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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）
Android Touch事件传递机制引发的血案
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