android 事件分发机制（转载）

转载自:http://blog.csdn.net/chenzujie/article/details/13277081
直接进入正题，在android开发中，手势触碰是经常使用到的。这两天刚好在看这方面的资料，在这里写篇文章做个小总结。首先必须知道五大布局Layout：LineaLayout，RelativeLayout，FrameLayout，AbsoluteLayout都继承自ViewGroup，而TableLayout继承LinearLayout，也相当于继承于ViewGroup。而ViewGroup继承自View。而我们最终在屏幕上显示控制的控件如TextView，Button等都直接或间接继承自View。View中有两个方法参与到Touch事件分发onDispatchTouchEvent(MotionEvent event)和onTouch(MotionEvent event)ViewGroup有三个方法参与到Touch事件分发onDispatchTouchEvent(MotionEvent event)，onInterceptTouchEvent(MotionEvent ev)，onTouch(MotionEvent event)

当我们手指出门屏幕，手势事件最早被传递到Layout布局的dispatchTouchEvent，即ViewGroup的dispatchTouchEvent[java] view plaincopy
public boolean dispatchTouchEvent(MotionEvent ev) {

final int action = ev.getAction();

final float xf = ev.getX();

final float yf = ev.getY();

final float scrolledXFloat = xf + mScrollX;

final float scrolledYFloat = yf + mScrollY;

final Rect frame = mTempRect;

boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;

if (action == MotionEvent.ACTION_DOWN) {

if (mMotionTarget != null) {

mMotionTarget = null;

}

if (disallowIntercept || !onInterceptTouchEvent(ev)) {

ev.setAction(MotionEvent.ACTION_DOWN);

final int scrolledXInt = (int) scrolledXFloat;

final int scrolledYInt = (int) scrolledYFloat;

final View[] children = mChildren;

final int count = mChildrenCount;

for (int i = count - 1; i >= 0; i--) {

final View child = children[i];

if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE

|| child.getAnimation() != null) {

child.getHitRect(frame);

if (frame.contains(scrolledXInt, scrolledYInt)) {

final float xc = scrolledXFloat - child.mLeft;

final float yc = scrolledYFloat - child.mTop;

ev.setLocation(xc, yc);

child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;

if (child.dispatchTouchEvent(ev)) {

mMotionTarget = child;

return true;

}

}

}

}

}

}

boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||

(action == MotionEvent.ACTION_CANCEL);

if (isUpOrCancel) {

mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;

}

final View target = mMotionTarget;

if (target == null) {

ev.setLocation(xf, yf);

if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {

ev.setAction(MotionEvent.ACTION_CANCEL);

mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;

}

return super.dispatchTouchEvent(ev);

}

if (!disallowIntercept && onInterceptTouchEvent(ev)) {

final float xc = scrolledXFloat - (float) target.mLeft;

final float yc = scrolledYFloat - (float) target.mTop;

mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;

ev.setAction(MotionEvent.ACTION_CANCEL);

ev.setLocation(xc, yc);

if (!target.dispatchTouchEvent(ev)) {

}

mMotionTarget = null;

return true;

}

if (isUpOrCancel) {

mMotionTarget = null;

}

final float xc = scrolledXFloat - (float) target.mLeft;

final float yc = scrolledYFloat - (float) target.mTop;

ev.setLocation(xc, yc);

if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {

ev.setAction(MotionEvent.ACTION_CANCEL);

target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;

mMotionTarget = null;

}

return target.dispatchTouchEvent(ev);

}

此段代码叫长，挑重点看，第9行，但是ACTION_DOWN事件时，会进入到第13行的判断，第一个参数disallowIntercept是指是否禁用掉事件拦截的功能，默认是false，也可以通过调用requestDisallowInterceptTouchEvent方法对这个值进行修改。而第二个参数！onInterceptTouchEvent（ev）即是否进行拦截的判断，如果我们不复写这个方法，默认为false，就会进入if判断，并获得点击区域有的view，在第29行将手势事件传递给被点击到的view，如果view的dispatchTouchEvent(ev)返回true，表示手势事件被这个view消费了（view的dispatchTouchEvent后续分析），并把该view置给mMotionTarget后返回true。

这里，正是因为有view消费了ACTION_DOWN事件，使得当后续的ACTION_MOVE ACTION_UP传递到这个ViewGroup的时候，mMotionTarget不为空，使得47行的target不为空，最后都会执行74行的target.dispatchTouchEvent(ev),即ACTION_DOWN事件被哪个view的onTouch或者onTouchEvent返回了true，那么后续的ACTION_MOVE,ACTION_UP事件将会直接传递给这个view执行。（onTouch和onTouchEvent后续分析）继续分析这个ViewGroup的dispatchTouchEvent，如果13行的onInterceptTouchEvent被我们复写返回true代表ACTION_DOWN被这个ViewGroup拦截下来，将不会进入if判断，因此mMotionTarget将为null使得target也为null，最后执行了50行的super.dispatchTouchEvent即ViewGroup的父类View的dispatchTouchEvent。并在其中调用onTouch或者onTouchEvent。如果我们不复写onInterceptTouchEvent让其进入if判断，但如果child的dispatchTouchEvent返回的是false，表示child没有把这个事件消费了，最终也还是调用了50行的super.dispatchTouchEvent(ev)。
现在再来看看上面分析中经常说到的View的dispatchTouchEvent[java] view plaincopy
public boolean dispatchTouchEvent(MotionEvent event) {

if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&

mOnTouchListener.onTouch(this, event)) {

return true;

}

return onTouchEvent(event);

}

非常简短的几行代码，是不是比看了ViewGroup的dispatchTouchEvent轻松多了！

首先先判断mOnTouchListener是否为空，这个mOnTouchListener是啥呢，看下源码就知道了[java] view plaincopy
public void setOnTouchListener(OnTouchListener l) {

mOnTouchListener = l;

}

显然，就是我们设置OnTouchListener的接口实例，并在其中复写了onTouch方法。

if判断的第二个参数用来判断当前view是否是enable为状态。而第三个参数就是我们复写的onTouch方法了，当我们返回了true，这个view的dispatchtouchEvent将返回true，从而使得调用这个dispatchTouchEvent的ViewGroup的dispatchTouchEvent确定了mMotionTarget,确定了消费了ACTION_DOWN事件的view。而如果我们没有设置OnTouchEvent接口或者onTouch返回false则调用了onTouchEvent。继续看下view的onTouchEvent[java] view plaincopy
public boolean onTouchEvent(MotionEvent event) {

final int viewFlags = mViewFlags;

if ((viewFlags & ENABLED_MASK) == DISABLED) {

// A disabled view that is clickable still consumes the touch

// events, it just doesn't respond to them.

return (((viewFlags & CLICKABLE) == CLICKABLE ||

(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));

}

if (mTouchDelegate != null) {

if (mTouchDelegate.onTouchEvent(event)) {

return true;

}

}

if (((viewFlags & CLICKABLE) == CLICKABLE ||

(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {

switch (event.getAction()) {

case MotionEvent.ACTION_UP:

boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;

if ((mPrivateFlags & PRESSED) != 0 || prepressed) {

// take focus if we don't have it already and we should in

// touch mode.

boolean focusTaken = false;

if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {

focusTaken = requestFocus();

}

if (!mHasPerformedLongPress) {

// This is a tap, so remove the longpress check

removeLongPressCallback();

// Only perform take click actions if we were in the pressed state

if (!focusTaken) {

// Use a Runnable and post this rather than calling

// performClick directly. This lets other visual state

// of the view update before click actions start.

if (mPerformClick == null) {

mPerformClick = new PerformClick();

}

if (!post(mPerformClick)) {

performClick();

}

}

}

if (mUnsetPressedState == null) {

mUnsetPressedState = new UnsetPressedState();

}

if (prepressed) {

mPrivateFlags |= PRESSED;

refreshDrawableState();

postDelayed(mUnsetPressedState,

ViewConfiguration.getPressedStateDuration());

} else if (!post(mUnsetPressedState)) {

// If the post failed, unpress right now

mUnsetPressedState.run();

}

removeTapCallback();

}

break;

case MotionEvent.ACTION_DOWN:

if (mPendingCheckForTap == null) {

mPendingCheckForTap = new CheckForTap();

}

mPrivateFlags |= PREPRESSED;

mHasPerformedLongPress = false;

postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());

break;

case MotionEvent.ACTION_CANCEL:

mPrivateFlags &= ~PRESSED;

refreshDrawableState();

removeTapCallback();

break;

case MotionEvent.ACTION_MOVE:

final int x = (int) event.getX();

final int y = (int) event.getY();

// Be lenient about moving outside of buttons

int slop = mTouchSlop;

if ((x < 0 - slop) || (x >= getWidth() + slop) ||

(y < 0 - slop) || (y >= getHeight() + slop)) {

// Outside button

removeTapCallback();

if ((mPrivateFlags & PRESSED) != 0) {

// Remove any future long press/tap checks

removeLongPressCallback();

// Need to switch from pressed to not pressed

mPrivateFlags &= ~PRESSED;

refreshDrawableState();

}

}

break;

}

return true;

}

return false;

}

这段代码同样很长，同样挑重点看。

第14行当这个view是可点击的就会进入if判断，否则直接返回false，因此很有可能这个onTouchEvent的代码如果在第14行进入了if判断，很可能里面会有点击事件的执行。我们往下看，第38行执行ACTION_UP手势时，执行的performClick[java] view plaincopy
public boolean performClick() {

sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

if (mOnClickListener != null) {

playSoundEffect(SoundEffectConstants.CLICK);

mOnClickListener.onClick(this);

return true;

}

return false;

}

没错，performClick确实执行了mOnClickListener.onClikc(this);就是执行我们设置的onClick事件。并且在onTouchEvent的89行返回了true。

在这部分分析中我们也发现了一个小秘密，如果我们复写了onTouchListener中的onTouch让它返回false，这样dispatchTouchEvent将不会执行到onTouchEvent方法，使得onClick事件永远得不到执行。
好了，全部分析完毕，做几点小总结1、onTouch优先于onTouchEvent执行，且onTouch返回true将不执行onTouchEvent,并导致onTouchEvent里的点击事件得不到执行2、手势事件从最外部的ViewGroup的dispatchTouchEvent开始分发，如果onInterceptTouchEvent拦截了该手势事件，即返回true，手势事件将不会往下分发，即不执行view的dispatchTouchEvent，而是让该ViewGroup的的父类dispatchTouchEvent来消费该手势事件。3、如果View的dispatchTouchEvent消费了ACTION_DOWN事件，即对ACTION_DOWN返回true，后续的的ACTION_MOVE和ACTION_UP将直接被分发到这个view来消费。
最后感谢 Android事件分发机制完全解析，带你从源码的角度彻底理解(上)Android事件分发机制完全解析，带你从源码的角度彻底理解(下)
两篇文章的作者，我就是在看了这两篇文章后，再看了些资料才了解了android手势分发事件