Android 事件分发机制
转载自：http://blog.csdn.net/singwhatiwanna/article/details/17339857
概述
一直想写篇关于Android事件派发机制的文章，却一直没写。这两天刚好是周末。有时间了，想想写一篇吧。不然总是仅仅停留在会用的层次上可是无法了解其内部机制。
我用的是4.4源代码，打开看看，挺复杂的，尤其是事件是怎么从Activity派发出来的。太费解了。了解Windows消息机制的人会发现。认为Android的事件派发机制和Windows的消息派发机制挺像的，事实上这是一种典型的消息“冒泡”机制，非常多平台採用这个机制，消息最先到达最底层View，然后它先进行推断是不是它所须要的，否则就将消息传递给它的子View。这样一来，消息就从水底的气泡一样向上浮了一点距离。以此类推。气泡达到顶部和空气接触，破了（消息被处理了）。当然也有气泡浮出到顶层了。还没破（消息无人处理）。这个消息将由系统来处理。对于Android来说。会由Activity来处理。
Android点击事件的派发机制
1. 从Activity传递究竟层View
点击事件用MotionEvent来表示。当一个点击操作发生时，事件最先传递给当前Activity，由Activity的dispatchTouchEvent来进行事件派发，详细的工作是由Activity内部的Window来完毕的，Window会将事件传递给decor view，decor view一般就是当前界面的底层容器（即setContentView所设置的View的父容器），通过Activity.getWindow.getDecorView（）能够获得。另外。看以下代码的的时候。主要看我凝视的地方。代码非常多非常复杂，我无法一一说明。可是我凝视的地方都是关键点，是博主细致读代码总结出来的。
源代码解读：
事件是由哪里传递给Activity的。这个我还不清楚。可是不要紧。我们从activity開始分析，已经足够我们了解它的内部实现了。
Code：Activity#dispatchTouchEvent
/**
* Called to process touch screen events. You can override this to
* intercept all touch screen events before they are dispatched to the
* window. Be sure to call this implementation for touch screen events
* that should be handled normally.
*
* @param ev The touch screen event.
*
* @return boolean Return true if this event was consumed.
*/
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
//这个函数事实上是个空函数。啥也没干，假设你没重写的话，不用关心
onUserInteraction();
}
//这里事件開始交给Activity所附属的Window进行派发，假设返回true。整个事件循环就结束了
//返回false意味着事件没人处理。全部人的onTouchEvent都返回了false，那么Activity就要来做最后的收场。
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
//这里，Activity来收场了，Activity的onTouchEvent被调用
return onTouchEvent(ev);
}
Window是怎样将事件传递给ViewGroup的
Code：Window#superDispatchTouchEvent
/**
* Used by custom windows, such as Dialog, to pass the touch screen event
* further down the view hierarchy. Application developers should
* not need to implement or call this.
*
*/
public abstract boolean superDispatchTouchEvent(MotionEvent event);这居然是一个抽象函数，还注明了应用开发人员不要实现它或者调用它。这是什么情况？再看看例如以下类的说明，大意是说：这个类能够控制顶级View的外观和行为策略。并且还说这个类的唯一一个实现位于android.policy.PhoneWindow。当你要实例化这个Window类的时候。你并不知道它的细节，由于这个类会被重构，仅仅有一个工厂方法能够使用。好吧。还是非常模糊啊，不太懂，只是我们能够看一下android.policy.PhoneWindow这个类。虽然实例化的时候此类会被重构，可是重构而已，功能是类似的。
Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.
The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation.
Code：PhoneWindow#superDispatchTouchEvent
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}这个逻辑非常清晰了。PhoneWindow将事件传递给DecorView了。这个DecorView是啥呢，请看以下
private final class DecorView extends FrameLayout implements RootViewSurfaceTaker
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
@Override
public final View getDecorView() {
if (mDecor == null) {
installDecor();
}
return mDecor;
}
顺便说一下。平时Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通过Activity来得到内部的View。
这个mDecor显然就是getWindow().getDecorView()返回的View。而我们通过setContentView设置的View是它的一个子View。
眼下事件传递到了DecorView 这里。因为DecorView 继承自FrameLayout且是我们的父View。所以终于事件会传递给我们的View，原因先无论了。换句话来说，事件肯定会传递到我们的View，不然我们的应用怎样响应点击事件呢。
只是这不是我们的重点，重点是事件到了我们的View以后应该怎样传递。这是对我们更实用的。从这里開始。事件已经传递到我们的顶级View了，注意：顶级View实际上是最底层View，也叫根View。
2.底层View对事件的分发过程
点击事件究竟层View（通常是一个ViewGroup）以后，会调用ViewGroup的dispatchTouchEvent方法，然后的逻辑是这种：假设底层ViewGroup拦截事件即onInterceptTouchEvent返回true。则事件由ViewGroup处理。这个时候。假设ViewGroup的mOnTouchListener被设置，则会onTouch会被调用，否则，onTouchEvent会被调用。也就是说，假设都提供的话，onTouch会屏蔽掉onTouchEvent。在onTouchEvent中。假设设置了mOnClickListener，则onClick会被调用。
假设顶层ViewGroup不拦截事件，则事件会传递给它的在点击事件链上的子View，这个时候，子View的dispatchTouchEvent会被调用。到此为止，事件已经从最底层View传递给了上一层View。接下来的行为和其底层View一致。如此循环，完毕整个事件派发。另外要说明的是，ViewGroup默认是不拦截点击事件的，其onInterceptTouchEvent返回false。
源代码解读：
Code：ViewGroup#dispatchTouchEvent
@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;
// 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) {
//这里推断是否拦截点击事件，假设拦截。则intercepted=true
intercepted = onInterceptTouchEvent(ev);
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;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
//这里面一大堆是派发事件到子View，假设intercepted是true，则直接跳过
if (!canceled && !intercepted) {
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)) {
continue;
}
newTouchTarget = getTouchTarget(child);
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();
//注意以下两句。假设有子View处理了点击事件。则newTouchTarget会被赋值。
//同一时候alreadyDispatchedToNewTouchTarget也会为true。这两个变量是直接影响以下的代码逻辑的。
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.
//这里假设当前ViewGroup拦截了事件，或者其子View的onTouchEvent都返回了false。则事件会由ViewGroup处理
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
//这里就是ViewGroup对点击事件的处理
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 (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 (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}
以下再看ViewGroup对点击事件的处理
Code：ViewGroup#dispatchTransformedTouchEvent
/**
* 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.
*/
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) {
//这里就是ViewGroup对点击事件的处理，其调用了View的dispatchTouchEvent方法
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;
}再看
Code：View#dispatchTouchEvent
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}这段代码比較简单，View对事件的处理是这种：假设设置了OnTouchListener就调用onTouch，否则就直接调用onTouchEvent，而onClick是在onTouchEvent内部通过performClick触发的。简单来说。事件假设被ViewGroup拦截或者子View的onTouchEvent都返回了false，则事件终于由ViewGroup处理。3.无人处理的点击事件
假设一个点击事件。子View的onTouchEvent返回了false，则父View的onTouchEvent会被直接调用，以此类推。假设全部的View都不处理，则终于会由Activity来处理。这个时候，Activity的onTouchEvent会被调用。
这个问题已经在1和2中做了说明。
