Android4.0 Launcher 源码分析3——WorkSpace结构（滑动）

3、WorkSpace结构（滑动）

　　桌面的左右滑动功能主要是在PagedView类中实现的，而WorkSpace是PagedView类的子类，所以会继承PagedView中的方法。当我们的手指点击WorkSpace时，首先就会触发PageView中的onInterceptTouchEvent()方法，会根据相应的条件来判断是否对Touch事件进行拦截，如果onInterceptTouchEvent()方法返回为true，则会对Touch事件进行拦截，PageView类的onTouch方法会进行响应从而得到调用。如果返回false，就分两钟情况：

1. 我们是点击在它的子控键上进行滑动时，比如我们是点击在桌面的图标上进行左右滑动的，workspace则会把Touch事件分发给它的子控件。
2. 而如果仅仅是点击到桌面的空白出Touch 事件就不会发生响应。

WorkSpace滑动事件：

1. MotionEvent.ACTION_DOWN：在我们手指第一次触摸到屏幕时，首先会对onInterceptTouchEvent中的事件进行判断，如果是按下事件 (MotionEvent.ACTION_DOWN)，则会记录按下时的X坐标、Y坐标等等数据，同时改变现在Workspace的状态为滚动状态(TOUCH_STATE_SCROLLING)，这时会返回 ture，把事件交给onTouchEvent函数来处理，onTouchEvent中同样会对事件类型进行判断，当事件方法为 (MotionEvent.ACTION_DOWN)的时候，就可以开始显示滚动的指示条了(就是Hotseat上PageIndirector显示第几屏的屏点)。
2. MotionEvent.ACTION_MOVE：当我们按着屏幕不放进行滑动的时候，又会在onInterceptTouchEvent进行事件拦截，但是现在的事件类型变为了MotionEvent.ACTION_MOVE，因为是移动的操作，所以会在拦截的时候取消桌面长按的事件的响应，同时转到onTouchEvent中对ACTION_MOVE事件的响应中，判断我们移动了多少距离，使用scrollBy方法来对桌面进行移动，并刷新屏幕。
3. MotionEvent.ACTION_UP：最后我们放开手后会触发 onTouchEvent中的MotionEvent.ACTION_UP事件，这时会根据滑动的情况来判断是朝左滑动还是朝右滑动，如果手指只滑动了屏 幕宽度的少一半距离，则会弹回原来的页面，滑动多于屏幕宽度的一半则会进行翻页。

　　同时要注意无论在什么情况下触发了WorkSpace滑动的事件，则系统会不断调用computeScroll()方法，我们重写这个方法同时在这个方法中调用刷新界面等操作。

　　滑动过程中所要注意的主要方法如下，具体见代码注释。

     //对Touch事件进行拦截   主要用于在拦截各种Touch事件时，设置mTouchState的各种状态
     @Override
     public boolean onInterceptTouchEvent(MotionEvent ev) {
         /*
          * This method JUST determines whether we want to intercept the motion.
          * If we return true, onTouchEvent will be called and we do the actual
          * scrolling there.
          * 这个方法仅仅决定了我们是否愿意去对滑动事件进行拦截，如果返回为true,则会调用onTouchEvent我们将会在那里进行事件处理
          */
         //对滑动的速率进行跟踪。 

         acquireVelocityTrackerAndAddMovement(ev); 

         // Skip touch handling if there are no pages to swipe
         // 如果没有页面，则跳过操作。
         if (getChildCount() <= 0) return super.onInterceptTouchEvent(ev); 

         /*
          * Shortcut the most recurring case: the user is in the dragging
          * state and he is moving his finger.  We want to intercept this
          * motion.
          * shortcut最常见的情况是：用户处于拖动的状态下，同时在移动它的手指，这时候我们需要拦截这个动作。
          *
          */
         final int action = ev.getAction();
         //如果是在MOVE的情况下，则进行Touch事件拦截
         if ((action == MotionEvent.ACTION_MOVE) &&
                 (mTouchState == TOUCH_STATE_SCROLLING)) {
             return true;
         } 

         switch (action & MotionEvent.ACTION_MASK) {
             case MotionEvent.ACTION_MOVE: {
                 /*
                  * mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
                  * whether the user has moved far enough from his original down touch.
                  * 如果mIsBeingDragged==false ,否则快捷方式应该捕获到该事件，检查一下用户从它点击的地方位移是否足够
                  */
                 if (mActivePointerId != INVALID_POINTER) {
                     //根据移动的距离判断是翻页还是移动一段位移，同时设置lastMotionX或者mTouchState这些值。同时取消桌面长按事件。
                     determineScrollingStart(ev);
                     break;
                 }
                 // if mActivePointerId is INVALID_POINTER, then we must have missed an ACTION_DOWN
                 // event. in that case, treat the first occurence of a move event as a ACTION_DOWN
                 // i.e. fall through to the next case (don't break)
                 // (We sometimes miss ACTION_DOWN events in Workspace because it ignores all events
                 // while it's small- this was causing a crash before we checked for INVALID_POINTER)
                 // 如果mActivePointerId 是 INVALID_POINTER，这时候我们应该已经错过了ACTION_DOWN事件。在这种情况下，把
                 // 第一次发生移动的事件当作ACTION——DOWN事件，直接进入下一个情况下。
                 // 我们有时候会错过workspace中的ACTION_DOWN事件，因为在workspace变小的时候会忽略掉所有的事件。
             } 

             case MotionEvent.ACTION_DOWN: {
                 final float x = ev.getX();
                 final float y = ev.getY();
                 // Remember location of down touch
                 // 记录按下的位置
                 mDownMotionX = x;
                 mLastMotionX = x;
                 mLastMotionY = y;
                 mLastMotionXRemainder = 0;
                 mTotalMotionX = 0;
                 //Return the pointer identifier associated with a particular pointer data index is this event.
                 //The identifier tells you the actual pointer number associated with the data,
                 //accounting for individual pointers going up and down since the start of the current gesture.
                 //返回和这个事件关联的触点数据id，计算单独点的id会上下浮动，因为手势的起始位置挥发声改变。
                 mActivePointerId = ev.getPointerId(0);
                 mAllowLongPress = true; 

                 /*
                  * If being flinged and user touches the screen, initiate drag;
                  * otherwise don't.  mScroller.isFinished should be false when
                  * being flinged.
                  * 如果被拖动同时用户触摸到了屏幕，就开始初始化拖动，否则便不会。
                  * 当拖动完成后mScroller.isFinished就应该设置为false.
                  *
                  */
                 final int xDist = Math.abs(mScroller.getFinalX() - mScroller.getCurrX()); 

                 final boolean finishedScrolling = (mScroller.isFinished() || xDist < mTouchSlop);
                 if (finishedScrolling) {
                     //标记为TOUCH_STATE_REST状态
                     mTouchState = TOUCH_STATE_REST;
                     //取消滚动动画
                     mScroller.abortAnimation();
                 } else {
                     //状态为TOUCH_STATE_SCROLLING
                     mTouchState = TOUCH_STATE_SCROLLING;
                 } 

                 // check if this can be the beginning of a tap on the side of the pages
                 // to scroll the current page
                 // 检测此事件是不是开始于点击页面的边缘来对当前页面进行滚动。
                 if (mTouchState != TOUCH_STATE_PREV_PAGE && mTouchState != TOUCH_STATE_NEXT_PAGE) {
                     if (getChildCount() > 0) {
                         //根据触点的点位来判断是否点击到上一页，从而更新相应的状态
                         if (hitsPreviousPage(x, y)) {
                             mTouchState = TOUCH_STATE_PREV_PAGE;
                         } else if (hitsNextPage(x, y)) {
                             mTouchState = TOUCH_STATE_NEXT_PAGE;
                         }
                     }
                 }
                 break;
             } 

             case MotionEvent.ACTION_UP:
             case MotionEvent.ACTION_CANCEL:
                 //触点不被相应时，所做的动作
                 mTouchState = TOUCH_STATE_REST;
                 mAllowLongPress = false;
                 mActivePointerId = INVALID_POINTER;
                 //释放速率跟踪
                 releaseVelocityTracker();
                 break; 

             case MotionEvent.ACTION_POINTER_UP:
                 onSecondaryPointerUp(ev);
                 releaseVelocityTracker();
                 break;
         } 

         /*
          * The only time we want to intercept motion events is if we are in the
          * drag mode.
          * 我们唯一会去对移动事件进行拦截的情况时我们在拖动模式下
          */
         if(DEBUG) Log.d(TAG, "onInterceptTouchEvent "+(mTouchState != TOUCH_STATE_REST));
         //只要是mTouchState的状态不为TOUCH_STATE_REST,那么就进行事件拦截
         return mTouchState != TOUCH_STATE_REST;
     } 

　　onTouchEvent方法，详细见代码注释：

 @Override
 public boolean onTouchEvent(MotionEvent ev) {
     // Skip touch handling if there are no pages to swipe
     // 如果没有子页面，就直接跳过
     if (getChildCount() <= 0) return super.onTouchEvent(ev); 

     acquireVelocityTrackerAndAddMovement(ev); 

     final int action = ev.getAction(); 

     switch (action & MotionEvent.ACTION_MASK) {
     case MotionEvent.ACTION_DOWN:
         /*
          * If being flinged and user touches, stop the fling. isFinished
          * will be false if being flinged.
          * 如果在滑动的过程中下用户又点击桌面，则取消滑动，从而响应当前的点击。
          * 在滑动的isFinished将返回false.
          */
         if (!mScroller.isFinished()) {
             mScroller.abortAnimation();
         } 

         // Remember where the motion event started
         mDownMotionX = mLastMotionX = ev.getX();
         mLastMotionXRemainder = 0;
         mTotalMotionX = 0;
         mActivePointerId = ev.getPointerId(0);
         //主要用来显示滚动条，表明要开始滚动了，这里可以进行调整，滚动条时逐渐显示还是立刻显示。
         if (mTouchState == TOUCH_STATE_SCROLLING) {
             pageBeginMoving();
         }
         break; 

     case MotionEvent.ACTION_MOVE:
         if (mTouchState == TOUCH_STATE_SCROLLING) {
             // Scroll to follow the motion event
             final int pointerIndex = ev.findPointerIndex(mActivePointerId);
             final float x = ev.getX(pointerIndex);
             final float deltaX = mLastMotionX + mLastMotionXRemainder - x;
             //总共移动的距离
             mTotalMotionX += Math.abs(deltaX); 

             // Only scroll and update mLastMotionX if we have moved some discrete amount.  We
             // keep the remainder because we are actually testing if we've moved from the last
             // scrolled position (which is discrete).
             // 如果我们移动了一小段距离，我们则移动和更新mLastMotionX 。我们保存Remainder变量是因为会检测我们 

             //是否是从最后的滚动点位移动的。
             if (Math.abs(deltaX) >= 1.0f) {
                 mTouchX += deltaX;
                 mSmoothingTime = System.nanoTime() / NANOTIME_DIV;
                 if (!mDeferScrollUpdate) {
                     scrollBy((int) deltaX, 0);
                     if (DEBUG) Log.d(TAG, "onTouchEvent().Scrolling: " + deltaX);
                 } else {
                     invalidate();
                 }
                 mLastMotionX = x;
                 mLastMotionXRemainder = deltaX - (int) deltaX;
             } else {
             //Trigger the scrollbars to draw. When invoked this method starts an animation to fade the
             //scrollbars out after a default delay. If a subclass provides animated scrolling,
             //the start delay should equal the duration of the scrolling animation.
             //触发scrollbar进行绘制。 使用这个方法来启动一个动画来使scrollbars经过一段时间淡出。如果子类提供了滚动的动画，则
             //延迟的时间等于动画滚动的时间。
                 awakenScrollBars();
             }
         } else {
             determineScrollingStart(ev);
         }
         break; 

     case MotionEvent.ACTION_UP:
         if (mTouchState == TOUCH_STATE_SCROLLING) {
             final int activePointerId = mActivePointerId;
             final int pointerIndex = ev.findPointerIndex(activePointerId);
             final float x = ev.getX(pointerIndex);
             final VelocityTracker velocityTracker = mVelocityTracker;
             velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
             int velocityX = (int) velocityTracker.getXVelocity(activePointerId);
             final int deltaX = (int) (x - mDownMotionX);
             final int pageWidth = getScaledMeasuredWidth(getPageAt(mCurrentPage));
             // 屏幕的宽度*0.4f
             boolean isSignificantMove = Math.abs(deltaX) > pageWidth *
                     SIGNIFICANT_MOVE_THRESHOLD;
             final int snapVelocity = mSnapVelocity; 

             mTotalMotionX += Math.abs(mLastMotionX + mLastMotionXRemainder - x); 

             boolean isFling = mTotalMotionX > MIN_LENGTH_FOR_FLING &&
                     Math.abs(velocityX) > snapVelocity; 

             // In the case that the page is moved far to one direction and then is flung
             // in the opposite direction, we use a threshold to determine whether we should
             // just return to the starting page, or if we should skip one further.
             // 这钟情况是页面朝一个方向移动了一段距离，然后又弹回去了。我们使用一个阀值来判断是进行翻页还是返回到初始页面
             boolean returnToOriginalPage = false;
             if (Math.abs(deltaX) > pageWidth * RETURN_TO_ORIGINAL_PAGE_THRESHOLD &&
                     Math.signum(velocityX) != Math.signum(deltaX) && isFling) {
                 returnToOriginalPage = true;
             } 

             int finalPage;
             // We give flings precedence over large moves, which is why we short-circuit our
             // test for a large move if a fling has been registered. That is, a large
             // move to the left and fling to the right will register as a fling to the right.
             //朝右移动
             if (((isSignificantMove && deltaX > 0 && !isFling) ||
                     (isFling && velocityX > 0)) && mCurrentPage > 0) {
                 finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage - 1;
                 snapToPageWithVelocity(finalPage, velocityX);
             //朝左移动
             } else if (((isSignificantMove && deltaX < 0 && !isFling) ||
                     (isFling && velocityX < 0)) &&
                     mCurrentPage < getChildCount() - 1) {
                 finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage + 1;
                 snapToPageWithVelocity(finalPage, velocityX);
             //寻找离屏幕中心最近的页面移动
             } else {
                 snapToDestination();
             }
         }
          //直接移动到前一页
          else if (mTouchState == TOUCH_STATE_PREV_PAGE) {
             // at this point we have not moved beyond the touch slop
             // (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so
             // we can just page
             int nextPage = Math.max(0, mCurrentPage - 1);
             if (nextPage != mCurrentPage) {
                 snapToPage(nextPage);
             } else {
                 snapToDestination();
             }
         }
          //直接移动到下一页
          else if (mTouchState == TOUCH_STATE_NEXT_PAGE) {
             // at this point we have not moved beyond the touch slop
             // (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so
             // we can just page
             int nextPage = Math.min(getChildCount() - 1, mCurrentPage + 1);
             if (nextPage != mCurrentPage) {
                 snapToPage(nextPage);
             } else {
                 snapToDestination();
             }
         } else {
             onUnhandledTap(ev);
         }
         mTouchState = TOUCH_STATE_REST;
         mActivePointerId = INVALID_POINTER;
         releaseVelocityTracker();
         break;
      //对事件不响应
     case MotionEvent.ACTION_CANCEL:
         if (mTouchState == TOUCH_STATE_SCROLLING) {
             snapToDestination();
         }
         mTouchState = TOUCH_STATE_REST;
         mActivePointerId = INVALID_POINTER;
         releaseVelocityTracker();
         break; 

     case MotionEvent.ACTION_POINTER_UP:
         onSecondaryPointerUp(ev);
         break;
     } 

     return true;
 }

scrollTo和scrollBy的区别

　　我们查看View类的源代码如下所示，mScrollX记录的是当前View针对屏幕坐标在水平方向上的偏移量，而mScrollY则是记录的时当前View针对屏幕在竖值方向上的偏移 量。

　　从以下代码我们可以得知：

1. scrollTo就是把View移动到屏幕的X和Y位置，也就是绝对位置。
2. 而scrollBy其实就是调用的scrollTo，但是参数是当前mScrollX和mScrollY加上X和Y的位置，所以ScrollBy调用的是相对于mScrollX和 mScrollY的位置。

　　我们在上面的代码中可以看到当我们手指不放移动屏幕时，就会调用scrollBy来移动一段相对的距离。而当我们手指松开后，会调用 mScroller.startScroll(mUnboundedScrollX, 0, delta, 0, duration); 来产生一段动画来移动到相应的页面，在这个过程中系统回不断调用computeScroll()，我们再使用scrollTo来把View移动到当前Scroller所在的绝对位置。

 /**
    * Set the scrolled position of your view. This will cause a call to
    * {@link #onScrollChanged(int, int, int, int)} and the view will be
    * invalidated.
    * @param x the x position to scroll to
    * @param y the y position to scroll to
    */
   public void scrollTo(int x, int y) {
       if (mScrollX != x || mScrollY != y) {
           int oldX = mScrollX;
           int oldY = mScrollY;
           mScrollX = x;
           mScrollY = y;
           invalidateParentCaches();
           onScrollChanged(mScrollX, mScrollY, oldX, oldY);
           if (!awakenScrollBars()) {
               invalidate(true);
           }
       }
   }
   /**
    * Move the scrolled position of your view. This will cause a call to
    * {@link #onScrollChanged(int, int, int, int)} and the view will be
    * invalidated.
    * @param x the amount of pixels to scroll by horizontally
    * @param y the amount of pixels to scroll by vertically
    */
   public void scrollBy(int x, int y) {
       scrollTo(mScrollX + x, mScrollY + y);
   }

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