概述：
前面三篇博客已将分发事件的原理和简单应用介绍了下，那么有什么比较常用的控件能够更加深入且更加具体的说明这个问题呢。所以这篇，主要是介绍源码中事件分发的例子。大家在开发的时候，很多场合下会用到侧滑菜单，而android在V4支持库中也提供了一个非常好用的侧滑控件-DrawerLayout，一般在这个控件中，我们会设置一个侧滑栏，然后设置一个内容布局，这样在侧滑的时候便会将左侧隐藏的布局显示出来。这个效果相对来说比较简单，理解起来也比较容易，但是实现却不简单。那么这篇博客就将简单分析DrawerLayout的事件分发的实现。

效果

我们在创建布局的时候，android会提供一个Navigation Drawer Activity的一个默认Activity的实现。我们就先来看下这个默认实现的效果。

1.gif

这个效果应该在很大部分的应用中都有，那么滑动怎么实现的呢，这里就来分析下吧（其他如测量和布局会在相应主题的博客中介绍）

拦截事件

事件分发事件在源码解析那篇中已经有了介绍，一般不会去重写。这里从介绍onInterceptTouchEvent方法开始。如下是源码：

@Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);

        // "|" used deliberately here; both methods should be invoked.
        final boolean interceptForDrag = mLeftDragger.shouldInterceptTouchEvent(ev) |
                mRightDragger.shouldInterceptTouchEvent(ev);

        boolean interceptForTap = false;

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                mInitialMotionX = x;
                mInitialMotionY = y;
                if (mScrimOpacity > 0) {
                    final View child = mLeftDragger.findTopChildUnder((int) x, (int) y);
                    if (child != null && isContentView(child)) {
                        interceptForTap = true;
                    }
                }
                mDisallowInterceptRequested = false;
                mChildrenCanceledTouch = false;
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                // If we cross the touch slop, don't perform the delayed peek for an edge touch.
                if (mLeftDragger.checkTouchSlop(ViewDragHelper.DIRECTION_ALL)) {
                    mLeftCallback.removeCallbacks();
                    mRightCallback.removeCallbacks();
                }
                break;
            }

            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP: {
                closeDrawers(true);
                mDisallowInterceptRequested = false;
                mChildrenCanceledTouch = false;
            }
        }

        return interceptForDrag || interceptForTap || hasPeekingDrawer() || mChildrenCanceledTouch;
    }

可以发现，代码不长。首先比较重要的是下面一行代码，这行代码中mRightDragger和mLeftDragger（这两个都是ViewDragHelper类）的shouldInterceptTouchEvent方法用来判断是否应该去拦截事件。

  // "|" used deliberately here; both methods should be invoked.
        final boolean interceptForDrag = mLeftDragger.shouldInterceptTouchEvent(ev) |
                mRightDragger.shouldInterceptTouchEvent(ev);

方法里面判断所点击的View，然后在MOVE事件中调用tryCaptureViewForDrag，然后CallBack的的tryCaptureView方法判断是否是可以Drag的View。然后设置标签为STATE_DRAGGING，如果是STATE_DRAGGING的话，shouldInterceptTouchEvent返回true。

boolean tryCaptureViewForDrag(View toCapture, int pointerId) {
        if (toCapture == mCapturedView && mActivePointerId == pointerId) {
            // Already done!
            return true;
        }
        if (toCapture != null && mCallback.tryCaptureView(toCapture, pointerId)) {
            mActivePointerId = pointerId;
            captureChildView(toCapture, pointerId);
            return true;
        }
        return false;
    }

判断是否是相匹配的View

        @Override
        public boolean tryCaptureView(View child, int pointerId) {
            // Only capture views where the gravity matches what we're looking for.
            // This lets us use two ViewDragHelpers, one for each side drawer.
            return isDrawerView(child) && checkDrawerViewAbsoluteGravity(child, mAbsGravity)
                    && getDrawerLockMode(child) == LOCK_MODE_UNLOCKED;
        }

如果获取到了View的话，那么interceptForDrag值变为true，说明将拦截点击事件。

然后再往下看InterceptTouchEvent的代码，接下去是获取坐标点，然后给成员变量初始的X和Y值赋值，这没什么好说的。

final float x = ev.getX();
final float y = ev.getY();
mInitialMotionX = x;
mInitialMotionY = y;

然后是一段比较重要的代码，首先用mScrimOpacity判断是否有侧滑菜单显示在界面上，如果有的话，找到点击区域内的顶端的控件控件。如果顶端的控件满足要求，那么拦截。

if (mScrimOpacity > 0) {
�    final View child = mLeftDragger.findTopChildUnder((int) x, (int) y);
�    if (child != null && isContentView(child)) {
�        interceptForTap = true;
�    }
}

最后判断是否拦截，几个条件中是否有一个满足。

return interceptForDrag || interceptForTap || hasPeekingDrawer() || mChildrenCanceledTouch;

TouchEvent事件

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        mLeftDragger.processTouchEvent(ev);
        mRightDragger.processTouchEvent(ev);

        final int action = ev.getAction();
        boolean wantTouchEvents = true;

        switch (action & MotionEventCompat.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                mInitialMotionX = x;
                mInitialMotionY = y;
                mDisallowInterceptRequested = false;
                mChildrenCanceledTouch = false;
                break;
            }

            case MotionEvent.ACTION_UP: {
                final float x = ev.getX();
                final float y = ev.getY();
                boolean peekingOnly = true;
                final View touchedView = mLeftDragger.findTopChildUnder((int) x, (int) y);
                if (touchedView != null && isContentView(touchedView)) {
                    final float dx = x - mInitialMotionX;
                    final float dy = y - mInitialMotionY;
                    final int slop = mLeftDragger.getTouchSlop();
                    if (dx * dx + dy * dy < slop * slop) {
                        // Taps close a dimmed open drawer but only if it isn't locked open.
                        final View openDrawer = findOpenDrawer();
                        if (openDrawer != null) {
                            peekingOnly = getDrawerLockMode(openDrawer) == LOCK_MODE_LOCKED_OPEN;
                        }
                    }
                }
                closeDrawers(peekingOnly);
                mDisallowInterceptRequested = false;
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                closeDrawers(true);
                mDisallowInterceptRequested = false;
                mChildrenCanceledTouch = false;
                break;
            }
        }

        return wantTouchEvents;
    }

这段处理Touch事件的代码中，我们主要看的是第一行

mLeftDragger.processTouchEvent(ev);

来看下这个方法内的MOVE事件：

public void processTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);
        final int actionIndex = MotionEventCompat.getActionIndex(ev);

        if (action == MotionEvent.ACTION_DOWN) {
            // Reset things for a new event stream, just in case we didn't get
            // the whole previous stream.
            cancel();
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                final int pointerId = MotionEventCompat.getPointerId(ev, 0);
                final View toCapture = findTopChildUnder((int) x, (int) y);

                saveInitialMotion(x, y, pointerId);

                // Since the parent is already directly processing this touch event,
                // there is no reason to delay for a slop before dragging.
                // Start immediately if possible.
                tryCaptureViewForDrag(toCapture, pointerId);

                final int edgesTouched = mInitialEdgesTouched[pointerId];
                if ((edgesTouched & mTrackingEdges) != 0) {
                    mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_DOWN: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                final float x = MotionEventCompat.getX(ev, actionIndex);
                final float y = MotionEventCompat.getY(ev, actionIndex);

                saveInitialMotion(x, y, pointerId);

                // A ViewDragHelper can only manipulate one view at a time.
                if (mDragState == STATE_IDLE) {
                    // If we're idle we can do anything! Treat it like a normal down event.

                    final View toCapture = findTopChildUnder((int) x, (int) y);
                    tryCaptureViewForDrag(toCapture, pointerId);

                    final int edgesTouched = mInitialEdgesTouched[pointerId];
                    if ((edgesTouched & mTrackingEdges) != 0) {
                        mCallback.onEdgeTouched(edgesTouched & mTrackingEdges, pointerId);
                    }
                } else if (isCapturedViewUnder((int) x, (int) y)) {
                    // We're still tracking a captured view. If the same view is under this
                    // point, we'll swap to controlling it with this pointer instead.
                    // (This will still work if we're "catching" a settling view.)

                    tryCaptureViewForDrag(mCapturedView, pointerId);
                }
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                if (mDragState == STATE_DRAGGING) {
                    // If pointer is invalid then skip the ACTION_MOVE.
                    if (!isValidPointerForActionMove(mActivePointerId)) break;

                    final int index = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
                    final float x = MotionEventCompat.getX(ev, index);
                    final float y = MotionEventCompat.getY(ev, index);
                    final int idx = (int) (x - mLastMotionX[mActivePointerId]);
                    final int idy = (int) (y - mLastMotionY[mActivePointerId]);

                    dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);

                    saveLastMotion(ev);
                } else {
                    // Check to see if any pointer is now over a draggable view.
                    final int pointerCount = MotionEventCompat.getPointerCount(ev);
                    for (int i = 0; i < pointerCount; i++) {
                        final int pointerId = MotionEventCompat.getPointerId(ev, i);

                        // If pointer is invalid then skip the ACTION_MOVE.
                        if (!isValidPointerForActionMove(pointerId)) continue;

                        final float x = MotionEventCompat.getX(ev, i);
                        final float y = MotionEventCompat.getY(ev, i);
                        final float dx = x - mInitialMotionX[pointerId];
                        final float dy = y - mInitialMotionY[pointerId];

                        reportNewEdgeDrags(dx, dy, pointerId);
                        if (mDragState == STATE_DRAGGING) {
                            // Callback might have started an edge drag.
                            break;
                        }

                        final View toCapture = findTopChildUnder((int) x, (int) y);
                        if (checkTouchSlop(toCapture, dx, dy) &&
                                tryCaptureViewForDrag(toCapture, pointerId)) {
                            break;
                        }
                    }
                    saveLastMotion(ev);
                }
                break;
            }

            case MotionEventCompat.ACTION_POINTER_UP: {
                final int pointerId = MotionEventCompat.getPointerId(ev, actionIndex);
                if (mDragState == STATE_DRAGGING && pointerId == mActivePointerId) {
                    // Try to find another pointer that's still holding on to the captured view.
                    int newActivePointer = INVALID_POINTER;
                    final int pointerCount = MotionEventCompat.getPointerCount(ev);
                    for (int i = 0; i < pointerCount; i++) {
                        final int id = MotionEventCompat.getPointerId(ev, i);
                        if (id == mActivePointerId) {
                            // This one's going away, skip.
                            continue;
                        }

                        final float x = MotionEventCompat.getX(ev, i);
                        final float y = MotionEventCompat.getY(ev, i);
                        if (findTopChildUnder((int) x, (int) y) == mCapturedView &&
                                tryCaptureViewForDrag(mCapturedView, id)) {
                            newActivePointer = mActivePointerId;
                            break;
                        }
                    }

                    if (newActivePointer == INVALID_POINTER) {
                        // We didn't find another pointer still touching the view, release it.
                        releaseViewForPointerUp();
                    }
                }
                clearMotionHistory(pointerId);
                break;
            }

            case MotionEvent.ACTION_UP: {
                if (mDragState == STATE_DRAGGING) {
                    releaseViewForPointerUp();
                }
                cancel();
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                if (mDragState == STATE_DRAGGING) {
                    dispatchViewReleased(0, 0);
                }
                cancel();
                break;
            }
        }
    }

dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);这个方法最后会调用offsetLeftAndRight()来对mLeft和mRight等进行偏移。

在UP事件中，会回调CallBack的方法，如果位置不在指定的位置的话，会执行动画。

  @Override
        public void onViewReleased(View releasedChild, float xvel, float yvel) {
            // Offset is how open the drawer is, therefore left/right values
            // are reversed from one another.
            final float offset = getDrawerViewOffset(releasedChild);
            final int childWidth = releasedChild.getWidth();

            int left;
            if (checkDrawerViewAbsoluteGravity(releasedChild, Gravity.LEFT)) {
                left = xvel > 0 || xvel == 0 && offset > 0.5f ? 0 : -childWidth;
            } else {
                final int width = getWidth();
                left = xvel < 0 || xvel == 0 && offset > 0.5f ? width - childWidth : width;
            }

            mDragger.settleCapturedViewAt(left, releasedChild.getTop());
            invalidate();
        }

动画在mDragger.settleCapturedViewAt(left, releasedChild.getTop());中执行
settleCapturedViewAt中最后会执行

mScroller.startScroll(startLeft, startTop, dx, dy, duration);

在computeScroll中

 @Override
    public void computeScroll() {
        final int childCount = getChildCount();
        float scrimOpacity = 0;
        for (int i = 0; i < childCount; i++) {
            final float onscreen = ((LayoutParams) getChildAt(i).getLayoutParams()).onScreen;
            scrimOpacity = Math.max(scrimOpacity, onscreen);
        }
        mScrimOpacity = scrimOpacity;

        // "|" used on purpose; both need to run.
        if (mLeftDragger.continueSettling(true) | mRightDragger.continueSettling(true)) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
    }

来看下mLeftDragger.continueSettling(true) 这个重要的动画方法

public boolean continueSettling(boolean deferCallbacks) {
        if (mDragState == STATE_SETTLING) {
            boolean keepGoing = mScroller.computeScrollOffset();
            final int x = mScroller.getCurrX();
            final int y = mScroller.getCurrY();
            final int dx = x - mCapturedView.getLeft();
            final int dy = y - mCapturedView.getTop();

            if (dx != 0) {
                ViewCompat.offsetLeftAndRight(mCapturedView, dx);
            }
            if (dy != 0) {
                ViewCompat.offsetTopAndBottom(mCapturedView, dy);
            }

            if (dx != 0 || dy != 0) {
                mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);
            }

            if (keepGoing && x == mScroller.getFinalX() && y == mScroller.getFinalY()) {
                // Close enough. The interpolator/scroller might think we're still moving
                // but the user sure doesn't.
                mScroller.abortAnimation();
                keepGoing = false;
            }

            if (!keepGoing) {
                if (deferCallbacks) {
                    mParentView.post(mSetIdleRunnable);
                } else {
                    setDragState(STATE_IDLE);
                }
            }
        }

        return mDragState == STATE_SETTLING;
    }

可以看到，执行的还是offsetLeftAndRight这个方法来改变子控件的布局。

总结

这里对DrawerLayout大致的流程总结了一下，相对来说这个控件的实现比较复杂，这里也不可能全部分析一遍。但是可以通过我们的分析，来了解android源码中时如何去处理拦截和点击事件的。