onInterceptTouchEvent 与 onTouchEvent 分析与MotionEvent在ViewGroup与View中的分发

onInterceptTouchEvent 与 onTouchEvent 分析与MotionEvent在ViewGroup与View中的分发

Notice：本文将紧接着

Android 触屏事件 OnTouch
onClick onTouchEvent对于触屏事件的处理和分发

这一片博文来分析，如果您还没有读过这一片博文，强烈建议你先读一次上述博文

OK，言归正传，我们开始吧

最近，一直听到有人在争论关于dispatchTouchEvent这个函数 和 onInterceptTouchEvent这两个函数到底是那一这真正的决定了MotionEvent的分发。这里我还是统一给出答案吧。我们都知道如果我们不希望我们的ViewGroup阻拦我们的View获得MotionEvent，我们一般只需要在onInterceptTouchEvent这个函数中return false（并且该函数默认return false）。或者，在特定的情况下，如果我们希望某些时候交给我们的我们ViewGroup来处理有些情况下我们又希望我们的View来处理MotionEvent。这种情况下，我们应该怎么来处理呢。我想大家都知道，我们需要在onInterceptTouchEvent做一些处理，因为大家都知道onInterceptTouchEvent是用来做MotionEvent事件的处理对象判断的。

那么，答案很明显了，处理MotionEvent只有dispatchTouchEvent（其实这一点从函数名称都可以看出来，更不要说onInterceptTouchEvent还只是ViewGroup特有的方法）。

好，我们先来整理我们目前已知的，并且提出疑问然后再开始分析

一、Android 中的MotionEvent事件分发处理是由dispatchTouchEvent来完成的（底层直接调用该方法，我们这里不分析）

二、通过上一篇博文我们知道dispatchTouchEvent > onTouch > onTouchEvent > onClick

这里我还是简单啰嗦一下吧，其实onTouchListener 和 onClickListener 只是为了提供给上层便捷的处理接口，他们的存在只是为了对于开发提供了便捷，但是在Android中我们通常采用onInterceptTouchEvent和onTouchEvent
联合来判断MotionEvent的处理对象

三、我们在ViewGroup中采用onInterceptTouchEvent和onTouchEvent
联合来判断MotionEvent的处理对象

OK，结论到此结束，我们应该提出问题。

一、既然MotionEvent事件是由dispatchTouchEvent来分发的，那么他和onInterceptTouchEvent又是什么关系呢

二、onInterceptTouchEvent和onTouchEvent
是怎样来完成事件分发的

那么，到了现在，我们的主题只有一个，解决上面的两个问题即可。

对于第一个问题，最好的解决方式，很明显我们需要靠源码来解决

下面，我先给出ViewGroup重写之后的dispatchTouchEvent函数，这里我们需要注意的事他并没有调用super.dispatchTouchEvent。而是实实在在的重写了所有逻辑（至于View源码的dispatchTouchEvent，各位看官去我的上一篇博文里面去看看吧）

[java] view
plaincopy

@Override

public boolean dispatchTouchEvent(MotionEvent ev) {

if (mInputEventConsistencyVerifier != null) {

mInputEventConsistencyVerifier.onTouchEvent(ev, 1);

}

if (DBG_MOTION || DBG_TOUCH) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent 1: ev = " + ev + ",mFirstTouchTarget = "

+ mFirstTouchTarget + ",this = " + this);

}

boolean handled = false;

if (onFilterTouchEventForSecurity(ev)) { //上一篇博文中提到的遮蔽判断其实是错误的，这里指的应该是窗口被其他窗口遮挡，很明显一般情况下是不会的

final int action = ev.getAction();

final int actionMasked = action & MotionEvent.ACTION_MASK;

// Handle an initial down.

if (actionMasked == MotionEvent.ACTION_DOWN) {

// Throw away all previous state when starting a new touch gesture.

// The framework may have dropped the up or cancel event for the previous gesture

// due to an app switch, ANR, or some other state change.

cancelAndClearTouchTargets(ev);

resetTouchState();

}

// Check for interception.

final boolean intercepted;

if (actionMasked == MotionEvent.ACTION_DOWN

|| mFirstTouchTarget != null) {

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

if (!disallowIntercept) {

intercepted = onInterceptTouchEvent(ev);//这里开始判断

/// M : add log to help debugging

if (intercepted == true) {

if (DBG_TOUCH) {

Xlog.d(TAG, "Touch event was intercepted event = " + ev + ",this = " + this);

}

}

ev.setAction(action); // restore action in case it was changed

} else {

intercepted = false;

}

} else {

// There are no touch targets and this action is not an initial down

// so this view group continues to intercept touches.

intercepted = true;

}

// Check for cancelation.

final boolean canceled = resetCancelNextUpFlag(this)

|| actionMasked == MotionEvent.ACTION_CANCEL;

// Update list of touch targets for pointer down, if needed.

final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;

if (DBG_MOTION) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent 2: actionMasked = " + actionMasked

+ ",intercepted = " + intercepted + ",canceled = " + canceled + ",split = "

+ split + ",mChildrenCount = " + mChildrenCount + ",mFirstTouchTarget = "

+ mFirstTouchTarget + ",this = " + this);

}

TouchTarget newTouchTarget = null;

boolean alreadyDispatchedToNewTouchTarget = false;

if (!canceled && !intercepted) { //依据interceptd 分析转折点

if (actionMasked == MotionEvent.ACTION_DOWN

|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)

|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {

final int actionIndex = ev.getActionIndex(); // always 0 for down

final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)

: TouchTarget.ALL_POINTER_IDS;

// Clean up earlier touch targets for this pointer id in case they

// have become out of sync.

removePointersFromTouchTargets(idBitsToAssign);

final int childrenCount = mChildrenCount;

if (newTouchTarget == null && childrenCount != 0) {

final float x = ev.getX(actionIndex);

final float y = ev.getY(actionIndex);

// Find a child that can receive the event.

// Scan children from front to back.

final View[] children = mChildren;

final boolean customOrder = isChildrenDrawingOrderEnabled();

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

final int childIndex = customOrder ?

getChildDrawingOrder(childrenCount, i) : i;

final View child = children[childIndex];

if (!canViewReceivePointerEvents(child)

|| !isTransformedTouchPointInView(x, y, child, null)) {

if (DBG_MOTION) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent continue 6: i = "

+ i + ",count = " + childrenCount + ",child = " + child

+ ",this = " + this);

}

continue;

}

newTouchTarget = getTouchTarget(child);

if (DBG_MOTION) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent to child 3: child = "

+ child + ",childrenCount = " + childrenCount + ",i = " + i

+ ",newTouchTarget = " + newTouchTarget + ",idBitsToAssign = "

+ idBitsToAssign + ",mFirstTouchTarget = " + mFirstTouchTarget

+ ",this = " + this);

}

if (newTouchTarget != null) {

// Child is already receiving touch within its bounds.

// Give it the new pointer in addition to the ones it is handling.

newTouchTarget.pointerIdBits |= idBitsToAssign;

break;

}

resetCancelNextUpFlag(child);

if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // -------

// Child wants to receive touch within its bounds.

mLastTouchDownTime = ev.getDownTime();

mLastTouchDownIndex = childIndex;

mLastTouchDownX = ev.getX();

mLastTouchDownY = ev.getY();

newTouchTarget = addTouchTarget(child, idBitsToAssign);

alreadyDispatchedToNewTouchTarget = true;

break;

}

}

}

if (newTouchTarget == null && mFirstTouchTarget != null) {

// Did not find a child to receive the event.

// Assign the pointer to the least recently added target.

newTouchTarget = mFirstTouchTarget;

while (newTouchTarget.next != null) {

newTouchTarget = newTouchTarget.next;

}

newTouchTarget.pointerIdBits |= idBitsToAssign;

}

}

}

// Dispatch to touch targets.

if (mFirstTouchTarget == null) {

if (DBG_MOTION) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent mFirstTouchTarget = null, canceled = "

+ canceled + ",this = " + this);

}

// No touch targets so treat this as an ordinary view.

handled = dispatchTransformedTouchEvent(ev, canceled, null, //--------

TouchTarget.ALL_POINTER_IDS);

} else {

// Dispatch to touch targets, excluding the new touch target if we already

// dispatched to it. Cancel touch targets if necessary.

TouchTarget predecessor = null;

TouchTarget target = mFirstTouchTarget;

while (target != null) {

final TouchTarget next = target.next;

if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {

handled = true;

} else {

final boolean cancelChild = resetCancelNextUpFlag(target.child)

|| intercepted;

if (dispatchTransformedTouchEvent(ev, cancelChild, //----------

target.child, target.pointerIdBits)) {

handled = true;

}

if (DBG_MOTION) {

Xlog.d(TAG, "dispatchTouchEvent middle 5: cancelChild = " + cancelChild

+ ",mFirstTouchTarget = " + mFirstTouchTarget + ",target = "

+ target + ",predecessor = " + predecessor + ",next = " + next

+ ",this = " + this);

}

if (cancelChild) {

if (predecessor == null) {

mFirstTouchTarget = next;

} else {

predecessor.next = next;

}

target.recycle();

target = next;

continue;

}

}

predecessor = target;

target = next;

}

}

// Update list of touch targets for pointer up or cancel, if needed.

if (canceled

|| actionMasked == MotionEvent.ACTION_UP

|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {

resetTouchState();

} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {

final int actionIndex = ev.getActionIndex();

final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);

removePointersFromTouchTargets(idBitsToRemove);

}

}

if (DBG_MOTION) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent end 4: handled = " + handled + ",mFirstTouchTarget = "

+ mFirstTouchTarget + ",this = " + this);

}

if (!handled && mInputEventConsistencyVerifier != null) {

mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);

}

return handled;

}

通过上面我标红色的代码(做标记地方，显示有问题)，我们很明显的看得出来通过onInterceptTouchEvent最终交给了dispatchTransformedTouchEvent来处理，那么dispatchTransformedTouchEvent又做了什么呢

[java] view
plaincopy

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,

View child, int desiredPointerIdBits) {

final boolean handled;

// Canceling motions is a special case. We don't need to perform any transformations

// or filtering. The important part is the action, not the contents.

final int oldAction = event.getAction();

if (DBG_MOTION) {

Xlog.d(TAG, "dispatchTransformedTouchEvent 1: event = " + event + ",cancel = "

+ cancel + ",oldAction = " + oldAction + ",desiredPointerIdBits = "

+ desiredPointerIdBits + ",mFirstTouchTarget = " + mFirstTouchTarget

+ ",child = " + child + ",this = " + this);

}

if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {

event.setAction(MotionEvent.ACTION_CANCEL);

if (child == null) {

handled = super.dispatchTouchEvent(event);

} else {

handled = child.dispatchTouchEvent(event);

}

event.setAction(oldAction);

if (DBG_MOTION) {

Xlog.d(TAG, "Dispatch cancel action end: handled = " + handled + ",oldAction = "

+ oldAction + ",child = " + child + ",this = " + this);

}

return handled;

}

// Calculate the number of pointers to deliver.

final int oldPointerIdBits = event.getPointerIdBits();

final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

// If for some reason we ended up in an inconsistent state where it looks like we

// might produce a motion event with no pointers in it, then drop the event.

if (newPointerIdBits == 0) {

Xlog.i(TAG, "Dispatch transformed touch event without pointers in " + this);

return false;

}

// If the number of pointers is the same and we don't need to perform any fancy

// irreversible transformations, then we can reuse the motion event for this

// dispatch as long as we are careful to revert any changes we make.

// Otherwise we need to make a copy.

final MotionEvent transformedEvent;

if (newPointerIdBits == oldPointerIdBits) {

if (child == null || child.hasIdentityMatrix()) {

if (child == null) {

handled = super.dispatchTouchEvent(event);

} else {

final float offsetX = mScrollX - child.mLeft;

final float offsetY = mScrollY - child.mTop;

event.offsetLocation(offsetX, offsetY);

handled = child.dispatchTouchEvent(event);

event.offsetLocation(-offsetX, -offsetY);

}

if (DBG_MOTION) {

Xlog.d(TAG, "dispatchTransformedTouchEvent 2 to child " + child

+ ",handled = " + handled + ",mScrollX = " + mScrollX + ",mScrollY = "

+ mScrollY + ",mFirstTouchTarget = " + mFirstTouchTarget + ",event = "

+ event + ",this = " + this);

}

return handled;

}

transformedEvent = MotionEvent.obtain(event);

} else {

transformedEvent = event.split(newPointerIdBits);

}

// Perform any necessary transformations and dispatch.

if (child == null) {

handled = super.dispatchTouchEvent(transformedEvent);

} else {

final float offsetX = mScrollX - child.mLeft;

final float offsetY = mScrollY - child.mTop;

transformedEvent.offsetLocation(offsetX, offsetY);

if (! child.hasIdentityMatrix()) {

transformedEvent.transform(child.getInverseMatrix());

}

handled = child.dispatchTouchEvent(transformedEvent);

}

if (DBG_MOTION) {

Xlog.d(TAG, "dispatchTransformedTouchEvent 3 to child " + child + ",handled = "

+ handled + ",mScrollX = " + mScrollX + ",mScrollY = " + mScrollY

+ ",mFirstTouchTarget = " + mFirstTouchTarget + ",transformedEvent = "

+ transformedEvent + ",this = " + this);

}

// Done.

transformedEvent.recycle();

return handled;

}

从上面的代码来判断我们假设我们不重载

[java] view
plaincopy

public boolean onInterceptTouchEvent(MotionEvent ev) {

return false;

}

那么很明显了，我们的ViewGroup不会吃掉本次MotionEvent

从上一博文中的MyImageView被点击来判断

当我们点击我们ImageView时



我们继续接着我们的Log来分析

MotionEvent事件首先传递给了我们的MyFrameLayout，这时，我们要注意虽然我们设置了onTouchListener和OnClickListener，但是因为我们没有重载onInterceptTouchEvent默认的onInterceptTouchEvent直接return
false;所以我们的MyFrameLayout没有资格消费本次的MotionEvent事件，最终经过判断dispatchTransformedTouchEvent把事件交给了我们的MyImageView

[java] view
plaincopy

handled = child.dispatchTouchEvent(transformedEvent);

当然 因为我们的MyImageView 也设置了onTouchListener和onClickListener，所以他顺理成章的消费了本次MotionEvent

当我们再点击我们FrameLayout



这时虽然我们的onInterceptTouchEvent还是return false；但是我们的touchTarget是空的，所以

[java] view
plaincopy

if (child == null) {

handled = super.dispatchTouchEvent(transformedEvent);

} else

我们的MyFrameLayout 也还是最终享受到了MotionEvent
所以说 不一定我们的onInterceptTouchEvent 返回了false 我们的ViewGroup就不能接收到OnTouchEvent了

这里，我们再次考虑 当我们的onInterceptTouchEvent 被我们重载
并返回了true 那么走的路线就不一样了

[java] view
plaincopy

if (!canceled && !intercepted) { //依据interceptd 分析转折点</span>

[java] view
plaincopy

if (mFirstTouchTarget == null) {

if (DBG_MOTION) {

Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent mFirstTouchTarget = null, canceled = "

+ canceled + ",this = " + this);

}

// No touch targets so treat this as an ordinary view.

handled = dispatchTransformedTouchEvent(ev, canceled, null,

TouchTarget.ALL_POINTER_IDS);

这里注意，因为我们的onInterceptTouchEvent return true;我们直接跳过了Child的判断并最终导致我们的child == null;那么很明星我们的MotionEvent只能被我们的ViewGroup消费了。

那么，到了这里我们开始大胆的做出假设性结论：

一、只要我们的onInterceptTouchEvent return
true 那么我们的MotionEvent 与ChildView 无缘

二、如果我们的onInterceptTouchEvent return false；那么我们的ChildView 会优先获得MotionEvent ，但是当我们的ChildView 并不在TouchTarget上，我们的ViewGroup依然有机会得到本次MotionEvent
。

那么接下来，我们所有做的就只能验证我们的结论了。
这里我们先验证第一条// 这里的验证我们将依据ACTION的不同来分析。

这里我们对我们的MyFrameLayout 做出改变（重载onInterceptTouchEvent ）

[java] view
plaincopy

@Override

public boolean onInterceptTouchEvent(MotionEvent event) {

switch(event.getAction()){

case MotionEvent.ACTION_DOWN:{

Log.d(TAG,"MyFrameLayout onInterceptTouchEvent ACTION_DOWN return true");

return true;

}

case MotionEvent.ACTION_MOVE:{

Log.d(TAG,"MyFrameLayout onInterceptTouchEvent ACTION_MOVE return true");

return true;

}

case MotionEvent.ACTION_UP:{

Log.d(TAG,"MyFrameLayout onInterceptTouchEvent ACTION_UP return true");

return true;

}

}

return true;

}

简而言之，就是不论什么情况下，我们都要我们的onInterceptTouchEvent return true；
并分别改写我们的FrameLayout 和ImageView的OnTouchEvent函数

[java] view
plaincopy

@Override

public boolean onTouchEvent(MotionEvent event) {

Log.d(TAG,"MyFrameLayout onTouchEvent"+event.getAction());

switch(event.getAction()){

case MotionEvent.ACTION_DOWN:{

Log.d(TAG,"MyFrameLayout onTouchEvent ACTION_DOWN return true");

break;

}

case MotionEvent.ACTION_MOVE:{

Log.d(TAG,"MyFrameLayout onTouchEvent ACTION_MOVE return true");

break;

}

case MotionEvent.ACTION_UP:{

Log.d(TAG,"MyFrameLayout onTouchEvent ACTION_UP return true");

break;

}

}

return super.onTouchEvent(event);

}

[java] view
plaincopy

@Override

public boolean onTouchEvent(MotionEvent event) {

Log.d(TAG,"MyImageView onTouchEvent"+event.getAction());

switch(event.getAction()){

case MotionEvent.ACTION_DOWN:{

Log.d(TAG,"MyImageView onTouchEvent ACTION_DOWN return true");

break;

}

case MotionEvent.ACTION_MOVE:{

Log.d(TAG,"MyImageView onTouchEvent ACTION_MOVE return true");

break;

}

case MotionEvent.ACTION_UP:{

Log.d(TAG,"MyImageView onTouchEvent ACTION_UP return true");

break;

}

}

return super.onTouchEvent(event);

}

首先我们点击我们的MyImageView,我们来看一下Log



很明显，一旦我们的onInterceptTouchEvent
return true,我们的子View就与MotionEvent 无缘了

接下来，我们继续点击我们的FrameLayout
Log 图，我就不贴了，和上面一模一样，为什么呢，很显然我们的子View就与MotionEvent 无缘了

接下来，我们继续做出改变
我们在FrameLayout中将onInterceptTouchEvent 中的MotionEvent.ACTION_DOWN returnfalse；
依然点击我们的点击我们的MyImageView



这里我们来分析一下Log 首先我们需要知道3 == MotionEvent.ACTION_CANCEL
那么，首先因为我们的onInterceptTouchEvent 在MotionEvent.ACTION_DOWN returnfalse；所以我们的FrameLayout没办法继续往下走了（不会走onTouch 和 onTouchEvent）转而MotionEvent事件转交给我们的ImageView
所以我们的ImageView总算是得到了MotionEvent.ACTION_DOWN
但是在经过MotionEvent.ACTION_MOVE时，因为我们的onInterceptTouchEvent
返回了true，所以很明显我们的FrameLayout告诉了系统 这次的MotionEvent事件我要了，所以我们的ImageView的到了MotionEvent.ACTION_CANCEL 反而我们的FrameLayout 可以继续往下走了。
所以说我们的ImageView猜中了开头缺有猜中结局，虽然得到了ACTION_DOWN，但却什么都做不了只能以ACTION_CANCEL草草收场，所以也不会触发onClickListener

同理我们的FrameLayout虽然得到了ACTION_UP 但是没有ACTION_DOWN 也是徒劳。同样不会OnClickListener

到了这里，我们同样也是可以类推的
如果我们的FrameLayout在onInterceptTouchEvent 在ACTION_MOVE中返回false，我们的ImageView可以接受到ACTION_MOVE 但是最终接受的还是ACTION_CANCEL依然接受不到ACTION_UP
，反而我们的FrameLayout 只能得到ACTION_UP ，简而言之就是说依据onInterceptTouchEvent 的返回值不同各MotionEvent事件最终要么被ViewGroup 要么被ChildView获取

从上面的Log，我们依然可以看到的是 onInterceptTouchEvent
并不会在每一次 MotionEvent事件（ACTION_DOWN、ACTION_MOVE、ACTION_UP 等）调用，例如果在ACTION_DOWN
return true 交给了ViewGroup 而View没有得到的话，ACTION_MOVE时就不会调用，但是如果return false，ChildView 得到了ACTION_MOVE时就会再次调用