深入源码理解Android事件分发机制
如果对Android事件分发机制不理解，可以首先看图解 Android 事件分发机制
- 对于 dispatchTouchEvent，onTouchEvent，return true是终结事件传递。return false 是回溯到父View的onTouchEvent方法。
- ViewGroup 想把自己分发给自己的onTouchEvent，需要拦截器onInterceptTouchEvent方法return true 把事件拦截下来。
- ViewGroup 的拦截器onInterceptTouchEvent 默认是不拦截的，所以return super.onInterceptTouchEvent()=return false；
- View 没有拦截器，为了让View可以把事件分发给自己的onTouchEvent，View的dispatchTouchEvent默认实现（super）就是把事件分发给自己的onTouchEvent。
下面来将深入源码理解Android事件分发机制，首先看下ViewGroup下面的dispatchTouchEvent()事件
事件分发，从按下DOWN事件开始，从ViewGroup.dispatchTouch()往下分发
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);
// 初始化所以touch状态，主要 mFirstTouchTarget = null;
resetTouchState();
}
ACTION_DOWN事件，会调用resetTouchState()函数，这个函数主要设置mFirstTouchTarget == null
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
...
newTouchTarget = getTouchTarget(child);
...
// 主要运行child.dispatchTouchEvent()方法，判断自己是否需要需要继续分发
// ViewGroud继续运行child.dispatchTouchEvent()方法，直到底部为View为止
// View.dispatchTouchEvent()，首先判断onTouch，再判断onTouchEvent
// 必须是child.dispatchTouchEvent() = true，不能是孙子View
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
...
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
// Adds a touch target for specified child to the beginning of the list.
// 把newTouchTarget放在链表头部，这里给mFirstTouchTarget赋值，不为空了
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);
}
ACTION_DOWN事件，会进入for循环遍历子View，把Down事件往下分发，dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)事件，其中childe不为null，
所以dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign) == child.dispatchTouchEvent()，把Down事件分发到childView中，主要分两种情况。
1. 子View消费了事件
子View消费了事件，child.dispatchTouchEvent() == true（在这里假设子View的onTouchEvent()消费事件）
所以，dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)也返回true
调用addTouchTarget(child, idBitsToAssign)，把child保存在mFirstTouchTarget中，mFirstTouchTarget != null
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
// 运行super.dispatchTouchEvent()，也就是View.dispatchTouchEvent()
// handled返回dispatchTouchEvent()事件结果，如果未消费为false
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else { ... }
因为mFirstTouchTarget != null，就不调用dispatchTransformedTouchEvent()，也即不调用View.dispatchTouchEvent()
看一下View.dispatchTouchEvent()里面主要做了什么
主要先调用onTouch()方法，如果不返回true，在调用onTouchEvent()
ListenerInfo li = mListenerInfo;
// 首先执行onTouch()事件
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
// 然后执行onTouchEvent()事件
if (!result && onTouchEvent(event)) {
result = true;
}
所以GroupView的onTouch和onTouchEvent都不会执行了，也即到了子View的onTouchEvent事件就截止了，不会往上继续回调父View的onTouchEvent事件
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
// Down事件mFirstTouchTarget != null的话，alreadyDispatchedToNewTouchTarget = true
// Up事件mFirstTouchTarget != null的话，alreadyDispatchedToNewTouchTarget = false
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
// Up事件，直接运行上次处理Down事件的child.dispatchTouchEvent()
// 并且handled = true, 直接消费，不会继续往下分发
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
...
}
}
因为alreadyDispatchedToNewTouchTarget == true，进入if判断里面，设置handled = true，ViewGroup.dispatchTouchEvent()返回true。
activity的也会记录ViewGroup到mFirstTouchTarget里
2. 子View不消费事件
子View不消费事件，child.dispatchTouchEvent() == false
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
// 运行super.dispatchTouchEvent()，也就是View.dispatchTouchEvent()
// handled返回dispatchTouchEvent()事件结果，如果未消费为false
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else { ... }
因为前面调用了resetTouchState()函数，所以mFirstTouchTarget==null，所以调用dispatchTransformedTouchEvent(ev, canceled, null, xx)方法，而
因为child == null，调用super.dispatchTouchEvent(transformedEvent)，也就是View.dispatchTouchEvent()，就会执行GroupView的onTouch和onTouchEvent事件，往上继续回调父View的onTouchEvent事件
接着UP事件，从ViewGroup.dispatchTouch()往下分发
UP事件，不进入for循环遍历，所以不会再循环执行子View的dispatchTouchEvent()事件
1. DOWN事件，子View消费了事件
mFirstTouchTarget != null，进入else判断里面
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
// Down事件mFirstTouchTarget != null的话，alreadyDispatchedToNewTouchTarget = true
// Up事件mFirstTouchTarget != null的话，alreadyDispatchedToNewTouchTarget = false
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
// Up事件，直接运行上次处理Down事件的child.dispatchTouchEvent()
// 并且handled = true, 直接消费，不会继续往下分发
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
...
}
}
因为DOWN事件已经记录子View，消费了事件，并且记录在mFirstTouchTarget链表里，UP事件只会执行mFirstTouchTarget链表里的所有子View的事件分发，而不执行GroupView的onTouch和onTouchEvent事件。
所以这个时候UP事件，不管子View的onTouchEvent事件返不返回false，都不会往上继续回调父View的onTouchEvent事件
2. DOWN事件，子View不消费事件
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
...
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
UP事件，而且mFirstTouchTarget == null，所以直接拦截，肯定不会走for遍历循环。
mFirstTouchTarget == null， 调用dispatchTransformedTouchEvent(ev, canceled, null, xx)方法，而
因为child == null，调用super.dispatchTouchEvent(transformedEvent)，也就是View.dispatchTouchEvent()，就会执行GroupView的onTouch和onTouchEvent事件，也不会往上继续回调父View的onTouchEvent事件
View.dispatchTouchEvent()完整源码分析
public boolean dispatchTouchEvent(MotionEvent ev) {
...
boolean handled = false;
// Filter the touch event to apply security policies
// 这里过滤判断里面 是判断窗口是否被遮挡，如果被遮挡则终止处理返回false
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
// 触摸的动作：按下，抬起，滑动，多点按下，多点抬起
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);
// 初始化所以touch状态，主要 mFirstTouchTarget = null;
resetTouchState();
}
// Check for interception.
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
// 判断是否不允许拦截
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
// onInterceptTouchEvent获知是否拦截
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) {
...
// 一般只有MotionEvent.ACTION_DOWN的时候进入
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
...
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
...
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
...
newTouchTarget = getTouchTarget(child);
...
// 主要运行child.dispatchTouchEvent()方法，判断自己是否需要需要继续分发
// ViewGroud继续运行child.dispatchTouchEvent()方法，直到底部为View为止
// View.dispatchTouchEvent()，首先判断onTouch，再判断onTouchEvent
// 必须是child.dispatchTouchEvent() = true，不能是孙子View
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
...
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
// Adds a touch target for specified child to the beginning of the list.
// 把newTouchTarget放在链表头部，这里给mFirstTouchTarget赋值，不为空了
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.
// Down事件如果，dispatchTouchEvent返回true了，mFirstTouchTarget != null，就不调用dispatchTransformedTouchEvent()
// Up事件如果childView，dispatchTouchEvent返回True了，mFirstTouchTarget != null
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
// 运行super.dispatchTouchEvent()，也就是View.dispatchTouchEvent()
// handled返回dispatchTouchEvent()事件结果，如果未消费为false
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;
// Down事件mFirstTouchTarget != null的话，alreadyDispatchedToNewTouchTarget = true
// Up事件mFirstTouchTarget != null的话，alreadyDispatchedToNewTouchTarget = false
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
// Up事件，直接运行上次处理Down事件的child.dispatchTouchEvent()
// 并且handled = true, 直接消费，不会继续往下分发
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.
// Up事件重置Touch事件状态
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);
}
}
...
return handled;
}
其中的dispatchTransformedTouchEvent()事件主要用来执行dispatchTouchEvent()事件
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
...
// child == null，运行View.dispatchTouchEvent()
// child != null，运行child.dispatchTouchEvent()
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;
}
再来看下View下面的dispatchTouchEvent()事件
public boolean dispatchTouchEvent(MotionEvent event) {
...
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
// 首先执行onTouch()事件
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
// 然后执行onTouchEvent()事件
if (!result && onTouchEvent(event)) {
result = true;
}
}
...
return result;
}
最后看下View下的onTouchEvent()事件
public boolean onTouchEvent(MotionEvent event) {
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
...
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
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;
// 如果未被获焦，获焦
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);
}
// 如果不是长按事件，执行点击事件performClick()
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();
}
}
}
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
// 如果不可点击，判断是否是长按事件，直接返回
if (!clickable) {
checkForLongClick(0, x, y);
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// checkForLongClick()：postDelay(长按事件)
// 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;
case MotionEvent.ACTION_CANCEL:
if (clickable) {
setPressed(false);
}
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
break;
case MotionEvent.ACTION_MOVE:
if (clickable) {
drawableHotspotChanged(x, y);
}
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
// Remove any future long press/tap checks
removeTapCallback();
removeLongPressCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}
break;
}
return true;
}
return false;
}
结论
1. onTouch()和onTouchEvent()的区别
该2个方法都是在View.dispatchTouchEvent()中调用
但onTouch()优先于onTouchEvent()执行；若手动复写在onTouch()中返回true（即将事件消费掉），将不会再执行onTouchEvent()
2. onClick()和onLongClick()
该2个方法都是在View.onTouchEvent()中调用
和onLongClick()优先于onClick()执行，以为在Down事件里面就会发送postDealy(onLongClick())，Up如果不执行onLongClick()才执行onClick()事件
3. Touch事件的后续事件（MOVE、UP）层级传递
当dispatchTouchEvent()事件分发时，只有前一个事件（如ACTION_DOWN）返回true，才会收到后一个事件（ACTION_MOVE和ACTION_UP）
即如果在执行ACTION_DOWN时返回false，后面一系列的ACTION_MOVE、ACTION_UP事件都不会执行
ACTION_DOWN事件在哪个控件消费了（return true）， 那么ACTION_MOVE和ACTION_UP就会从上往下（通过dispatchTouchEvent）做事件分发往下传，就只会传到这个控件，不会继续往下传
如果ACTION_DOWN事件是在dispatchTouchEvent消费，那么事件到此为止停止传递，如果ACTION_DOWN事件是在onTouchEvent消费的，那么会把ACTION_MOVE或ACTION_UP事件传给该控件的onTouchEvent处理并结束传递。
可以看图示如下图示(图片来自图解 Android 事件分发机制)
在ViewGroup2的dispatchTouchEvent返回false，并且在ViewGroup1的onTouchEvent返回true消费这次事件
红色的箭头代表ACTION_DOWN 事件的流向
蓝色的箭头代表ACTION_MOVE 和 ACTION_UP 事件的流向
