ViewGroup源码解读
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我们之前刚刚分析完事件传递机制和view的源码，如果没有看过的，建议看完View的事件拦截机制浅析以及View的事件源码解析。这次我们来分析下viewgroup的。
可能有人会想，怎么又是源码分析，肯定又是一大通。其实没你想的那么复杂。仔细分析一波就行了。
解读ViewGroup
我们都知道，一个事件完整的流程是从dispatchTouchevent–>onInterceptTouchevent–>onTouchEvent。我们先不说事件监听的问题。上述三个步骤就是正常一个点击的流程。前面我们分析view的时候发现它并没有onInterceptTouchevent这个方法。这个我之前有提到，view已经是最底层了，所以就不需要拦截了。而这一整套的机制就是在ViewGroup中体现出来的。我们先来看一张图： 
触摸事件发生后，在Activity内最先接收到事件的是Activity自身的dispatchTouchEvent，然后Activity传递给Activity的Window。接着Window传递给最顶端的View，也就是DecorView。接下来才是我们熟悉的触摸事件流程：首先是最顶端的ViewGroup(这边便是DecorView)的dispatchTouchEvent接收到事件。并通过onInterceptTouchEvent判断是否需要拦截。如果拦截则分配到ViewGroup自身的onTouchEvent，如果不拦截则查找位于点击区域的子View(当事件是ACTION_DOWN的时候，会做一次查找并根据查找到的子View设定一个TouchTarget，有了TouchTarget以后，后续的对应id的事件如果不被拦截都会分发给这一个TouchTarget)。查找到子View以后则调用dispatchTransformedTouchEvent把MotionEvent的坐标转换到子View的坐标空间，这不仅仅是x，y的偏移，还包括根据子View自身矩阵的逆矩阵对坐标进行变换(这就是使用setTranslationX,setScaleX等方法调用后，子View的点击区域还能保持和自身绘制内容一致的原因。使用Animation做变换点击区域不同步是因为Animation使用的是Canvas的矩阵而不是View自身的矩阵来做变换)。
dispatchTouchevent分析
我们先放上dispatchTouchevent的源码，然后一步一步来分析：
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);
}
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;
}
// 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;
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;
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 ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
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;
}
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);
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();
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;
}
}
}
// 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;
}
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是不是整个人都蒙蔽了，这么长一串。其实整段代码可以缩减成几句话，就是这样：
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean result = false;
if (!onInterceptTouchEvent(ev)) {
result = child.dispatchTouchEvent(ev);
}
if (!result) {
result = onTouchEvent(ev);
}
return result;
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默认不消耗事件，如果本身没有拦截，就交给子类的dispatch事件，如果事件没有消费，就调用自身的onTouchEvent事件。你们仔细想想，流程是不是这样的？
好了，我们现在开始分析整个dispatch事件。具体说明和代码，你们自己对应= =因为太长了。
对action_down的处理：
我们发现，刚进方法的时候有个判断，第一次按下的时候，他会通过 cancelAndClearTouchTargets(ev)取消并且清除所有的手势操作，并且通过resetTouchState()把手势状态设置成默认状态。
接下来的操作，当然就是检查是否需要拦截事件拉。既然是拦截，当然就会走onInterceptTouchEvent这个方法了。我们来看看，viewgroup的onInterceptTouchEvent方法是怎么处理的。
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
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我们可以发现，他默认就是false的。那么我们继续回到dispatch看。判断是否拦截后，我们发现他还执行了一句话ev.setAction(action) 官方说明是恢复操作，防止被更改。
事件处理
接下来就是检查事件是否取消咯。如果没有取消并且没有拦截就执行正常的事件处理。
如果事件是针对可访问性焦点视图，我们将其提供给具有可访问性焦点的视图。如果它不处理它，我们清除该标志并像往常一样将事件分派给所有的 ChildView。我们检测并避免保持这种状态，因为这些事非常罕见。这段是官方的解释。我们继续向下看，他执行这样一个方法removePointersFromTouchTargets(idBitsToAssign)。是为了防止指针不同步，清除之前的触摸标识。自我认为可能会和多指触控有关，先不管他，我们继续向下分析。
接下来就是打造了，他会先得到触摸点的坐标位置，然后在当前位置查找可接触的ChildView。然后重点！！！他的查找顺序是从后向前查找。什么意思呢？就是如果A和B有重叠的部分，并且B在A的上面，那么他处理的便是B的事件了。而不处理A的事件。
如果子View可以接受事件，那么我们就给他一个触摸的标识。接下来他会通过调用dispatchTransformedTouchEvent把事件分配给子View。
最后他会判断是否有touchtarget。如果没有的话，那就处理子view的事件。否则就会遍历touchtarget处理事件，也就是之前说的多点触控。在往后就是对action_up和cancel做的一些处理了，譬如：重置手势状态，移除多指操作等等。
dispatchTransformedTouchEvent分析
前面我们说到了，会通过这个方法把事件分发给子view。我们还是先来看代码：
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();
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;
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这段代码就比之前简单很多了。我们会发现，他先判断状态是否取消，如果取消了，把当前事件变成取消状态，然后在判断是否有子view。如果有子view的话直接调用子view的dispatch事件。下面就是多指了，一个pointer对应一个ID，防止处理冲突。我印象中能简单粗暴的处理多指，应该是ViewDragHelper了。具体，你们可以自己去看。后面就如之前一样，判断child是否为null。然后得到是执行自身的事件还是child的事件。
总结
1.ViewGroup包涵多个子view的时候，我们是从后遍历，判断当前view是否可以点击，然后分发给需要处理的子view。
2.我们可以在onInterceptTouchEvent中进行事件拦截。
3.我们可以发现ViewGroup没有onTouchEvent事件，说明他的处理逻辑和View是一样的。
4.子view如果消耗了事件，那么ViewGroup就不会在接受到事件了。
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