本文详细解析了Android中View事件在ViewGroup和ViewTree中的传递机制，包括touch事件的处理流程、事件拦截、事件分发及最终到达Activity的onTouchEvent过程。通过代码分析，展示了事件从触摸开始到最终被Activity处理的完整路径。
前面总结了touch事件在View中的传递，以及在一个viewTree中传递的流程。
但是在ViewGroup以及ViewTree中的传递是根据log看出来的，那么在代码中到底是如何处理的？
今天就看下代码，主要就是从viewGroup中dispatchEvent开始。
首先还是会调用当前activity的dispatchTouchEvent();然后走走走,...
就走到了rootView的dispatchTouchEvent();--并且rootView还是viewGroup
先上代码，随后慢慢分析。
@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;
// 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);
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;
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 (childrenCount != 0) {
// 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);
for (int i = childrenCount - 1; i >= 0; i--) {
final View child = children[i];
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 = i;
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) {
// 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 (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;
}下面就逐步分析这个代码，我们分阶段来看，有些细节就不说了。
首先是
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
} /**
* Consistency verifier for debugging purposes.
* @hide
*/
protected final InputEventConsistencyVerifier mInputEventConsistencyVerifier =
InputEventConsistencyVerifier.isInstrumentationEnabled() ?
new InputEventConsistencyVerifier(this, 0) : null;
然后就来到了下一步:
if (onFilterTouchEventForSecurity(ev)) {
//判断是不是需要将事件安全过滤，一般返回true不用过滤，不过当touch模糊或者当前window模糊时会过滤掉
//如果过滤掉，则直接执行到下面这一行：
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled; if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;事件就传递给了activity的onTouchEvent了。
注意，事件一旦由activity的dispatchTouchEvent传递给Activity的onTouchEvent后，
那么这一次事件中的所有活动就直接是由Activity的dispatchTouchEvent到Activity的onTouchEvent了
那么如果按正常情况，是可以进行事件传递的，代码如下
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;MotionEvent.ACTION_MASK是255，255与任何数相与都是那个数本身，
因此actionMasked也就是action。
代码继续往下走:
// 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();
}具体实现了什么暂时不用深究，反正知道当按下后就是世界的重新开始了，
android中是可以重来的(多美好),最重要的就是将mFirstTouchTarget置为了null
代码继续往下走
// 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);
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;
}注释说的很明白，就是是否进行拦截的一个判断
因为是按下，所以actionMasked == MotionEvent.ACTION_DOWN为true.
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
FLAG_DISALLOW_INTERCEPT是横为true的，mGroupFlags这个的值由什么决定
由它的这个viewTree中的子view的方法
child.getParent().requestDisallowInterceptTouchEvent(true);
参数为true,则FLAG_DISALLOW_INTERCEPT。
因此因此，当你的子view不想让父view拦截时，也可以不重写父view的方法，而是直接调用
getParent().requestDisallowInterceptTouchEvent(true);就可以了
进行了disallowIntercept赋值后，代码继续往下走
如果disallowIntercept = false.则进入第一层
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed我们可以重写返回true,则intercepted就为true,这就是拦截的标记
默认viewGroup中是不拦截的
如果disallowIntercept = true,则直接默认不拦截
如果如果if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
这一层不满足，也就是说不是按下，并且mFirstTouchTarget==null
那么
else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}也就是拦截，这个就是解释如果第一次拦截了(就没有touchTARGET,mFirstTouchTarget==null)，
那么在这次事件中的以后就都拦截，并且不走onIntercepetTouchEvent了
但重新开始down，这些又会重新判断
第三部分，现在intercepted这个标志决定了，也知道了onInterceptTouchEvent是什么时候执行的
我们看下面做什么
先是不是特别重要的代码块:就是判断是不是取消手势或其他，然后到了下面
if (!canceled && !intercepted) {
//首先肯定不是cancel的手势，那！canceled就为true,主要就是判断!intercepted的值了，这个值在上面决定过
//如果interctepted为true，则就是说父控件拦截，那么按理论是直接执行父控件的onTouch事件，我们看代码来查看是怎样做到的
// 拦截，不走if，就跳到了下面
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
...就走了if，执行了dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);
先上这个方法的完整代码
<span style="color:#333333;"> /**
* 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) {
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;
}</span><span style="color:#ff0000;">
</span>我们看这个方法里主要做了什么，基本上viewTree中事件的分发都和这个方法有关，这个方法非常重要
上面是这个方法的所有，那么我们在这时调用这个方法，我们要走下这个方法的流程了
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
//第一个cancel是传递过来的，而传时这个cancel的值也是是否取消手势有关，因为不是，所以就不走if,走到了下面
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
}newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
desiredPointerIdBits传过来的是TouchTarget.ALL_POINTER_IDS---从名字看是所有，就是都为各个位都为1
任何一个数与这样的数相与都为本身，因此old=true，因此第一个if成立
走第二个if
child是传过来的值，确实为null
因此就走了super.dispatchTouchEvent(event)
注意，此时的super是指类的继承关系中的super，也就是viewGroup的父类view的dispatchTouchEvent，
而不是viewTree中child和parent，要分清
那么就走到了view.dispatchTouchEvent，那么狠清楚了，走到了我们上一次所说的
也就是onDispatch--onTOUCH--ONTouchEvent---onclick()
这个是在当前viewGroup这一层执行的，因此也就执行了
viewGroup的onTouch方法(也就是view的)，如果不消费，就返回给上一层，让上一层执行onTouchEvent了
然后就返回handled,这个handled的值就是由view的dispatchTouchEvent决定的。
那么那么，如果Intercept为false，也就是不拦截，就进入了
//那么那么，如果Intercept为false，也就是不拦截，就进入了
if (!canceled && !intercepted) {//中，那按情况是执行子view的dispatchTouchEvent，将事件分发下去，那么是怎么做到的，看代码
//if代码块中
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE)
//第一次为按下，所以会进入代码块
final int actionIndex = ev.getActionIndex(); // always 0 for down
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
//得到这次事件的索引以及索引对应的fingerID
final int childrenCount = mChildrenCount;//得到当前viewGroup的子view数量
if (childrenCount != 0) {
// 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);
//如果子view数量不为0，就记录下这次事件的坐标。
for (int i = childrenCount - 1; i >= 0; i--) {
final View child = children[i];
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
continue;
}
//对这些子view进行循环，判断当前事件坐标是不是在这个子view中，如果不是，就继续循环下面是如果是的代码
关键的在这
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
mLastTouchDownIndex = i;
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}根据参数走到了这边
// 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);
}handled = child.dispatchTouchEvent(transformedEvent);--开始执行了子child的dispatchTouchEvent,将handled返回！
这样就将事件传递给子view或者子viewGroup了
下面还有一些事件的分发细节，今天暂不说了，这个代码很复杂，考虑的很多，
主要就是根据源码有一个印象关于事件传递
那需要最后总结下了
当在一个屏幕上down时
activity.dispatchTouchEvent()--rootView.dispatchTouchEvent()
没有getParent.requestDisallowInterceptTouchEvent(true).并且onIntercpetTouchEvent返回为false(默认)
childView.dispatchTouchEvent()--...--最后一个childView.dispatchTouchEvent--childView.onTouchEvent()
如果childView.onTouchEvent返回true,则消费了事件，事件就到此终止了。
如果childView.onTouchEvent返回false,则响应事件，则不消费事件，事件又向他的父view传递，调用父view.onTouchEvent，重复上述行为
直到到了
rootView.onTouchEvent();
如果设置了getParent.requestDisallowInterceptTouchEvent(true)或者onIntercpetTouchEvent返回true
则直接执行rootView.onTouchEvent();
如果rootView.onTouchEvent()返回false，则整个viewTree中没有任何事件的消费，
就又返回到了activity中的onTouchEvent();
那这个是时候进行up操作，就不在执行view事件传递了！直接就由
activity.dispatchTouchEvent（）--activity.onTouchEvent().直到下一次事件的开始。。
framework层的探测很懒惰，第一次不消费，就再不问你们是否消费了。
以上是分析，在实际中基本就是这个流程，最后推荐如果更清楚的搞清这个传递，看下面链接视频
http://v.youku.com/v_show/id_XODQ1MjI2MDQ0.html?f=23088492
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