Android Touch事件传递机制引发的血案

尊重原创：http://blog.csdn.net/yuanzeyao/article/details/38942135

关于Android Touch事件传递机制我之前也写过两篇文章，自觉得对Touche事件还是理解得比較清楚的，可是近期遇到的一个问题，让我再次对Android Touche事件进行一次学习。

我的关于Android Touche事件传递机制的文章例如以下：

http://blog.csdn.net/yuanzeyao/article/details/37961997

http://blog.csdn.net/yuanzeyao/article/details/38025165

我在这两篇文章中得出过下面结论：

1、假设一个view是clickable的，那么这个View的onTouchEvent是一定会返回true的，也就是说不论什么触摸事件都会被消费掉

2、假设一个View对于ACTION_DOWN事件没有消费掉(onTouchEvent 返回false)，那么兴许的ACTION_MOVE，ACTION_UP是都不会接受到的，也就是没有机会处理这些事件，这些事件都是在父View里面给处理了

3、假设一个ViewGroup想要拦截事件(不让事件传递到子View),那么它只须要改写ViewGroup的onInterceptTouchEvent(MotionEvent ev) 方法，让他返回true,或者调用requestDisallowInterceptTouchEvent(true);

4、Android中的Touche事件是从底层向上层传递的 Activity->DecorView->ViewGroup->View

理解了上面的问题，我们就開始看看我所遇到的问题吧，

在使用SlideMenu的时候，在中的Activity中只放置一个TextView,你会发现SlideMenu无法滑动，当时通过顶部的Title能够滑动，因为对SlideMenu用的不是非常熟，当时以为是SlideMenu的哪个属性用错了，后来一直没有解决这个问题，直到一位网友说设置TextView的clickable为true就能够解决这个问题，我尝试了一下，还真行！哈哈。。。，这个里面的原因你理解了吗？假设没有理解，请继续往下看

依照我之前对Touche事件的理解，假设设置clickable，那么Touche事件肯定就被TextView给消费掉了，假设被TextView消费掉了，那么SlideMenu怎样实现滑动？要解开这个问题答案，还是看看SlideMenu的源代码吗

我们首先看看SlideMenu中CustomViewAbove和Touche有关的方法

@Override
	public boolean onInterceptTouchEvent(MotionEvent ev) {

		if (!mEnabled)
			return false;

		final int action = ev.getAction() & MotionEventCompat.ACTION_MASK;

		if (action == MotionEvent.ACTION_DOWN && DEBUG)
			Log.v(TAG, "Received ACTION_DOWN");

		if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP
				|| (action != MotionEvent.ACTION_DOWN && mIsUnableToDrag)) {
			endDrag();
			return false;
		}

		switch (action) {
		case MotionEvent.ACTION_MOVE:
			try{
				final int activePointerId = mActivePointerId;
				if (activePointerId == INVALID_POINTER)
					break;
				final int pointerIndex = this.getPointerIndex(ev, activePointerId);
				final float x = MotionEventCompat.getX(ev, pointerIndex);
				final float dx = x - mLastMotionX;
				final float xDiff = Math.abs(dx);
				final float y = MotionEventCompat.getY(ev, pointerIndex);
				final float yDiff = Math.abs(y - mLastMotionY);
				if (DEBUG) Log.v(TAG, "onInterceptTouch moved to:(" + x + ", " + y + "), diff:(" + xDiff + ", " + yDiff + "), mLastMotionX:" + mLastMotionX);
				if (xDiff > mTouchSlop && xDiff > yDiff && thisSlideAllowed(dx)) {
					if (DEBUG) Log.v(TAG, "Starting drag! from onInterceptTouch");
					startDrag();
					mLastMotionX = x;
					setScrollingCacheEnabled(true);
				} else if (yDiff > mTouchSlop) {
					mIsUnableToDrag = true;
				}
			}
			catch(IllegalArgumentException e)
			{
				e.printStackTrace();
			}
			break;

		case MotionEvent.ACTION_DOWN:
			mActivePointerId = ev.getAction() & ((Build.VERSION.SDK_INT >= 8) ? MotionEvent.ACTION_POINTER_INDEX_MASK :
				MotionEvent.ACTION_POINTER_INDEX_MASK);
			mLastMotionX = mInitialMotionX = MotionEventCompat.getX(ev, mActivePointerId);
			mLastMotionY = MotionEventCompat.getY(ev, mActivePointerId);
			if (thisTouchAllowed(ev)) {
				mIsBeingDragged = false;
				mIsUnableToDrag = false;
				if (isMenuOpen() && mViewBehind.menuTouchInQuickReturn(mContent, mCurItem, ev.getX() + mScrollX)) {
					mQuickReturn = true;
				}
			} else {
				mIsUnableToDrag = true;
			}
			break;
		case MotionEventCompat.ACTION_POINTER_UP:
			onSecondaryPointerUp(ev);
			break;
		}

		if (!mIsBeingDragged) {
			if (mVelocityTracker == null) {
				mVelocityTracker = VelocityTracker.obtain();
			}
			mVelocityTracker.addMovement(ev);
		}
		return mIsBeingDragged || mQuickReturn;
	}

看看这种方法，这种方法里面有个逻辑就是当滑动到一定距离，就会返回true,也就是说会拦截滑动事件，第一个ACTION_DOWN肯定不会拦截。

再看看onToucheEvent.java

	@Override
	public boolean onTouchEvent(MotionEvent ev) {

		if (!mEnabled)
			return false;

		//		if (!mIsBeingDragged && !thisTouchAllowed(ev))
		//			return false;

		if (!mIsBeingDragged && !mQuickReturn)
			return false;

		final int action = ev.getAction();

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

		switch (action & MotionEventCompat.ACTION_MASK) {
		case MotionEvent.ACTION_DOWN:
			/*
			 * If being flinged and user touches, stop the fling. isFinished
			 * will be false if being flinged.
			 */
			completeScroll();

			// Remember where the motion event started
			mLastMotionX = mInitialMotionX = ev.getX();
			mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
			break;
		case MotionEvent.ACTION_MOVE:
			if (!mIsBeingDragged) {
				if (mActivePointerId == INVALID_POINTER)
					break;
				final int pointerIndex = getPointerIndex(ev, mActivePointerId);
				final float x = MotionEventCompat.getX(ev, pointerIndex);
				final float dx = x - mLastMotionX;
				final float xDiff = Math.abs(dx);
				final float y = MotionEventCompat.getY(ev, pointerIndex);
				final float yDiff = Math.abs(y - mLastMotionY);
				if (DEBUG) Log.v(TAG, "onTouch moved to:(" + x + ", " + y + "), diff:(" + xDiff + ", " + yDiff + ")\nmIsBeingDragged:" + mIsBeingDragged + ", mLastMotionX:" + mLastMotionX);
				if ((xDiff > mTouchSlop || (mQuickReturn && xDiff > mTouchSlop / 4))
						&& xDiff > yDiff && thisSlideAllowed(dx)) {
					if (DEBUG) Log.v(TAG, "Starting drag! from onTouch");
					startDrag();
					mLastMotionX = x;
					setScrollingCacheEnabled(true);
				} else {
					if (DEBUG) Log.v(TAG, "onTouch returning false");
					return false;
				}
			}
			if (mIsBeingDragged) {
				// Scroll to follow the motion event
				final int activePointerIndex = getPointerIndex(ev, mActivePointerId);
				if (mActivePointerId == INVALID_POINTER) {
					break;
				}
				final float x = MotionEventCompat.getX(ev, activePointerIndex);
				final float deltaX = mLastMotionX - x;
				mLastMotionX = x;
				float oldScrollX = getScrollX();
				float scrollX = oldScrollX + deltaX;
				final float leftBound = getLeftBound();
				final float rightBound = getRightBound();
				if (scrollX < leftBound) {
					scrollX = leftBound;
				} else if (scrollX > rightBound) {
					scrollX = rightBound;
				}
				// Don't lose the rounded component
				mLastMotionX += scrollX - (int) scrollX;
				scrollTo((int) scrollX, getScrollY());
				pageScrolled((int) scrollX);
			}
			break;
		case MotionEvent.ACTION_UP:
			if (mIsBeingDragged) {
				final VelocityTracker velocityTracker = mVelocityTracker;
				velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
				int initialVelocity = (int) VelocityTrackerCompat.getXVelocity(
						velocityTracker, mActivePointerId);
				final int scrollX = getScrollX();
				//				final int widthWithMargin = getWidth();
				//				final float pageOffset = (float) (scrollX % widthWithMargin) / widthWithMargin;
				// TODO test this. should get better flinging behavior
				final float pageOffset = (float) (scrollX - getDestScrollX(mCurItem)) / getBehindWidth();
				final int activePointerIndex = getPointerIndex(ev, mActivePointerId);
				if (mActivePointerId != INVALID_POINTER) {
					final float x = MotionEventCompat.getX(ev, activePointerIndex);
					final int totalDelta = (int) (x - mInitialMotionX);
					int nextPage = determineTargetPage(pageOffset, initialVelocity, totalDelta);
					setCurrentItemInternal(nextPage, true, true, initialVelocity);
				} else {
					setCurrentItemInternal(mCurItem, true, true, initialVelocity);
				}
				mActivePointerId = INVALID_POINTER;
				endDrag();
			} else if (mQuickReturn && mViewBehind.menuTouchInQuickReturn(mContent, mCurItem, ev.getX() + mScrollX)) {
				// close the menu
				setCurrentItem(1);
				endDrag();
			}
			break;
		case MotionEvent.ACTION_CANCEL:
			if (mIsBeingDragged) {
				setCurrentItemInternal(mCurItem, true, true);
				mActivePointerId = INVALID_POINTER;
				endDrag();
			}
			break;
		case MotionEventCompat.ACTION_POINTER_DOWN: {
			final int index = MotionEventCompat.getActionIndex(ev);
			final float x = MotionEventCompat.getX(ev, index);
			mLastMotionX = x;
			mActivePointerId = MotionEventCompat.getPointerId(ev, index);
			break;
		}
		case MotionEventCompat.ACTION_POINTER_UP:
			onSecondaryPointerUp(ev);
			int pointerIndex = this.getPointerIndex(ev, mActivePointerId);
			if (mActivePointerId == INVALID_POINTER)
				break;
			mLastMotionX = MotionEventCompat.getX(ev, pointerIndex);
			break;
		}
		return true;
	}

我们重点观察ACTION_DWON事件，对于ACTION_DWON事件，SlideMenu是没有拦截的，所以传递到了TextView，因为默认TextView是没有clickable的，所以是不会消费这个事件，假设TextView不消费，那么事件就传递到了SlideMenu,可是我们发如今SlideMenu中也没有消费这个事件，还记得我们上面的结论2吗，依据结论2，我们知道后面的事件是传递只是来的，所以导致了SlideMenu无法滑动。

假设我们设置了clickable,那么第一个ACTION_DOWN就被TextView处理了，所以后面每一个事件都会传递到TextView（前提是不被拦截，实际结果是被拦截，并被SlideMenu处理，所以SlideMenu滑动了）