已经记不清有多久了，貌似自从接触Android开发开始，Android的事件分发机制一直伴随着我们。网上各种大神的各种分析，看完了可能还是会晕晕乎乎的。没办法，谁让咱是菜鸡呢（对！我菜鸡我还有有理了。。）。
前面写过一篇关于Android事件的由来，介绍了Android输入事件从产生到发送到View的过程。今天，就来说说关于事件分发的流程。
1. 事件产生的整个流程（Java层）
2. ViewGroup的事件分发
我们都知道ViewGroup事件分发是Parent向Child进行分发，如果Child消费了事件，则返回true告诉Parent；否则，返回false。如果Child都没有消费，那么此时会看Parent是否有消费。
@Override
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;
// 是否过滤此次事件
if (onFilterTouchEventForSecurity(ev)) {
// 获得Action
final int action = ev.getAction();
// Action & 0xff, 正常是的Action包括了1-12，其他的会有ACTION_POINTER_1_DOWN等Action
// 这里 & 0xff则需要获得1-12的Action
final int actionMasked = action & MotionEvent.ACTION_MASK;
// 如果是按下事件
// 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.
// 取消并清除触摸事件的目标
cancelAndClearTouchTargets(ev);
// 重置触摸状态
resetTouchState();
}
// Check for interception.
// 是否被拦截
final boolean intercepted;
// 如果是按下事件或者第一个触摸目标不为空
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
// 是否允许拦截，根据标志位FLAG_DISALLOW_INTERCEPT来判断
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
// 为false说明允许拦截，根据onInterceptTouchEvent的返回值来判断是否拦截触摸事件
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
// 重新设置MotionEvent的Action防止其被改变
ev.setAction(action); // restore action in case it was changed
} else {
// 不允许拦截的话则设置为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.
// 如果没有触摸目标并且这个事件不是按下事件，这个ViewGroup需要继续拦截这个事件
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.
// 检查是否取消了事件
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// Update list of touch targets for pointer down, if needed.
// FLAG_SPLIT_MOTION_EVENTS在initViewGroup()初始化中设置
//if (mContext.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB) {
// mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS;
//}
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
// 是否已经分发给新的目标
boolean alreadyDispatchedToNewTouchTarget = false;
// 如果事件没有被取消并且也没有被拦截，则进行分发
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.
// 辅助功能
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
// 如果是按下事件或者是指针按下或者是光标移动的Action，则需要进行分发
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
// 按下事件总是0
final int actionIndex = ev.getActionIndex(); // always 0 for down
// split为true，idBitsToAssign = 1 << ev.getPointerId(actionIndex)
// 每一个触摸点Pointer都会有一个当次动作序列的唯一Id和Index.MotionEvent中多个手指的操作API大部分都是通过pointerIndex来进行的，
// 如：获取不同Pointer的触碰位置,getX(int pointerIndex);获取PointerId等等。大部分情况下，pointerId = pointerIndex
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.
// 为这个点的id清除之前的触摸目标，防止不同步
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
// 新的目标为null并且childCount!=0
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.
// 构建触摸分发list，构建的时候有个条件hasChildWithZ，如果没有则返回null，一般返回null
final ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
// 获得childIndex
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
// 获得childView
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;
}
// 如果View不能接收PointerEvents（不是VISIBLE或者child.getAnimation() != null）或者该事件的点不在View中
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
// 通过child获得touchTarget，如果有的话说明Child已经接收事件了
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.
// 设置新的pointerIdBits
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
// 这里是分发给child，如果返回true，则说明接收事件
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();
// 生成一个touchTarget
newTouchTarget = addTouchTarget(child, idBitsToAssign);
// 设置已经接收事件
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();
}
// 没有找到可以接收事件的child，所以把这个指派给最近添加的目标
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.
// mFirstTouchTarget不等于null是在addTouchTarget中赋值，这个需要事件分发的时候返回true
// 这时候可以知道此次事件是被View接收，而上面一部分只是关于DOWN事件的处理，所以一旦有View接收了DOWN事件，
// 那么接下来的事件都将交给此View处理
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
// 没有触摸目标，所以把它认为是普通View，调用super.dispatchTouchEvent
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;
// 临时变量tagert
TouchTarget target = mFirstTouchTarget;
// 链表遍历，target = target.next
while (target != null) {
final TouchTarget next = target.next;
// 这个条件为真的话，是在DOWN事件是有View接收，此时alreadyDispatchedToNewTouchTarget = true
// 并且mFirstTouchTarget = target = newTouchTarget
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
// 是否取消，判断flag PFLAG_CANCEL_NEXT_UP_EVENT
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
// 到这边的话此次事件已经不再是DOWN事件，而是MOVE事件，此时分发给target.child，知道最终接收DOWN事件的View
// 如果返回true，则设置handled = true
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.
// 抬起事件重置状态
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;
}
一言不合就扔一堆代码，我能怎么办啊？我也很无奈啊。。要想答案来，唯有码中求。下面还是分析一波：**
1. 通过安全策略检查是否需要过滤此次事件（检查Window是否被覆盖），如果过滤了，则不会处理此次事件；否则进行第2步。
2. 获得事件的Action，并且如果是按下事件的话，则需要清除触摸事件的目标并且重置触摸状态。
3. 检查是否需要拦截此次事件，这时候先判断标志位FLAG_DISALLOW_INTERCEPT是否存在，如果存在则说明不允许进行拦截，否则的话根据onInterceptTouchEvent的返回值来判断是否拦截触摸事件。这里我们可以清楚一点：如果ViewGroup的onInterceptTouchEvent返回true，则不会对事件的进行向下分发。
4. 检查事件是否被取消。
5. 如果事件没有被取消并且没有被拦截，则需要获取事件的id。每一个触摸点Pointer都会有一个当次动作序列的唯一Id。获取不同Pointer的触碰位置可以通过getX(int pointerIndex)方法。大部分情况下，pointerId = pointerIndex。
6. 构建分发列表，构建的时候有个条件hasChildWithZ，如果没有则返回null，一般返回null。
7. 对子View进行遍历，如果可以接收事件，那么先去获得触摸目标（getTouchTarget）。如果返回值不为null，则说明子View已经接收事件了，跳出循环；否则，调用dispatchTransformedTouchEvent方法将事件传递给子View。
8. 根据dispatchTransformedTouchEvent返回值来判断是否子View接收了事件。true：接收事件；false：没有接收。
9. 如果没有接收事件，则mFirstTouchTarget = null，所以会调用dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS)这里会调用super.dispatchTouchEvent交给View去处理这次touch事件；如果接收了这次事件，则mFirstTouchTarget !=null 并且alreadyDispatchedToNewTouchTarget = true&&target == newTouchTarget成立，最终handled设置为true。
10. 如果事件不是DOWN事件，那么去寻找是否有TouchTarget，如果有的话，说明有View消费了事件，那么接下来的该事件的其他操作将交由此View处理
说完dispatchTouchEvent方法，这里需要说下dispatchTransformedTouchEvent方法：
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.
// 事件取消，如果child不为空则向child分发ACTION_CANCEL事件；否则调用super.dispatchTouchEvent
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.
// 通过getPointerIdBits获取事件的触碰点的id和传入的进行&运算，如果此时仍和oldPointerIdBits相同，
// 则说明desiredPointerIdBits包括了oldPointerIdBits，可以理解为是同一个事件
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.
// 如果新的事件和旧的事件的getPointerIdBits & desiredPointerIdBits相同，则设置offsetLocation并且直接去进行事件的分发
// 一般来说单点触摸事件会进行这里
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
// 如果没有child的话，则调用View的dispatchTouchEvent方法
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
// 对child的dispatchTouchEvent方法进行事件分发：
// 1. 如果child是ViewGroup，则会进行ViewGroup的事件分发过程
// 2. 如果child是View，则进行View的事件分发过程
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.
// 执行必要的的转换和分发
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;
}
这个方法其实总得来说就执行了两个过程：
1. 传入的子View不为null，将事件分发给子View。
2. 传入的子View为null，调用super.dispatchTouchEvent方法去处理。
3. View的dispatchTouchEvent方法
上面说到，如果子View为null，则会调用super.dispatchTouchEvent方法。这里看下View的dispatchTouchEvent方法：
public boolean dispatchTouchEvent(MotionEvent event) {
......
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
// 通过安全策略检查是否需要过滤
if (onFilterTouchEventForSecurity(event)) {
// 如果View是enable并且正在处理ScrollBar的鼠标拖拽事件，设置结果为true
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
// 判断ListenerInfo中的OnTouchListener是否为null
// 不为null，并且ENABLED，则调用mOnTouchListener.onTouch方法
// 如果mOnTouchListener.onTouch返回true，则结果设为true
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
// 如果结果为false并且onTouchEvent方法返回值为true，结果设为true
if (!result && onTouchEvent(event)) {
result = true;
}
}
......
return result;
}
这个方法就简单了很多，因为它作为最终的View（View不包括子View），所能进行的操作要么消费事件要么不消费事件。从方法的实现中也可以看到：
- 如果设置了OnTouchListener，那么在执行过程中会先执行OnTouchListener的onTouch方法，接着根据返回值来确定是否需要执行onTouchEvent方法。
- onTouchEvent是否需要调用是和result的值有关：如果result为true，则不调用；反之，则调用。
4. 验证
验证过程其是很简单，只要能够进行debug看方法的调用即可。我这里测试的代码如下：
5. 总结
从源码可以看出：
- 事件的分发最重要的是对ACTION_DOWN事件的分发，在分发过程中：如果该ViewGroup没有拦截，那么会对其子View进行事件分发。如果子View没有消费事件（返回值为true），那么交由上一级处理。
- 同一事件的其他操作，例如ACTION_MOVE这个是在处理完ACTION_DOWN事件后进行的，主要是通过查找TouchTarget是否存在来判断是否事件需要传递。
- View中OnTouchListener的onTouch方法优先级高于onTouchEvent方法，并且onTouchEvent在result为true的时候不会调用。
