dispatchTouchEvent源码解析

1. dispatchTouchEvent是处理触摸事件分发,Android中所有的事件都必须经过这个方法的分发，然后决定是自身消费当前事件还是继续往下分发给子控件处理。返回true表示不继续分发，事件没有被消费。返回false则继续往下分发，如果是ViewGroup则分发给onInterceptTouchEvent进行判断是否拦截该事件，事件(多数情况)是从Activity的dispatchTouchEvent开始的。执行dispatchTouchEvent(ev)，事件向下分发。



2. onInterceptTouchEvent是ViewGroup中才有的方法，View中没有，它的作用是负责事件的拦截，返回true的时候表示拦截当前事件，不继续往下分发，交给自身的onTouchEvent进行处理。返回false则不拦截，继续往下传。这是ViewGroup特有的方法，因为ViewGroup中可能还有子View，而在Android中View中是不能再包含子View的（iOS可以）



3. onTouchEvent是View中提供的方法，ViewGroup也有这个方法，view中不提供onInterceptTouchEvent。view中默认返回true，表示消费了这个事件，onTouchEvent方法用于事件的处理，返回true表示消费处理当前事件，返回false则不处理，交给子控件进行继续分发。



知识点：

1.Android中事件传递按照从上到下进行层级传递，事件处理从Activity开始到ViewGroup再到View。



2.事件传递方法包括dispatchTouchEvent、onTouchEvent、onInterceptTouchEvent，其中前两个是View和ViewGroup都有的，最后一个是只有ViewGroup才有的方法。这三个方法的作用分别是负责事件分发、事件处理、事件拦截。





3.onTouch事件要先于onClick事件执行，onTouch在事件分发方法dispatchTouchEvent中调用，而onClick在事件处理方法onTouchEvent中被调用，onTouchEvent要后于dispatchTouchEvent方法的调用。



代码分析：



[java] view
plain copy

import android.view.MotionEvent;

import android.view.View;

public class ZeroDispatchTouchEvent {

/**

* dispatchTouchEvent()源码学习及其注释

* 常说事件传递中的流程是:dispatchTouchEvent->onInterceptTouchEvent->onTouchEvent

* 在这个链条中dispatchTouchEvent()是处在链首的位置当然也是最重要的.

* 在dispatchTouchEvent()决定了Touch事件是由自己的onTouchEvent()处理

* 还是分发给子View处理让子View调用其自身的dispatchTouchEvent()处理.

*

*

* 其实dispatchTouchEvent()和onInterceptTouchEvent()以及onTouchEvent()的关系

* 在dispatchTouchEvent()方法的源码中体现得很明显.

* 比如dispatchTouchEvent()会调用onInterceptTouchEvent()来判断是否要拦截.

* 比如dispatchTouchEvent()会调用dispatchTransformedTouchEvent()方法且在该方法中递归调用

* dispatchTouchEvent();从而会在dispatchTouchEvent()里最终调用到onTouchEvent()

*

*

*

* 重点关注:

* 1 子View对于ACTION_DOWN的处理十分重要!!!!!

* ACTION_DOWN是一系列Touch事件的开端,如果子View对于该ACTION_DOWN事件在onTouchEvent()中返回了false即未消费.

* 那么ViewGroup就不会把后续的ACTION_MOVE和ACTION_UP派发给该子View.在这种情况下ViewGroup就和普通的View一样了,

* 调用该ViewGroup自己的dispatchTouchEvent()从而调用自己的onTouchEvent();即不会将事件分发给子View.

* 详细代码请参见如下代码分析.

*

* 2 为什么子view对于Touch事件处理返回true那么其上层的ViewGroup就无法处理Touch事件了?????

* 这个想必大家都知道了,因为该Touch事件被子View消费了其上层的ViewGroup就无法处理该Touch事件了.

* 那么在源码中的依据是什么呢??请看下面的源码分析

*/

@Override

public boolean dispatchTouchEvent(MotionEvent ev) {

if (mInputEventConsistencyVerifier != null) {

mInputEventConsistencyVerifier.onTouchEvent(ev, 1);

}

boolean handled = false;

if (onFilterTouchEventForSecurity(ev)) {

final int action = ev.getAction();

final int actionMasked = action & MotionEvent.ACTION_MASK;

/**

* 第一步:对于ACTION_DOWN进行处理(Handle an initial down)

* 因为ACTION_DOWN是一系列事件的开端,当是ACTION_DOWN时进行一些初始化操作.

* 从源码的注释也可以看出来:清除以往的Touch状态(state)开始新的手势(gesture)

* cancelAndClearTouchTargets(ev)中有一个非常重要的操作:

* 将mFirstTouchTarget设置为null!!!!

* 随后在resetTouchState()中重置Touch状态标识

*/

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();

}

/**

* 第二步:检查是否要拦截(Check for interception)

* 在dispatchTouchEvent(MotionEventev)这段代码中

* 使用变量intercepted来标记ViewGroup是否拦截Touch事件的传递.

* 该变量在后续代码中起着很重要的作用.

*/

final boolean intercepted;

// 事件为ACTION_DOWN或者mFirstTouchTarget不为null(即已经找到能够接收touch事件的目标组件)时if成立

if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) {

//判断disallowIntercept(禁止拦截)标志位

//因为在其他地方可能调用了requestDisallowInterceptTouchEvent(boolean disallowIntercept)

//从而禁止执行是否需要拦截的判断(有点拗口~其实看requestDisallowInterceptTouchEvent()方法名就可明白)

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

//当没有禁止拦截判断时(即disallowIntercept为false)调用onInterceptTouchEvent(ev)方法

if (!disallowIntercept) {

//既然disallowIntercept为false那么就调用onInterceptTouchEvent()方法将结果赋值给intercepted

//常说事件传递中的流程是:dispatchTouchEvent->onInterceptTouchEvent->onTouchEvent

//其实在这就是一个体现,在dispatchTouchEvent()中调用了onInterceptTouchEvent()

intercepted = onInterceptTouchEvent(ev);

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

} else {

//当禁止拦截判断时(即disallowIntercept为true)设置intercepted = false

intercepted = false;

}

} else {

//当事件不是ACTION_DOWN并且mFirstTouchTarget为null(即没有Touch的目标组件)时

//设置 intercepted = true表示ViewGroup执行Touch事件拦截的操作。

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

//so this view group continues to intercept touches.

intercepted = true;

}

/**

* 第三步:检查cancel(Check for cancelation)

*

*/

final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL;

/**

* 第四步:事件分发(Update list of touch targets for pointer down, if needed)

*/

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

TouchTarget newTouchTarget = null;

boolean alreadyDispatchedToNewTouchTarget = false;

//不是ACTION_CANCEL并且ViewGroup的拦截标志位intercepted为false(不拦截)

if (!canceled && !intercepted) {

//处理ACTION_DOWN事件.这个环节比较繁琐.

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 (childrenCount != 0) {

// 依据Touch坐标寻找子View来接收Touch事件

// Find a child that can receive the event.

// Scan children from front to back.

final View[] children = mChildren;

final float x = ev.getX(actionIndex);

final float y = ev.getY(actionIndex);

final boolean customOrder = isChildrenDrawingOrderEnabled();

// 遍历子View判断哪个子View接受Touch事件

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)) {

continue;

}

newTouchTarget = getTouchTarget(child);

if (newTouchTarget != null) {

// 找到接收Touch事件的子View!!!!!!!即为newTouchTarget.

// 既然已经找到了,所以执行break跳出for循环

// 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);

/**

* 如果上面的if不满足,当然也不会执行break语句.

* 于是代码会执行到这里来.

*

* 调用方法dispatchTransformedTouchEvent()将Touch事件传递给子View做

* 递归处理(也就是遍历该子View的View树)

* 该方法很重要,看一下源码中关于该方法的描述:

* Transforms a motion event into the coordinate space of a particular child view,

* filters out irrelevant pointer ids, and overrides its action if necessary.

* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.

* 将Touch事件传递给特定的子View.

* 该方法十分重要!!!!在该方法中为一个递归调用,会递归调用dispatchTouchEvent()方法！！！！！！！！！！！！！！

* 在dispatchTouchEvent()中:

* 如果子View为ViewGroup并且Touch没有被拦截那么递归调用dispatchTouchEvent()

* 如果子View为View那么就会调用其onTouchEvent(),这个就不再赘述了.

*

*

* 该方法返回true则表示子View消费掉该事件,同时进入该if判断.

* 满足if语句后重要的操作有:

* 1 给newTouchTarget赋值

* 2 给alreadyDispatchedToNewTouchTarget赋值为true.

* 看这个比较长的英语名字也可知其含义:已经将Touch派发给新的TouchTarget

* 3 执行break.

* 因为该for循环遍历子View判断哪个子View接受Touch事件,既然已经找到了

* 那么就跳出该for循环.

* 4 注意:

* 如果dispatchTransformedTouchEvent()返回false即子View

* 的onTouchEvent返回false(即Touch事件未被消费)那么就不满足该if条件,也就无法执行addTouchTarget()

* 从而导致mFirstTouchTarget为null.那么该子View就无法继续处理ACTION_MOVE事件

* 和ACTION_UP事件!!!!!!!!!!!!!!!!!!!!!!

* 5 注意:

* 如果dispatchTransformedTouchEvent()返回true即子View

* 的onTouchEvent返回true(即Touch事件被消费)那么就满足该if条件.

* 从而mFirstTouchTarget不为null!!!!!!!!!!!!!!!!!!!

* 6 小结:

* 对于此处ACTION_DOWN的处理具体体现在dispatchTransformedTouchEvent()

* 该方法返回boolean,如下:

* true---->事件被消费----->mFirstTouchTarget!=null

* false--->事件未被消费---->mFirstTouchTarget==null

* 因为在dispatchTransformedTouchEvent()会调用递归调用dispatchTouchEvent()和onTouchEvent()

* 所以dispatchTransformedTouchEvent()的返回值实际上是由onTouchEvent()决定的.

* 简单地说onTouchEvent()是否消费了Touch事件(true or false)的返回值决定了dispatchTransformedTouchEvent()

* 的返回值!!!!!!!!!!!!!从而决定了mFirstTouchTarget是否为null!!!!!!!!!!!!!!!!从而进一步决定了ViewGroup是否

* 处理Touch事件.这一点在下面的代码中很有体现.

*

*

*/

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;

}

}

}

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/**

* 该if条件表示:

* 经过前面的for循环没有找到子View接收Touch事件并且之前的mFirstTouchTarget不为空

*/

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指向了最初的TouchTarget

newTouchTarget.pointerIdBits |= idBitsToAssign;

}

}

}

/**

* 分发Touch事件至target(Dispatch to touch targets)

*

* 经过上面对于ACTION_DOWN的处理后mFirstTouchTarget有两种情况:

* 1 mFirstTouchTarget为null

* 2 mFirstTouchTarget不为null

*

* 当然如果不是ACTION_DOWN就不会经过上面较繁琐的流程

* 而是从此处开始执行,比如ACTION_MOVE和ACTION_UP

*/

if (mFirstTouchTarget == null) {

/**

* 情况1：mFirstTouchTarget为null

*

* 经过上面的分析mFirstTouchTarget为null就是说Touch事件未被消费.

* 即没有找到能够消费touch事件的子组件或Touch事件被拦截了，

* 则调用ViewGroup的dispatchTransformedTouchEvent()方法处理Touch事件则和普通View一样.

* 即子View没有消费Touch事件,那么子View的上层ViewGroup才会调用其onTouchEvent()处理Touch事件.

* 在源码中的注释为:No touch targets so treat this as an ordinary view.

* 也就是说此时ViewGroup像一个普通的View那样调用dispatchTouchEvent(),且在dispatchTouchEvent()

* 中会去调用onTouchEvent()方法.

* 具体的说就是在调用dispatchTransformedTouchEvent()时第三个参数为null.

* 第三个参数View child为null会做什么样的处理呢?

* 请参见下面dispatchTransformedTouchEvent()的源码分析

*

* 这就是为什么子view对于Touch事件处理返回true那么其上层的ViewGroup就无法处理Touch事件了!!!!!!!!!!

* 这就是为什么子view对于Touch事件处理返回false那么其上层的ViewGroup才可以处理Touch事件!!!!!!!!!!

*

*/

handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);

} else {

/**

* 情况2：mFirstTouchTarget不为null即找到了可以消费Touch事件的子View且后续Touch事件可以传递到该子View

* 在源码中的注释为:

* 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;

//对于非ACTION_DOWN事件继续传递给目标子组件进行处理,依然是递归调用dispatchTransformedTouchEvent()

if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) {

handled = true;

}

if (cancelChild) {

if (predecessor == null) {

mFirstTouchTarget = next;

} else {

predecessor.next = next;

}

target.recycle();

target = next;

continue;

}

}

predecessor = target;

target = next;

}

}

/**

* 处理ACTION_UP和ACTION_CANCEL

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

* 在此主要的操作是还原状态

*/

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 (!handled && mInputEventConsistencyVerifier != null) {

mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);

}

return handled;

}

//=====================以上为dispatchTouchEvent()源码分析======================

//===============以下为dispatchTransformedTouchEvent()源码分析=================

/**

* 在dispatchTouchEvent()中调用dispatchTransformedTouchEvent()将事件分发给子View处理

*

* Transforms a motion event into the coordinate space of a particular child view,

* filters out irrelevant pointer ids, and overrides its action if necessary.

* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.

*

* 在此请着重注意第三个参数:View child

* 在dispatchTouchEvent()中多次调用了dispatchTransformedTouchEvent(),但是有时候第三个参数为null,有时又不是.

* 那么这个参数是否为null有什么区别呢？

* 在如下dispatchTransformedTouchEvent()源码中可见多次对于child是否为null的判断,并且均做出如下类似的操作:

* if (child == null) {

* handled = super.dispatchTouchEvent(event);

* } else {

* handled = child.dispatchTouchEvent(event);

* }

* 这个代码是什么意思呢??

* 当child == null时会将Touch事件传递给该ViewGroup自身的dispatchTouchEvent()处理.

* 即super.dispatchTouchEvent(event)正如源码中的注释描述的一样:

* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.

* 当child != null时会调用该子view(当然该view可能是一个View也可能是一个ViewGroup)的dispatchTouchEvent(event)处理.

* 即child.dispatchTouchEvent(event);

*

*

*/

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 (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);

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) {

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);

}

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);

}

// Done.

transformedEvent.recycle();

return handled;

}

}



参考资料：

Android中的dispatchTouchEvent()、onInterceptTouchEvent()和onTouchEvent()

Android事件传递机制

源码解析