一 Android View事件分发机制简析
1 事件分发的本质是什么?
简单的来说，就是当一个事件到来的时候，系统为我们分配究竟是哪一个view或者viewGroup来消费这个touch事件
2 事件分发机制的重要控制方法
（1）public boolean dispatchTouchEvent(MotionEvent ev)
如果事件分发到当前的view，那么这个方法一定会被调用，表示当前是否会消耗这个事件
（2）public boolean onInterceptTouchEvent(MotionEvent ev)
在dispatchTouchEvent（）方法中被调用，表示当前view是否拦截本次事件，注意，view没有这个事件，viewGroup才有
（3）public boolean onTouchEvent(MotionEvent event)
在dispatchTouchEvent（）方法中被调用，用来处理点击事件
二 从源码出发，开始分析
Step 1 Activity中的事件分发机制
由View的绘制流程我们可以知道，activity的组成大致可以分为Activity->PhoneWindow->DecorView->ViewGroup->View。所以我们首先进入Activity中，查看当前Activity的事件分发处理流程，也即是整个事件分发机制的处理入口
由以下代码我们开始分析
/**
* 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) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
由注释我们可以知道，这个方法是用来处理touch事件的，我们可以通过继承Activity，重现这个方法，在事件被分发到Window（PhoneWindow）层之前去拦截他们
在这段代码中，我们主要分析第二段，也即是：
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
点进去superDispatchTouchEvent（）方法，我们可以看到
/**
* 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);
superDispatchTouchEvent（）方法是一个抽象方法，我们知道，继承Window抽象类的，在这里知道就是PhoneWindow方法了，所以，这个时候，我们的事件分发已经传递到了window层
在PhoneWindow类中，我们可以看到这段代码：
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
由view的绘制流程相关知识我们可以知道，PhoneWindow其实就是DecorView+StatusBar组成，DecorView就是我们在Activity的onCreate()方法中，通过setContentView方法加载进去的。所以显然，这个时候，view的事件分发机制，已经分发到底层viewGroup中了。
Step 2：ViewGroup里的事件分发机制
由上面代码一步一步分析，我们终于进入到了viewGroup层，我们首先看：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
......
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
if (actionMasked == MotionEvent.ACTION_DOWN) {
cancelAndClearTouchTargets(ev);
resetTouchState();
}
......
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);
} else {
intercepted = false;
}
} else {
intercepted = true;
}
......
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) {
.......
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 (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) {
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;
}
备注：以上代码已经去除了部分简单易用理解或者跟本文无关的代码，代码太长，建议在android Studio中阅读
Step （1）
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;
}
在这里我们看到了全文开头的 onInterceptTouchEvent（）方法。那么什么情况下，回去执行事件分发机制的onInterceptTouchEvent（）方法呢？首先要满足三个条件
1 actionMasked == MotionEvent.ACTION_DOWN（这个简单，不做解释）
2 mFirstTouchTarget != null
3 (mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0
好了，我们先分析一下第2。往下查看代码，寻找mFirstTouchTarget是在哪赋值的，
/**
* Adds a touch target for specified child to the beginning of the list.
* Assumes the target child is not already present.
*/
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
可以知道，当我们的view不拦截事件，并且将这个事件赋给了子View的时候，mFirstTouchTarget是不为空的，反之，当我们的ViewGroup消费了这个事件的时候，mFirstTouchTarget是为空的，以为intercepted是true的，addTouchTarget（）方法是不会走的，也即是mFirstTouchTarget从没有被赋予过值！
好了，回归第三点 (mGroupFlags & FLAG_DISALLOW_INTERCEPT)==0的问题
@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
// We're already in this state, assume our ancestors are too
return;
}
if (disallowIntercept) {
mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
} else {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
// Pass it up to our parent
if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
}
}
可以看到，如果我们的ViewGroup的子View调用这个方法，设置disallowIntercept==true，那么我们的Viewgroup也不会去调用onInterceptTouchEvent(ev)方法
Step （2） 如果ViewGroup不拦截当前事件，将会把事件分发给对应的子View
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();
}
这段代码比较清晰，如果子View是可以接受touch事件的话，那么我们就把当前的事件的目标对象targetView赋值给当前的子View。如何判断View是可以接受当前touch事件的呢？我们看这个判断
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
continue;
}
也即是，
1 如果这个view当前在播放动画，
2 或者这个触摸点坐标刚好坐落在子View的范围内
都是符合条件的
到最后，我们来查看接下来的执行
` private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
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;
}
......
return handled;
}`
也即是到了View的dispatchTouchEvent（）方法中了
Step 3 Vew中的事件分发机制
public boolean dispatchTouchEvent(MotionEvent event) {
boolean result = false;
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
return result;
}
以上代码忽略了跟本文无关，或者易于理解的部分，只留下核心代码
1 首先，由代码可以看看出，顶层View的事件分发机制中，并没有onInterceptTouchEvent(ev)方法，其实也可以很容易理解，最顶层View的意思就是说没有子view了，也就是没有进一步拦截的必要了。
2 顶层View会去判断当前view是不是设置了touchListener，如果当前view监听了touchlistener，那么，result=false，也即是，onTouchEvent()方法不会被调用，所以可以看出，touchListener监听的优先级要高于onTouchEvent方法
最后，我们来分析onTouchEvent（）方法
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;
}
return true;
}
return false;
}
ok，onTouchEvent方法源码大概就是这样的，那么，我们可以看到，
1 当View设置了CLICKABLE 时候，表示当前View就会消耗这个点击事件，当事件处于MotionEvent.ACTION_UP的时候，view会触发performClick方法
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
return result;
}
好了，也就是说，我们平时的setOnClickListener，最后就跑到这里来了~~
2 当View设置了LONG_CLICKABLE 时候
case MotionEvent.ACTION_DOWN:
setPressed(true, x, y);
checkForLongClick(0, x, y);
break;
如果当view设置了longClickListener的时候，事件的触发机制并不在ACTION_UP触发，而是在ACTION_DOWN的时候，有个判断是否checkForLongClick的方法
private void checkForLongClick(int delayOffset, float x, float y) {
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
mHasPerformedLongPress = false;
if (mPendingCheckForLongPress == null) {
mPendingCheckForLongPress = new CheckForLongPress();
}
mPendingCheckForLongPress.setAnchor(x, y);
mPendingCheckForLongPress.rememberWindowAttachCount();
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}
}
public boolean postDelayed(Runnable action, long delayMillis) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.postDelayed(action, delayMillis);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
getRunQueue().postDelayed(action, delayMillis);
return true;
}
也即是说，系统判断当前是否符合长按的条件，就是在按下的时候，开始一个事件，然后去监听按下的事件，如果时间符合，那就是符合长按事件了
自此，View的事件分发机制分析完毕，从源码角度出发，确实比网上的一大堆片段的代码分析要容易记得多，所以还是建议大家多多读下源码，这对我们了解一些基本原理，还是十分有帮助
本文有一些解释和介绍思路参照了《安卓开发艺术探索》这本书，在此十分感谢此书的作者。
