上篇本来想把Activity的事件分发和本篇放在一起写，但ViewGroup的事件分发内容实在太多，所以分开了。本篇正式开始学习ViewGroup事件分发源码，ViewGroup事件分发的源码较长，所以下面分析的时候分段贴出源码。
ViewGroup的dispatchTouchEvent()源码:
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
//dispatchTouchEvent的返回值handle默认值为false
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// Handle an initial down.
//down事件就走进去
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.
//重要代码@1：清除TouchTarget目标控件
cancelAndClearTouchTargets(ev);
//重置touch状态（后面的禁止拦截状态就被重置了）
resetTouchState();
}
注释中标注的重要代码@1比较重要，ViewGroup在处理down事件的时候会首先清空touchTarget,也就是清空消费上一次touch事件的子View(事件传递链上所有target view）。 继续看下面的代码：
//重要代码@2:整段都重要
// Check for interception.
//是否拦截标志位，默认为false
final boolean intercepted;
//如果是down事件，或者FirstTouchTarget不为null,也就是有子控件消费touch事件
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//禁止拦截标志位,默认为false，在子控件可通过getParent().requestDisallowInterceptTouchEvent()修改其值
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
//不禁止拦截，也就是要拦截事件
if (!disallowIntercept) {
//修改拦截标志位
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
//不拦截事件，拦截标志位为false
intercepted = false;
}
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
//既不是down事件，并且也没有子控件消费touch事件，就拦截
intercepted = true;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// 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;
这一整段代码主要是检测是否拦截事件，通过重要代码@1，再结合上面的源码可以得出结论：
1，如果在子View中调用getParen().requestDisallowInterceptTouchEvent(true)不让父控件拦截事件，那么该方法必须写在子View的dispatchTouchEvent(),或者onTouch()亦或者onTouchEvent()中（如果子View的onTouch()返回true，则只能写在dispatchTouchEvent()或者onTouch()中，具体原因看View的事件分发）,因为如果写在这两个方法外面,touch事件的down事件传递到ViewGroup中的时候，会在重要代码1处重置禁止拦截标志位为false，相当于没改变禁止拦截标志位，父控件还是会拦截事件。
2，如果父View下面有子View消费touch事件，并且子View没有调用requestDisallowInterceptTouchEvent(true),无论是down，move,up事件，父View的onInterceptTouchEvent()都会执行，父View都会拦截
3，如果是down事件，即使父View下面没有子View消费touch事件，onInterceptTouchEvent()也会执行，也就是说，父View无论如何都会拦截down事件，子View调用requestDisallowInterceptTouchEvent(true)也不能控制
4，onInterceptTouchEvent()只有在2种情况下不会执行：
- 不是down事件，并且ViewGroup下面的子View没有消费touch事件
- 不是down事件，并且ViewGroup有子View消费touch事件，但是子View调用了requestDisallowInterceptTouchEvent(true)
源码继续：
//不是cancel事件，并且没有拦截
if (!canceled && !intercepted) {
// If the event is targeting accessiiblity focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
//如果是down事件，或者多个手指按下，或者鼠标在ViewGroup上移动，就开始分发事件
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.
//按Z轴的排列，从上到下倒序添加ViewGroup的直接子View到集合
final ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
//对ViewGroup的直接子View进行倒序遍历
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
// If there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
//判断是否在子View的范围内，或者子View是否在播放动画，都不符合，则继续遍历
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
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);
//重要代码@3：将事件分发给子View，如果有子View消费了Touch事件就进入if内部
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
if (preorderedList != null) {
// childIndex points into presorted list, find original index
for (int j = 0; j < childrenCount; j++) {
if (children[childIndex] == mChildren[j]) {
mLastTouchDownIndex = j;
break;
}
}
} else {
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
//有子View消费Touch事件，将它插入到事件传递链上，并给mFirstTouchTarget赋值
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
// The accessibility focus didn't handle the event, so clear
// the flag and do a normal dispatch to all children.
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
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;
}
}
}
ViewGroup的事件分发最关键的就是这段，这段主要是分发事件。从一进来的第一个if语句和第二个if语句，可以知道：只有在down事件，并且没有拦截的情况下，ViewGroup才会把down事件给分发给子View. 并且在分发的时候如果找到了有子View能够处理当前的down事件，newTouchTarget=mFirstTouchTarget,他们都不为null，并且alreadyDispatchedToNewTouchTarget的值也被置为true
看下dispatchTransformedTouchEvent()的源码：
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();
//cancel值为true，或者是cancel事件
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
//child为null,调用ViewGroup自己的父类，也就是View的dispatchTouchEvent()
handled = super.dispatchTouchEvent(event);
} else {
//child不为null，调用child的
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.
//跟上面的cancel事件一样
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;
}
在分发事件的步骤中重要代码@3处 :
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {....}
传递cancel为false,并且child不为null，所以dispatchTransformedTouchEvent()会调用最后的
handled = child.dispatchTouchEvent(transformedEvent(),也就是子View的dispatchTouchEvent(),如果子View也是ViewGroup,那么就又重新走一遍从本篇开始到此处的代码，如果子View不是ViewGroup,那么就走一遍View的事件分发。
通过事件的递归传递，就可以找到消费down事件的目标mFirstTouchTarget，如果找到了，mFirstTouchTarget不为null， 如果没找到，mFirstTouchTarget为null.
继续源码：
// Dispatch to touch targets.
//没有子View消费down事件
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
//交由ViewGroup自己处理
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;
//alreadyDispatchedToNewTouchTarget 每次进入dispatchTouchEvent()都会被置为false,newTouchTarget也会被置为null,
//只有在分发down事件时找到能够处理touch事件的目标子控件了，这两个条件才会被满足
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true; //分发down事件，找到目标子控件了，返回true
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted; //有子View消费了down事件，但是在move事件ViewGroup拦截了,cancelChild为true
//重要代码@4：cancelChild如果为true,分发一个cancel事件给child，并且ViewGroup自己返回true
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) { //找到了目标子控件，分发move和up事件给子view
handled = true; //如果子View的move和up事件也返回了true，viewGroup的dispatchTouchEvent()也返回true
}
//cancleChild为true,把mFirstTouchTarget 回收重置为null
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;
}
在上面的重要代码@4中，可以看到：
1，如果子View消费了down事件(mFirstTouchTarget不为null)，但是ViewGroup拦截了move事件(intercepted为true)，cancelChild 为true,dispatchTransformedTouchEvent()第二个参数就为true，move事件不会传递到子View，而是会传递一个cancel事件给子View来结束本次move事件的传递。并且cancelChild为true，会把mFirstTouchTarget置为null，直到下一次down事件到来之前，后面的任何事件(move,up)都会交个这个ViewGroup处理（调用自身的onTouchEvent()），不会再传递给子View。(子View的cancel事件返回值也会影响到Activity的onTouchEvent()是否触发)
2，如果子View消费了down事件，在move事件的时候，ViewGroup也没有拦截，但是子View自己不想处理move事件(dispatchTransformedTouchEvent()返回false)，handle这时候就为默认值false，ViewGroup的dispatchTouchEvent()会直接在最后返回false,不会去调用自身的onTouchEvent()去处理当前的move事件，而是把move事件层层往上传递到Activity，直接交给Activity处理。当前ViewGroup，以及中间的所有ViewGroup都不会处理当前以及后续的move事件。但是后续的up事件会正常下发到子view，不会交给Activity处理。
前面在具体的重要代码处已经总结了关键流程，懒得再总结了。
在这里还想谈下学习事件分发的感悟：
以前总是记别人的总结：“如果子view不处理touch事件，事件会回传给父View处理”,那是down，move,up事件的返回值统一都一样的情况下，才是对的，但是想上面的情况，我们可能会对不同的事件分别做不同的处理，这个结论就不对了。所以只记结论，只看什么“图解事件分发”的流程图，不看源码和测试，只会颠覆你的三观：为什么会这样，明明这个流程图就是这样的，怎么事件不传给父View了！！！ 哈哈，Reading the fucking source code! 答案在源码！
ps:源码比较复杂，有很多细节没有分析到（能力也有限），阅读源码主要把握流程，不能只见树木不见森林
