【开源项目5】测滑菜单MenuDrawer的使用以及解析

在安卓中左右侧滑菜单的使用用的比ios多得多，可能是谷歌带的头吧，几乎所有的谷歌应用都有侧滑菜单。谷歌没有开放这个源码，在一个成熟的开源代码出现之前，大家都是各自为战，偶尔能看到一个勉强实现了的。MenuDrawer和其他的侧滑代码不同，他是一个性能高效且成熟的库。

在menuDraer出现之前我还用过slidemenu，效果差不多，但感觉没有MenuDrawer流畅，后来看了MenuDrawer和slidemenu的源码之后，才知道其实他们的实现思路是基本一致的，不过MenuDrawer的代码写的更好些。

MenuDrawer支持左、右、上、下四种侧滑方式，同时支持Overlay和Sliding两种模式。如下图：

但是MenuDrawer在github上的项目是gradle的android studio做的，直接将library导入到项目中会出现资源文件找不到的错误。因此要学会在eclipse中使用的话需要点耐心。官方已经有了其使用的方法，我这里就暂时不多说。其实要学会用这个库很简单，但是我的目的是要学会他实现滑动的基本方法，学会修改这个库，运用在更多的场景中。

我想弄明白下面的疑问：

在一个界面中如何自如的通过手势控制布局，想隐藏多少隐藏多少。

MenuDrawer的代码很多，但是其实我并不需要里面的很多功能，比如和actionbar的关联，甚至是上下滑动我也不需要，我只需要一个实现左滑菜单的范例，然后其他的我自己来，我不想一个项目下来用了很多别人的库，包含着一大堆跟业务无关的代码，而且我还不知道是拿来干嘛的。

经过研究，发现只需反复研究三个几个文件就能知道MenuDrawer的实现过程。

这三个类以及他们的的继承关系如下：

MenuDrawer.java -> DraggableDrawer.java -> SlidingDrawer.java

MenuDrawer.java 这个是基本的类，继承自ViewGroup，他定义了许多公共方法和属性变量，MenuDrawer初始化也是在这个类中。其中包含如何加载菜单和主内容的layout，如何根据xml中的属性以及样式来获取一些初始化值，比如菜单的宽度，是否要阴影以及当前菜单的箭头指示资源文件等，绘制箭头和阴影的实现也在这个文件中。

DraggableDrawer.java实现了menu滑动的一些动画过度效果，其实感觉这里面的东西并不多。

SlidingDrawer.java这个是实现滑动的主要类，要知道如何通过手势滑动布局，这里就可以找到答案。

下面是SlidingDrawer的代码，不过是我修改过了的。

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package net.simonvt.menudrawer; import android.app.Activity; import android.content.Context; import android.graphics.Canvas; import android.util.AttributeSet; import android.view.MotionEvent; import android.view.VelocityTracker; public class SlidingDrawer extends DraggableDrawer {     private static final String TAG = "OverlayDrawer";     SlidingDrawer(Activity activity, int dragMode) {         super(activity, dragMode);     }     public SlidingDrawer(Context context) {         super(context);     }     public SlidingDrawer(Context context, AttributeSet attrs) {         super(context, attrs);     }     public SlidingDrawer(Context context, AttributeSet attrs, int defStyle) {         super(context, attrs, defStyle);     }     @Override     protected void initDrawer(Context context, AttributeSet attrs, int defStyle) {         super.initDrawer(context, attrs, defStyle);         super.addView(mMenuContainer, -1, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));         super.addView(mContentContainer, -1, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));     }     @Override     public void openMenu(boolean animate) {         int animateTo = 0;         switch (getPosition()) {             case LEFT:             case TOP:                 animateTo = mMenuSize;                 break;             case RIGHT:             case BOTTOM:                 animateTo = -mMenuSize;                 break;         }         animateOffsetTo(animateTo, 0, animate);     }     @Override     public void closeMenu(boolean animate) {         animateOffsetTo(mMenuCloseSize, 0, animate);     }     @Override     protected void onOffsetPixelsChanged(int offsetPixels) {         if (USE_TRANSLATIONS) {             switch (getPosition()) {                 case TOP:                 case BOTTOM:                     mContentContainer.setTranslationY(offsetPixels);                     break;                 default:                     mContentContainer.setTranslationX(offsetPixels);                     break;             }         } else {             switch (getPosition()) {                 case TOP:                 case BOTTOM:                     mContentContainer.offsetTopAndBottom(offsetPixels - mContentContainer.getTop());                     break;                 default:                     mContentContainer.offsetLeftAndRight(offsetPixels - mContentContainer.getLeft());                     break;             }         }         offsetMenu(offsetPixels);         invalidate();     }     @Override     protected void initPeekScroller() {         switch (getPosition()) {             case RIGHT:             case BOTTOM: {                 final int dx = -mMenuSize / 3;                 mPeekScroller.startScroll(0, 0, dx, 0, PEEK_DURATION);                 break;             }             default: {                 final int dx = mMenuSize / 3;                 mPeekScroller.startScroll(0, 0, dx, 0, PEEK_DURATION);                 break;             }         }     }     @Override     protected void onSizeChanged(int w, int h, int oldw, int oldh) {         super.onSizeChanged(w, h, oldw, oldh);         onOffsetPixelsChanged((int) mOffsetPixels);     }     @Override     protected void drawOverlay(Canvas canvas) {         final int width = getWidth();         final int height = getHeight();         final int offsetPixels = (int) mOffsetPixels;         final float openRatio = Math.abs(mOffsetPixels) / mMenuSize;         switch (getPosition()) {             case LEFT:                 mMenuOverlay.setBounds(0, 0, offsetPixels, height);                 break;             case RIGHT:                 mMenuOverlay.setBounds(width + offsetPixels, 0, width, height);                 break;             case TOP:                 mMenuOverlay.setBounds(0, 0, width, offsetPixels);                 break;             case BOTTOM:                 mMenuOverlay.setBounds(0, height + offsetPixels, width, height);                 break;         }         mMenuOverlay.setAlpha((int) (MAX_MENU_OVERLAY_ALPHA * (1.f - openRatio)));         mMenuOverlay.draw(canvas);     }     @Override     protected void onLayout(boolean changed, int l, int t, int r, int b) {         final int width = r - l;         final int height = b - t;         if (USE_TRANSLATIONS) {             mContentContainer.layout(0, 0, width, height);         } else {             final int offsetPixels = (int) mOffsetPixels;             if (getPosition() == Position.LEFT || getPosition() == Position.RIGHT) {                 mContentContainer.layout(offsetPixels, 0, width + offsetPixels, height);             } else {                 mContentContainer.layout(0, offsetPixels, width, height + offsetPixels);             }         }         switch (getPosition()) {             case LEFT:                 mMenuContainer.layout(0, 0, mMenuSize, height);                 break;             case RIGHT:                 mMenuContainer.layout(width - mMenuSize, 0, width, height);                 break;             case TOP:                 mMenuContainer.layout(0, 0, width, mMenuSize);                 break;             case BOTTOM:                 mMenuContainer.layout(0, height - mMenuSize, width, height);                 break;         }     }     /**      * Offsets the menu relative to its original position based on the position of the content.      *      * @param offsetPixels The number of pixels the content if offset.      */     private void offsetMenu(int offsetPixels) {         if (!mOffsetMenu || mMenuSize == 0) {             return;         }         final int width = getWidth();         final int height = getHeight();         final int menuSize = mMenuSize;         final int sign = (int) (mOffsetPixels / Math.abs(mOffsetPixels));         final float openRatio = Math.abs(mOffsetPixels) / menuSize;         final int offset = (int) (-0.25f * ((1.0f - openRatio) * menuSize) * sign);         switch (getPosition()) {             case LEFT: {                 if (USE_TRANSLATIONS) {                     if (offsetPixels > 0) {                         mMenuContainer.setTranslationX(offset);                     } else {                         mMenuContainer.setTranslationX(-menuSize);                     }                 } else {                     mMenuContainer.offsetLeftAndRight(offset - mMenuContainer.getLeft());                     mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);                 }                 break;             }             case RIGHT: {                 if (USE_TRANSLATIONS) {                     if (offsetPixels != 0) {                         mMenuContainer.setTranslationX(offset);                     } else {                         mMenuContainer.setTranslationX(menuSize);                     }                 } else {                     final int oldOffset = mMenuContainer.getRight() - width;                     final int offsetBy = offset - oldOffset;                     mMenuContainer.offsetLeftAndRight(offsetBy);                     mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);                 }                 break;             }             case TOP: {                 if (USE_TRANSLATIONS) {                     if (offsetPixels > 0) {                         mMenuContainer.setTranslationY(offset);                     } else {                         mMenuContainer.setTranslationY(-menuSize);                     }                 } else {                     mMenuContainer.offsetTopAndBottom(offset - mMenuContainer.getTop());                     mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);                 }                 break;             }             case BOTTOM: {                 if (USE_TRANSLATIONS) {                     if (offsetPixels != 0) {                         mMenuContainer.setTranslationY(offset);                     } else {                         mMenuContainer.setTranslationY(menuSize);                     }                 } else {                     final int oldOffset = mMenuContainer.getBottom() - height;                     final int offsetBy = offset - oldOffset;                     mMenuContainer.offsetTopAndBottom(offsetBy);                     mMenuContainer.setVisibility(offsetPixels == 0 ? INVISIBLE : VISIBLE);                 }                 break;             }         }     }     @Override     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {         final int widthMode = MeasureSpec.getMode(widthMeasureSpec);         final int heightMode = MeasureSpec.getMode(heightMeasureSpec);         if (widthMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.UNSPECIFIED) {             throw new IllegalStateException("Must measure with an exact size");         }         final int width = MeasureSpec.getSize(widthMeasureSpec);         final int height = MeasureSpec.getSize(heightMeasureSpec);                                                                                                                                                                                                                                                                                                                                                                                                                                                    /** by hejie */         mMenuSize = width;         mMenuCloseSize =  (int) (mMenuSize * 0.3f);;         //mMenuCloseSize =  (int) (mMenuSize * 0.4f);;        // if (mOffsetPixels == -1) openMenu(false);          openMenu(false);        /** */         int menuWidthMeasureSpec;         int menuHeightMeasureSpec;         switch (getPosition()) {             case TOP:             case BOTTOM:                 menuWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, width);                 menuHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec, 0, mMenuSize);                 break;             default:                 // LEFT/RIGHT                 menuWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, mMenuSize);                 menuHeightMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, height);         }         mMenuContainer.measure(menuWidthMeasureSpec, menuHeightMeasureSpec);         final int contentWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, width);         final int contentHeightMeasureSpec = getChildMeasureSpec(widthMeasureSpec, 0, height);         mContentContainer.measure(contentWidthMeasureSpec, contentHeightMeasureSpec);         setMeasuredDimension(width, height);         updateTouchAreaSize();     }     private boolean isContentTouch(int x, int y) {         boolean contentTouch = false;         switch (getPosition()) {             case LEFT:                 contentTouch = ViewHelper.getLeft(mContentContainer) < x;                 break;             case RIGHT:                 contentTouch = ViewHelper.getRight(mContentContainer) > x;                 break;             case TOP:                 contentTouch = ViewHelper.getTop(mContentContainer) < y;                 break;             case BOTTOM:                 contentTouch = ViewHelper.getBottom(mContentContainer) > y;                 break;         }         return contentTouch;     }     protected boolean onDownAllowDrag(int x, int y) {         switch (getPosition()) {             case LEFT:                 return (!mMenuVisible && mInitialMotionX <= mTouchSize)                         || (mMenuVisible && mInitialMotionX >= mOffsetPixels);             case RIGHT:                 final int width = getWidth();                 final int initialMotionX = (int) mInitialMotionX;                 return (!mMenuVisible && initialMotionX >= width - mTouchSize)                         || (mMenuVisible && initialMotionX <= width + mOffsetPixels);             case TOP:                 return (!mMenuVisible && mInitialMotionY <= mTouchSize)                         || (mMenuVisible && mInitialMotionY >= mOffsetPixels);             case BOTTOM:                 final int height = getHeight();                 return (!mMenuVisible && mInitialMotionY >= height - mTouchSize)                         || (mMenuVisible && mInitialMotionY <= height + mOffsetPixels);         }         return false;     }     protected boolean onMoveAllowDrag(int x, int y, float dx, float dy) {         switch (getPosition()) {             case LEFT:                 return (!mMenuVisible && mInitialMotionX <= mTouchSize && (dx > 0))                         || (mMenuVisible && x >= mOffsetPixels);             case RIGHT:                 final int width = getWidth();                 return (!mMenuVisible && mInitialMotionX >= width - mTouchSize && (dx < 0))                         || (mMenuVisible && x <= width + mOffsetPixels);             case TOP:                 return (!mMenuVisible && mInitialMotionY <= mTouchSize && (dy > 0))                         || (mMenuVisible && y >= mOffsetPixels);             case BOTTOM:                 final int height = getHeight();                 return (!mMenuVisible && mInitialMotionY >= height - mTouchSize && (dy < 0))                         || (mMenuVisible && y <= height + mOffsetPixels);         }         return false;     }     protected void onMoveEvent(float dx, float dy) {         switch (getPosition()) {             case LEFT:                 setOffsetPixels(Math.min(Math.max(mOffsetPixels + dx, mMenuCloseSize), mMenuSize));                 break;             case RIGHT:                 setOffsetPixels(Math.max(Math.min(mOffsetPixels + dx, 0), -mMenuSize));                 break;             case TOP:                 setOffsetPixels(Math.min(Math.max(mOffsetPixels + dy, 0), mMenuSize));                 break;             case BOTTOM:                 setOffsetPixels(Math.max(Math.min(mOffsetPixels + dy, 0), -mMenuSize));                 break;         }     }     protected void onUpEvent(int x, int y) {         final int offsetPixels = (int) mOffsetPixels;         switch (getPosition()) {             case LEFT: {                 if (mIsDragging) {                     mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);                     final int initialVelocity = (int) getXVelocity(mVelocityTracker);                     mLastMotionX = x;                     animateOffsetTo(initialVelocity > 0 ? mMenuSize : mMenuCloseSize, initialVelocity, true);                     // Close the menu when content is clicked while the menu is visible.                 } else if (mMenuVisible && x > offsetPixels) {                     closeMenu();                 }                 break;             }             case TOP: {                 if (mIsDragging) {                     mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);                     final int initialVelocity = (int) getYVelocity(mVelocityTracker);                     mLastMotionY = y;                     animateOffsetTo(initialVelocity > 0 ? mMenuSize : 0, initialVelocity, true);                     // Close the menu when content is clicked while the menu is visible.                 } else if (mMenuVisible && y > offsetPixels) {                     closeMenu();                 }                 break;             }             case RIGHT: {                 final int width = getWidth();                 if (mIsDragging) {                     mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);                     final int initialVelocity = (int) getXVelocity(mVelocityTracker);                     mLastMotionX = x;                     animateOffsetTo(initialVelocity > 0 ? 0 : -mMenuSize, initialVelocity, true);                     // Close the menu when content is clicked while the menu is visible.                 } else if (mMenuVisible && x < width + offsetPixels) {                     closeMenu();                 }                 break;             }             case BOTTOM: {                 if (mIsDragging) {                     mVelocityTracker.computeCurrentVelocity(1000, mMaxVelocity);                     final int initialVelocity = (int) getYVelocity(mVelocityTracker);                     mLastMotionY = y;                     animateOffsetTo(initialVelocity < 0 ? -mMenuSize : 0, initialVelocity, true);                     // Close the menu when content is clicked while the menu is visible.                 } else if (mMenuVisible && y < getHeight() + offsetPixels) {                     closeMenu();                 }                 break;             }         }     }     protected boolean checkTouchSlop(float dx, float dy) {         switch (getPosition()) {             case TOP:             case BOTTOM:                 return Math.abs(dy) > mTouchSlop && Math.abs(dy) > Math.abs(dx);             default:                 return Math.abs(dx) > mTouchSlop && Math.abs(dx) > Math.abs(dy);         }     }     public boolean onInterceptTouchEvent(MotionEvent ev) {         final int action = ev.getAction() & MotionEvent.ACTION_MASK;         if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {             mActivePointerId = INVALID_POINTER;             mIsDragging = false;             if (mVelocityTracker != null) {                 mVelocityTracker.recycle();                 mVelocityTracker = null;             }             if (Math.abs(mOffsetPixels) > mMenuSize / 2) {                 openMenu();             } else {                 closeMenu();             }             return false;         }         if (action == MotionEvent.ACTION_DOWN && mMenuVisible && isCloseEnough()) {             setOffsetPixels(0);             stopAnimation();             endPeek();             setDrawerState(STATE_CLOSED);             mIsDragging = false;         }         // Always intercept events over the content while menu is visible.         if (mMenuVisible) {             int index = 0;             if (mActivePointerId != INVALID_POINTER) {                 index = ev.findPointerIndex(mActivePointerId);                 index = index == -1 ? 0 : index;             }             final int x = (int) ev.getX(index);             final int y = (int) ev.getY(index);             if (isContentTouch(x, y)) {                 return true;             }         }         if (!mMenuVisible && !mIsDragging && mTouchMode == TOUCH_MODE_NONE) {             return false;         }         if (action != MotionEvent.ACTION_DOWN && mIsDragging) {             return true;         }         switch (action) {             case MotionEvent.ACTION_DOWN: {                 mLastMotionX = mInitialMotionX = ev.getX();                 mLastMotionY = mInitialMotionY = ev.getY();                 final boolean allowDrag = onDownAllowDrag((int) mLastMotionX, (int) mLastMotionY);                 mActivePointerId = ev.getPointerId(0);                 if (allowDrag) {                     setDrawerState(mMenuVisible ? STATE_OPEN : STATE_CLOSED);                     stopAnimation();                     endPeek();                     mIsDragging = false;                 }                 break;             }             case MotionEvent.ACTION_MOVE: {                 final int activePointerId = mActivePointerId;                 if (activePointerId == INVALID_POINTER) {                     // If we don't have a valid id, the touch down wasn't on content.                     break;                 }                 final int pointerIndex = ev.findPointerIndex(activePointerId);                 if (pointerIndex == -1) {                     mIsDragging = false;                     mActivePointerId = INVALID_POINTER;                     endDrag();                     closeMenu(true);                     return false;                 }                 final float x = ev.getX(pointerIndex);                 final float dx = x - mLastMotionX;                 final float y = ev.getY(pointerIndex);                 final float dy = y - mLastMotionY;                 if (checkTouchSlop(dx, dy)) {                     if (mOnInterceptMoveEventListener != null && (mTouchMode == TOUCH_MODE_FULLSCREEN || mMenuVisible)                             && canChildrenScroll((int) dx, (int) dy, (int) x, (int) y)) {                         endDrag(); // Release the velocity tracker                         requestDisallowInterceptTouchEvent(true);                         return false;                     }                     final boolean allowDrag = onMoveAllowDrag((int) x, (int) y, dx, dy);                     if (allowDrag) {                         setDrawerState(STATE_DRAGGING);                         mIsDragging = true;                         mLastMotionX = x;                         mLastMotionY = y;                     }                 }                 break;             }             case MotionEvent.ACTION_POINTER_UP:                 onPointerUp(ev);                 mLastMotionX = ev.getX(ev.findPointerIndex(mActivePointerId));                 mLastMotionY = ev.getY(ev.findPointerIndex(mActivePointerId));                 break;         }         if (mVelocityTracker == null) mVelocityTracker = VelocityTracker.obtain();         mVelocityTracker.addMovement(ev);         return mIsDragging;     }     @Override     public boolean onTouchEvent(MotionEvent ev) {         if (!mMenuVisible && !mIsDragging && mTouchMode == TOUCH_MODE_NONE) {             return false;         }         final int action = ev.getAction() & MotionEvent.ACTION_MASK;         if (mVelocityTracker == null) mVelocityTracker = VelocityTracker.obtain();         mVelocityTracker.addMovement(ev);         switch (action) {             case MotionEvent.ACTION_DOWN: {                 mLastMotionX = mInitialMotionX = ev.getX();                 mLastMotionY = mInitialMotionY = ev.getY();                 final boolean allowDrag = onDownAllowDrag((int) mLastMotionX, (int) mLastMotionY);                 mActivePointerId = ev.getPointerId(0);                 if (allowDrag) {                     stopAnimation();                     endPeek();                     startLayerTranslation();                 }                 break;             }             case MotionEvent.ACTION_MOVE: {                 final int pointerIndex = ev.findPointerIndex(mActivePointerId);                 if (pointerIndex == -1) {                     mIsDragging = false;                     mActivePointerId = INVALID_POINTER;                     endDrag();                     closeMenu(true);                     return false;                 }                 if (!mIsDragging) {                     final float x = ev.getX(pointerIndex);                     final float dx = x - mLastMotionX;                     final float y = ev.getY(pointerIndex);                     final float dy = y - mLastMotionY;                     if (checkTouchSlop(dx, dy)) {                         final boolean allowDrag = onMoveAllowDrag((int) x, (int) y, dx, dy);                         if (allowDrag) {                             setDrawerState(STATE_DRAGGING);                             mIsDragging = true;                             mLastMotionX = x;                             mLastMotionY = y;                         } else {                             mInitialMotionX = x;                             mInitialMotionY = y;                         }                     }                 }                 if (mIsDragging) {                     startLayerTranslation();                     final float x = ev.getX(pointerIndex);                     final float dx = x - mLastMotionX;                     final float y = ev.getY(pointerIndex);                     final float dy = y - mLastMotionY;                     mLastMotionX = x;                     mLastMotionY = y;                     onMoveEvent(dx, dy);                 }                 break;             }             case MotionEvent.ACTION_CANCEL:             case MotionEvent.ACTION_UP: {                 int index = ev.findPointerIndex(mActivePointerId);                 index = index == -1 ? 0 : index;                 final int x = (int) ev.getX(index);                 final int y = (int) ev.getY(index);                 onUpEvent(x, y);                 mActivePointerId = INVALID_POINTER;                 mIsDragging = false;                 break;             }             case MotionEvent.ACTION_POINTER_DOWN:                 final int index = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK)                         >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;                 mLastMotionX = ev.getX(index);                 mLastMotionY = ev.getY(index);                 mActivePointerId = ev.getPointerId(index);                 break;             case MotionEvent.ACTION_POINTER_UP:                 onPointerUp(ev);                 mLastMotionX = ev.getX(ev.findPointerIndex(mActivePointerId));                 mLastMotionY = ev.getY(ev.findPointerIndex(mActivePointerId));                 break;         }         return true;     }     private void onPointerUp(MotionEvent ev) {         final int pointerIndex = ev.getActionIndex();         final int pointerId = ev.getPointerId(pointerIndex);         if (pointerId == mActivePointerId) {             final int newPointerIndex = pointerIndex == 0 ? 1 : 0;             mLastMotionX = ev.getX(newPointerIndex);             mActivePointerId = ev.getPointerId(newPointerIndex);             if (mVelocityTracker != null) {                 mVelocityTracker.clear();             }         }     } }

onInterceptTouchEvent和onTouchEvent是捕捉用户滑动事件的方法，其中onInterceptTouchEvent的用途是在onTouchEvent之前截获事件，并提供一次改变事件传递方向的机会。当用户滑动屏幕，onTouchEvent会被调用并执行case MotionEvent.ACTION_MOVE:下面的代码，通过比较当前触摸点和上次触摸点的位置记录x和y方向上的偏移量，然后根据这个偏移量移动继承自ViewGroup的MenuDrawer。其实MenuDrawer的这个过程和ViewPager非常相似。

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final float x = ev.getX(pointerIndex); final float dx = x - mLastMotionX; final float y = ev.getY(pointerIndex); final float dy = y - mLastMotionY; mLastMotionX = x; mLastMotionY = y; onMoveEvent(dx, dy)

onMoveEvent就是移动ViewGroup的方法。

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protected void onMoveEvent(float dx, float dy) {     switch (getPosition()) {         case LEFT:             setOffsetPixels(Math.min(Math.max(mOffsetPixels + dx, mMenuCloseSize), mMenuSize));             break;         case RIGHT:             setOffsetPixels(Math.max(Math.min(mOffsetPixels + dx, 0), -mMenuSize));             break;         case TOP:             setOffsetPixels(Math.min(Math.max(mOffsetPixels + dy, 0), mMenuSize));             break;         case BOTTOM:             setOffsetPixels(Math.max(Math.min(mOffsetPixels + dy, 0), -mMenuSize));             break;     } }

根据上面的代码我们知道手指滑动屏幕的过程中onTouchEvent onMoveEvent和setOffsetPixels相继被调用。setOffsetPixels之后最终会在offsetMenu中用offsetLeftAndRight（）来移动ViewGroup。