. 		}
  57. 	} else {
  58. 		final int bottom = getHeight() - listPadding.bottom - incrementalDeltaY;
  59. 		for (int i = childCount - 1; i >= 0; i--) {
  60. 			final View child = getChildAt(i);
  61. 			if (child.getTop() <= bottom) {
  62. 				break;
  63. 			} else {
  64. 				start = i;
  65. 				count++;
  66. 				int position = firstPosition + i;
  67. 				if (position >= headerViewsCount && position < footerViewsStart) {
  68. 					mRecycler.addScrapView(child);
  69. 				}
  70. 			}
  71. 		}
  72. 	}
  73. 	mMotionViewNewTop = mMotionViewOriginalTop + deltaY;
  74. 	mBlockLayoutRequests = true;
  75. 	if (count > 0) {
  76. 		detachViewsFromParent(start, count);
  77. 	}
  78. 	offsetChildrenTopAndBottom(incrementalDeltaY);
  79. 	if (down) {
  80. 		mFirstPosition += count;
  81. 	}
  82. 	invalidate();
  83. 	final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
  84. 	if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
  85. 		fillGap(down);
  86. 	}
  87. 	if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
  88. 		final int childIndex = mSelectedPosition - mFirstPosition;
  89. 		if (childIndex >= 0 && childIndex < getChildCount()) {
  90. 			positionSelector(getChildAt(childIndex));
  91. 		}
  92. 	}
  93. 	mBlockLayoutRequests = false;
  94. 	invokeOnItemScrollListener();
  95. 	awakenScrollBars();
  96. 	return false;
  97. }

这种方法接收两个參数，deltaY表示从手指按下时的位置到当前手指位置的距离，incrementalDeltaY则表示据上次触发event事件手指在Y方向上位置的改变量。那么事实上我们就能够通过incrementalDeltaY的正负值情况来推断用户是向上还是向下滑动的了。如第34行代码所看到的，假设incrementalDeltaY小于0，说明是向下滑动。否则就是向上滑动。

以下将会进行一个边界值检測的过程。能够看到。从第43行開始，当ListView向下滑动的时候。就会进入一个for循环其中。从上往下依次获取子View，第47行其中，假设该子View的bottom值已经小于top值了。就说明这个子View已经移出屏幕了。所以会调用RecycleBin的addScrapView()方法将这个View加入到废弃缓存其中。并将count计数器加1，计数器用于记录有多少个子View被移出了屏幕。

那么假设是ListView向上滑动的话，事实上过程是基本相同的，仅仅只是变成了从下往上依次获取子View，然后推断该子View的top值是不是大于bottom值了，假设大于的话说明子View已经移出了屏幕，相同把它加入到废弃缓存中，并将计数器加1。

接下来在第76行。会依据当前计数器的值来进行一个detach操作，它的作用就是把全部移出屏幕的子View全部detach掉，在ListView的概念其中。全部看不到的View就没有必要为它进行保存，由于屏幕外还有成百上千条数据等着显示呢，一个好的回收策略才干保证ListView的高性能和高效率。

紧接着在第78行调用了offsetChildrenTopAndBottom()方法，并将incrementalDeltaY作为參数传入，这种方法的作用是让ListView中全部的子View都依照传入的參数值进行相应的偏移，这样就实现了随着手指的拖动，ListView的内容也会随着滚动的效果。

然后在第84行会进行推断，假设ListView中最后一个View的底部已经移入了屏幕，或者ListView中第一个View的顶部移入了屏幕，就会调用fillGap()方法，那么因此我们就能够猜出fillGap()方法是用来载入屏幕外数据的，进入到这种方法中瞧一瞧。例如以下所看到的：

  1. /**
  2.  * Fills the gap left open by a touch-scroll. During a touch scroll,
  3.  * children that remain on screen are shifted and the other ones are
  4.  * discarded. The role of this method is to fill the gap thus created by
  5.  * performing a partial layout in the empty space.
  6.  *
  7.  * @param down
  8.  *            true if the scroll is going down, false if it is going up
  9.  */
  10. abstract void fillGap(boolean down);

OK，AbsListView中的fillGap()是一个抽象方法，那么我们立马就能够想到。它的详细实现肯定是在ListView中完毕的了。回到ListView其中。fillGap()方法的代码例如以下所看到的： 

  1. void fillGap(boolean down) {
  2.     final int count = getChildCount();
  3.     if (down) {
  4.         final int startOffset = count > 0 ?
  5.  
  6. getChildAt(count - 1).getBottom() + mDividerHeight :
  7.                 getListPaddingTop();
  8.         fillDown(mFirstPosition + count, startOffset);
  9.         correctTooHigh(getChildCount());
  10.     } else {
  11.         final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
  12.                 getHeight() - getListPaddingBottom();
  13.         fillUp(mFirstPosition - 1, startOffset);
  14.         correctTooLow(getChildCount());
  15.     }
  16. }

down參数用于表示ListView是向下滑动还是向上滑动的，能够看到。假设是向下滑动的话就会调用fillDown()方法，而假设是向上滑动的话就会调用fillUp()方法。

那么这两个方法我们都已经很熟悉了，内部都是通过一个循环来去对ListView进行填充，所以这两个方法我们就不看了，可是填充ListView会通过调用makeAndAddView()方法来完毕，又是makeAndAddView()方法，但这次的逻辑再次不同了，所以我们还是回到这种方法瞧一瞧： 

  1. /**
  2.  * Obtain the view and add it to our list of children. The view can be made
  3.  * fresh, converted from an unused view, or used as is if it was in the
  4.  * recycle bin.
  5.  *
  6.  * @param position Logical position in the list
  7.  * @param y Top or bottom edge of the view to add
  8.  * @param flow If flow is true, align top edge to y. If false, align bottom
  9.  *        edge to y.
  10.  * @param childrenLeft Left edge where children should be positioned
  11.  * @param selected Is this position selected?
  12.  * @return View that was added
  13.  */
  14. private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
  15.         boolean selected) {
  16.     View child;
  17.     if (!mDataChanged) {
  18.         // Try to use an exsiting view for this position
  19.         child = mRecycler.getActiveView(position);
  20.         if (child != null) {
  21.             // Found it -- we're using an existing child
  22.             // This just needs to be positioned
  23.             setupChild(child, position, y, flow, childrenLeft, selected, true);
  24.             return child;
  25.         }
  26.     }
  27.     // Make a new view for this position, or convert an unused view if possible
  28.     child = obtainView(position, mIsScrap);
  29.     // This needs to be positioned and measured
  30.     setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
  31.     return child;
  32. }

无论怎么说。这里首先仍然是会尝试调用RecycleBin的getActiveView()方法来获取子布局，仅仅只是肯定是获取不到的了。由于在第二次Layout过程中我们已经从mActiveViews中获取过了数据，而依据RecycleBin的机制。mActiveViews是不能够反复利用的，因此这里返回的值肯定是null。

既然getActiveView()方法返回的值是null，那么就还是会走到第28行的obtainView()方法其中。代码例如以下所看到的：

  1. /**
  2.  * Get a view and have it show the data associated with the specified
  3.  * position. This is called when we have already discovered that the view is
  4.  * not available for reuse in the recycle bin. The only choices left are
  5.  * converting an old view or making a new one.
  6.  *
  7.  * @param position
  8.  *            The position to display
  9.  * @param isScrap
  10.  *            Array of at least 1 boolean, the first entry will become true
  11.  *            if the returned view was taken from the scrap heap, false if
  12.  *            otherwise.
  13.  *
  14.  * @return A view displaying the data associated with the specified position
  15.  */
  16. View obtainView(int position, boolean[] isScrap) {
  17. 	isScrap[0] = false;
  18. 	View scrapView;
  19. 	scrapView = mRecycler.getScrapView(position);
  20. 	View child;
  21. 	if (scrapView != null) {
  22. 		child = mAdapter.getView(position, scrapView, this);
  23. 		if (child != scrapView) {
  24. 			mRecycler.addScrapView(scrapView);
  25. 			if (mCacheColorHint != 0) {
  26. 				child.setDrawingCacheBackgroundColor(mCacheColorHint);
  27. 			}
  28. 		} else {
  29. 			isScrap[0] = true;
  30. 			dispatchFinishTemporaryDetach(child);
  31. 		}
  32. 	} else {
  33. 		child = mAdapter.getView(position, null, this);
  34. 		if (mCacheColorHint != 0) {
  35. 			child.setDrawingCacheBackgroundColor(mCacheColorHint);
  36. 		}
  37. 	}
  38. 	return child;
  39. }

这里在第19行会调用RecyleBin的getScrapView()方法来尝试从废弃缓存中获取一个View，那么废弃缓存有没有View呢？当然有。由于刚才在trackMotionScroll()方法中我们就已经看到了，一旦有不论什么子View被移出了屏幕，就会将它加入到废弃缓存中，而从obtainView()方法中的逻辑来看。一旦有新的数据须要显示到屏幕上。就会尝试从废弃缓存中获取View。所以它们之间就形成了一个生产者和消费者的模式，那么ListView奇妙的地方也就在这里体现出来了。无论你有随意多条数据须要显示。ListView中的子View事实上来来回回就那么几个，移出屏幕的子View会很快被移入屏幕的数据又一次利用起来，因而无论我们载入多少数据都不会出现OOM的情况，甚至内存都不会有所添加。

那么另外另一点是须要大家留意的。这里获取到了一个scrapView，然后我们在第22行将它作为第二个參数传入到了Adapter的getView()方法其中。那么第二个參数是什么意思呢？我们再次看一下一个简单的getView()方法演示样例：

  1. @Override
  2. public View getView(int position, View convertView, ViewGroup parent) {
  3. 	Fruit fruit = getItem(position);
  4. 	View view;
  5. 	if (convertView == null) {
  6. 		view = LayoutInflater.from(getContext()).inflate(resourceId, null);
  7. 	} else {
  8. 		view = convertView;
  9. 	}
  10. 	ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
  11. 	TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
  12. 	fruitImage.setImageResource(fruit.getImageId());
  13. 	fruitName.setText(fruit.getName());
  14. 	return view;
  15. }

第二个參数就是我们最熟悉的convertView呀。难怪平时我们在写getView()方法是要推断一下convertView是不是等于null，假设等于null才调用inflate()方法来载入布局，不等于null就能够直接利用convertView，由于convertView就是我们之间利用过的View。仅仅只是被移出屏幕后进入到了废弃缓存中，如今又又一次拿出来使用而已。然后我们仅仅须要把convertView中的数据更新成当前位置上应该显示的数据。那么看起来就好像是全新载入出来的一个布局一样，这背后的道理你是不是已经全然搞明确了？

之后的代码又都是我们熟悉的流程了。从缓存中拿到子View之后再调用setupChild()方法将它又一次attach到ListView其中，由于缓存中的View也是之前从ListView中detach掉的，这部分代码就不再反复进行分析了。

为了方便大家理解，这里我再附上一张图解说明：

那么到眼下为止。我们就把ListView的整个工作流程代码基本分析结束了。文章比較长，希望大家能够理解清晰，下篇文章中会解说我们平时使用ListView时遇到的问题，感兴趣的朋友请继续阅读 Android ListView异步载入图片乱序问题。原因分析及解决方式 。

blockquote{
border-left: 10px solid rgba(128,128,128,0.075);
background-color: rgba(128,128,128,0.05);
border-radius: 0 5px 5px 0;
padding: 15px 20px;
}

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