View、ViewGroup的事件分发机制
1、事件概念
当发生点击事件时，大致的调用顺序是先调用最外层View的dispatchTouchEvent方法，然后调用onInterceptTouchEvent方法，再调用onTouchEvent方法；
分发、拦截、消费，一个事件的所经历的就是这些处理的组合；
Activity和View没有onInterceptTouchEvent事件；
1.1、分发
表示事件是否会继续分发出去，默认返回false，返回true时表示事件不会再继续分发，甚至都不会分发到自身的onTouchEvent方法；
1.2、拦截
对事件传递做拦截，表示当前层级的View需要处理这个事件，将事件拦截下来，直接派给自己的onTouchEvent方法去处理，如果onTouchEvent方法处理了并且返回true，这个事件就结束了，如果onTouchEvent方法返回false，该事件还会继续传递到上层ViewGroup的onTouchEvent方发去处理；
1.3、消费
消费事件，如果该方法返回了true，表示对事件消费掉了，事件终止，如果返回false，事件会继续传递给上层ViewGroup方法处理；
2、关键方法
2.1、dispatchTouchEvent
分发事件，当该方法返回true时，该View不会继续分发事件，包括该事件不会继续分发到该View的onInterceptTouchEvent方法和onTouchEvent方法；
2.2、onInterceptTouchEvent
拦截事件的传递，是否会继续向子View、子ViewGroup传递，当该方法返回true时，事件不会继续向子View、子ViewGroup传递，相当于父级View把事件在此处截断了；
2.3、onTouchEvent
消费事件，对点击事件做相应的点击响应处理，具体执行点击后的操作，如果子View不做处理，即返回false，该事件还会继续传递到父View的onTouchEvent方法去处理，直到传递到组外层；
如果该方法返回true，表示这个事件被消费掉了，这个事件就此终止了，不会再有任何传递；
对于Activity的onTouchEvent事件，如果内部的控件没有对事件做任何处理，事件最终都会回到Activity的onTouchEvent方法去处理；
3、调用流程图
4、源码
4.1、dispatchTouchEvent
4.1.1、Activity
// Activity
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
public Window getWindow() {
return mWindow;
}
// attach方法中初始化mWindow对象
final void attach(Context context, ActivityThread aThread, ...) {
...
mWindow = new PhoneWindow(this, window, activityConfigCallback);
...
}
// PhoneWindow
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
// DecorView
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
Activity中的dispatchTouchEvent()方法中首先判断点击事件是否为按下，并调用onUserInteraction()方法，该方法为一个空实现；
其次会执行window的superDispatchTouchEvent()方法，在Activity的attach()方法中mWindow是new的一个PhoneWindow对象，在PhoneWindow中又是调用DecorView的superDispatchTouchEvent()方法，DecorView是一个继承自FrameLayout的类，所以他调用的父类的dispatchTouchEvent()方法将追溯到ViewGroup的dispatchTouchEvent()方法中，当子View和ViewGroup都不处理事件，即dispatchTouchEvent()方法都返回false，则会去调用Activity的onTouchEvent()方法处理事件；
4.1.2、ViewGroup
@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);
}
...
}
首先判断事件的焦点处理，输入事件响应，如果有输入焦点在，则交给输入去执行OnTouchEvent()事件；
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();
}
...
}
其次判断事件是否符合应用安全策略，将不符合的事件过滤掉；
当事件是ACTION_DOWN时，调用cancelAndClearTouchTargets()方法清除掉所有关联的目标，即按下事件被认为是一整个事件的开始，并且通过调用resetTouchState()方法重置触摸事件的状态为一个新的周期；
这里也会重置FLAG_DISALLOW_INTERCEPT这个标记位，这个标记位的作用是决定ViewGroup是否拦截除ACTION_DOWN之外的事件，无法拦截ACTION_DOWN事件，因为在ACTION_DOWN时会重置这个标记位；
// 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);
}
接着判断是否需要拦截，当事件不是ACTION_DOWN并且mFirstTouchTarget为空时，直接返回为true，表示该ViewGroup拦截下来了该事件，说明如果拦截，ViewGroup只能拦截到ACTION_DOWN事件，mFirstTouchTarget为空表示该ViewGroup没有子View去处理事件，所以自己拦截下来；
当ACTION_DWON或者mFirstTouchTarget不为空时，判断mGroupFlags & FLAG_DISALLOW_INTERCEPT，mGroupFlags是View的标记，FLAG_DISALLOW_INTERCEPT如果该变量设置了，说明该ViewGroup不该拦截事件，返回false，否则就会去调用ViewGroup的onInterceptTouchEvent()方法，ev.setAction(action)重置事件的action，在拦截方法中如果action有发生改变；
接着如果该事件被拦截，并且mFirstTouchTarget不为空（即事件被子View消费），将该事件的可访问性焦点标记为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;
判断事件是否被取消，例如划出屏幕之类的；
if (!canceled && !intercepted) {
// If the event is targeting accessibility 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 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;
}
}
}
接下来会去遍历子View，将事件分发到子View处理，判断子View是有可访问性焦点并且焦点在子View本身、通过调用canViewReceivePointerEvents()方法和isTransformedTouchPointInView()方法判断View是否可以接受事件（满足两个条件：事件发生在View的区域，子View没有正在播放的动画）、调用dispatchTransformedTouchEvent()方法筛选出可以接受事件的子View，结束循环，事件成功交给子View；
在调用dispatchTransformedTouchEvent()方法是，第三个参数不为空，即子View不为空时，调用子View的dispatchTouchEvent()方法去处理；
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
调用addTouchEvent()方法，为mFirstTouchTarget赋值；
// 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;
}
}
如果没有找到合适的可以接受事件的子View，则mFirstTouchTarget没有赋值，则ViewGroup自己调用dispatchTransformedTouchEvent()方法处理，第三个参数View child为null，查看该方法，当child为null时，调用父类的dispatchTouchEvent()方法，即调用View的dispatchTouchEvent()方法；
4.1.3、View
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}
...
}
View的dispatchTouchEvent()方法，会先判断该事件是否为带有焦点处理的，例如EditText的输入响应、Button的点击响应等，如果没有需要处理的则直接返回false，如果有则设置相关标记，继续进行正常的事件派发；
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
检查是否有输入性的事件，如果有，表示该View属于一个输入类型的View，则去调用该输入序列的onTouchEvent()方法去处理输入事件；
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
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;
}
}
public boolean onFilterTouchEventForSecurity(MotionEvent event) {
//noinspection RedundantIfStatement
if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0 && (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {
// Window is obscured, drop this touch.
return false;
}
return true;
}
接着调用onFilterTouchEventForSecurity()方法过滤不符合应用安全策略的事件，如果可以继续被分发则返回true，否则返回false；
接着判断View的各种状态，以及是否被滚动条事件消费掉等，handleScrollBarDragging()方法处理拖动滚动条事件，当该方法处理完之后返回true表示消费掉该事件；
接着判断mOnTouchListener是否为空，不为空表示用户自定义了OnTouchEvent事件（通过调用setOnTouchListener()方法），则会调用OnTouchListener的onTouchEvent()方法去处理事情并返回，所以设置OnTouchListener的优先级要高于本身的OnTouchEvent事件；
最后判断本身OnTouchEvent()方法处理返回；
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
如果最终事件没有处理并且是输入控件，则将该输入序列挂起为未处理，并重置该事件需要被忽略；
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
最终手势事件结束并且处理结果为false时，调用stopNestedScroll()方法，停止正在进行的嵌套滚动；
4.2、onInterceptTouchEvent
4.2.1、ViewGroup
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;
}
该方法判断是否需要对事件进行拦截下来处理，通过判断事件的来源、是否为ACTION_DOWN事件、是否是Button按钮按下以及滚动条事件等，来决定是否需要将该事件拦截下来；
4.3、onTouchEvent
4.3.1、Activity
public boolean onTouchEvent(MotionEvent event) {
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}
return false;
}
当屏幕触摸事件没有被任何子View消费时，最终会调用该方法来处理，该方法默认实现是返回为false；
4.3.2、View
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return clickable;
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
...
}
对View的各种标识进行判断，该View为可点击、长按的状态，clickable为true，当View为禁用点击时DISABLED，也会处理该事件，只是没有具体的响应，将状态设置为未点击，并刷新Drawable状态；
如果设置了触摸代理，即mTouchDelegate不为空，则交给mTouchDelegate对象去处理OnTouchEvent()事件并返回true，表示事件被消费，不再继续传递；
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
...
}
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
...
if (prepressed) {
...
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) {
...
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClickInternal();
}
}
}
...
if (prepressed) {
postDelayed(mUnsetPressedState, ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
如果该View有长按提示效果，则先处理掉该长按提示事件；
如果当前View是不可点击的，则清除掉View的各种相关状态；
PFLAG_PREPRESSED状态为嵌套滚动View，如果View被嵌套在一个滚动的View中，会检查其父View是否拦截事件，如果父View没有拦截则开始处理，调用setPressed()方法设置按下状态让用户明确感受到；
判断mHasPerformedLongPress和mIgnoreNextUpEvent是否为处理过长按事件，如果不是长按，调用removeLongPressCallback()方法移除掉长按事件的回调，接着开启一个子线程（new PerformClick()）去执行click事件，如果异步执行失败，则直接同步执行，接着调用ViewConfiguration.getPressedStateDuration()获得按下效果的显示时间设置View的显示状态（API 28 显示时间为64ms），同样post失败之后直接调用，移除掉Tap的回调，重置View状态；
performClickInternal()方法中调用performClick()方法，在这里会判断onClickListener是否为空，不为空则执行onClickListener的onClick()方法，即用户设置的onClickListener事件，优先级最低；
case MotionEvent.ACTION_DOWN:
if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
}
mHasPerformedLongPress = false;
if (!clickable) {
checkForLongClick(0, x, y);
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;
按下事件，如果View不可点击，检查长按事件，如果是则直接跳出，接着会检查是否在滚动容器中，如果是设置mPrivateFlags状态，接着new CheckForTap去检查是Tap还是长按事件，如果在滚动View中嵌套，则通过ViewConfiguration.getTapTimeout()去获取延迟返回时间，设置在规定时间内，如果用户没有移动则说明是点击事件（API 28中为100ms）；
case MotionEvent.ACTION_CANCEL:
if (clickable) {
setPressed(false);
}
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
break;
取消手势，重置View所有状态；
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;
移动手势，通过调用drawableHotspotChanged()方法，实时更新传递移动的坐标位置；
接着判断当前移动的手势是否在View中，如果移除手指，则移除掉对应的回调；
本文参考：
《Android开发艺术探索》
2019移动开发者峰会-陈冰-Android移动端手势交互原理