参考资料
前言
- 事件分发流程是我们解决页面复杂交互的基础，只要我们做的交互稍微复杂一些，就有可能用到事件分发和拦截，如果不能掌握，那就很难做好相关工作
- 老张我做面试官也有些年头了，事件分发可以说是必问的知识点，但是很少有候选人能全面掌握，总是欠缺点什么
- 关于事件分发的博客网上有很多，有一些重点在源码解析而缺少过程，有些重点在分发流程的分析而缺少代码分析，很多人看过后仍云山雾罩，不知其所以，本文期望通过源码和流程的分析，能够将事件分发讲明白一些
场景：
假设有一个Activity,它布局中有一个FrameLayout，FrameLayout有三个子FrameLayout,三个子FrameLayout又分别有三个子View，其结构如图所示：
image.png
我们用蓝色小圆代表dispatchTouchEvent的执行，红色小圆代表onInterceptTouchEvent的执行，黄色小圆代表onTouchEvent的执行。
事件分发流程
DOWN事件没有消费者的情况
DOWN事件的传递
DOWN时间可以说是事件分发过程中最重要的事件之一，它决定了这个事件是否被消费、事件传递链条的建立等等
首先假设上面的场景中没有任何View拦截事件，也没有任何View消费事件
下面看一下DOWN事件的分发过程：
我们从事件分发的起点Activity#dispatchTouchEvent开始说起：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
代码很简单，直接执行了Window#superDispatchTouchEvent，注意，这里的Window是且只能值PhoneWindow，跟下去：
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
这里执行了DecoreView#superDispatchTouchEvent:
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
decoreView是一个ViewGroup，这里调用的是ViewGroup#dispatchTouchEvent，这里是dispathTouchEvent过程的一个重头戏，我们不在这里分析，只需要知道它在几次执行后会调用到图示中v1的dispatchTouchEvent，具体为什么，看完本篇文章自然会明白。到这里场景中的事件的阶段如下图：
image.png
v1的dispathTouchEvent方法的实现在ViewGroup:
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;
// 不是Cancel事件 且 事件没有被拦截
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) {
// 获取Action的index
final int actionIndex = ev.getActionIndex(); // always 0 for down
// 获取Action的pointId
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;
}
// 该子View不能接收事件，或者事件触发点不在子View的坐标范围内
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
// 获取child的TouchTarget
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.
// 更新pointerIdBits
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
// 分发变换过的TouchEvent
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TouchTarget
newTouchTarget = addTouchTarget(child, idBitsToAssign);
// 已经将事件分发给新的touchTarget的标记
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;
// 如果在事件在本View这里被拦截了， 生成Cancel事件并向子View传递
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;
}
分段解析：
第18行起的这段if,表示如果DOWN事件传递进来，会将之前的保留的传递链条重置（如果有的话），至于传递链条是什么，后面会讲到
第28起的这段if,因为我们目前是DOWN，因此一定是符合actionMasked == MotionEvent.ACTION_DOWN这个条件的，会进入分支内部，在内部，如果本ViewGroup没有被禁止拦截事件，会执行本ViewGroup的onInterceptTouchEvent，ViewGroup的onInterceptTouchEvent方法比较简单，主要作用还是给子类继承来实现各种炫酷的效果,这里不多做分析，看下代码：
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;
}
到这里，场景中的事件分发阶段如下图：
image.png
接着分析ViewGroup#dispatchTouchEvent，因为我们假设onInterceptTouchEvent没有拦截事件，因此其返回值intercepted是false，接着看dispatchTouchEvent方法的59行，因为目前是DOWN事件且intercepted为false，代买进入该分支执行, actionIndex和idBitsToAssign主要是处理多点触摸，这里我们无需过多关注
看代码的第92行，这里从后向前遍历该VP的子View，我们看看这个分支处理的什么：
第110行，这句话判定该子View是否可以接收事件，且点击事件是否落该子View的范围内，如果两者都满足贼继续执行逻辑，否则继续遍历下一个子View
第117行，因为我们第一次进入，getTouchTarget会返回null,第130行，执行了dispatchTransformedTouchEvent：
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();
// 如果是Cancel事件，则传递cancel
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 {
// 修正点击坐标
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
// 执行子View的dispatchTouchEvent
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;
}
在dispatchTransformedTouchEvent的第45行，执行了子View的dispatchTouchEvent，因为我们是从后向前遍历的子View,因此这个子View一定是场景中的V4.到目前为止，事件分发到了下图的阶段：
image.png
V4也是一个ViewGroup,它的分发会重复上面的过程，在经历上面的阶段之后，调用到了V13的dispatchTouchEvent：
image.png
V13是一个View,它的dispatchTouchEvent如下：
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);
}
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
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;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
View#dispatchTouchEvent的第30行，是View的onTouchListener的执行，从这部分逻辑可以看出，如果onTouchListener#onTouch返回了true,则后面的onTouchEvent就不再执行了，如果不是，则会执行View#onTouchEvent
View#onTouchEvent的具体实现和本文关系不大，就不贴代码了，它的默认返回值是false.值得注意的是，View#setOnClickListener的回调是在onTouchEvent的UP事件中触发的
面试的时候经常有面试官会问onTouch onTouchEvent onClick的关系，读到这里我想大家就了然了
至此，我们场景中的事件分发阶段走到了如下所示：
image.png
言归正传接着看View#dispatchTouchEvent:
View#onTouchEvent的返回值false决定了View#dispatchTouchEvent的返回值也为false，View#dispatchTouchEvent的返回值false决定了ViewGroup#dispatchTransformedTouchEvent的返回值为false，
我们的调用流程回到了v4的子View遍历中，也就是ViewGroup#dispatchTouchEvent第92行开始的那部分逻辑继续执行循环，因为ViewGroup#dispatchTransformedTouchEvent为false，因此129后面那部分是不会执行的，继续循环会执行v12的dispatchTouchEvent和onTouchEvent以及v13的dispatchTouchEvent和onTouchEvent
这时场景中的事件分发阶段如下图：
image.png
此时V4的ViewGroup#dispatchTouchEvent中的子View遍历的逻辑执行完毕，逻辑来到了172行，这个mFirstTouchTarget的赋值是在129的if为true的情况，因为129行的判断为false，因此mFirstTouchTarget没有被赋值，这里会进入if分支执行174行执行V4的ViewGroup#dispatchTransformedTouchEvent，该参数child传值为null,在ViewGroup#dispatchTransformedTouchEvent中会调用V4的super.dispatchTouchEvent,V4的super就是View,因此会调用View#dispatchTouchEvent和View#onTouchEvent
此时，场景中的事件分发阶段如下图：
image.png
看到这里我想大家应该都看明白了，这不就是变相的多叉树的后序遍历么！！！！ 后面的流程就不一一分析了，我们用数字来标记各个方法的执行顺序：
image.png
这时候有朋友会说，光凭一张嘴我怎么相信你说的就是对的呢，万一你在信口开河，那我不上了鬼子的大当了么？俗话说 talking is cheap show me your code , 没图你说个** 我们就写个demo验证一下：
public class MotionEventActivity extends AppCompatActivity {
private RelativeLayout contentLayout;
private boolean onTouchEvent = false;
private boolean intercept = false;
private String viewName = "MotionEventActivity";
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_motion_event);
contentLayout = findViewById(R.id.contentLayout);
addView();
}
private void addView() {
MotionEventLayout view1 = new MotionEventLayout(this);
view1.setViewName("view1");
contentLayout.addView(view1);
view1.setLayoutParams(new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
MotionEventLayout view2 = new MotionEventLayout(this);
view2.setViewName("view2");
MotionEventLayout view3 = new MotionEventLayout(this);
view3.setViewName("view3");
MotionEventLayout view4 = new MotionEventLayout(this);
view4.setViewName("view4");
view1.addView(view2);
view2.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view1.addView(view3);
view3.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view1.addView(view4);
view4.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
MotionEventView view5 = new MotionEventView(this);
view5.setViewName("view5");
MotionEventView view6 = new MotionEventView(this);
view6.setViewName("view6");
MotionEventView view7 = new MotionEventView(this);
view7.setViewName("view7");
view2.addView(view5);
view5.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view2.addView(view6);
view6.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view2.addView(view7);
view7.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
MotionEventView view8 = new MotionEventView(this);
view8.setViewName("view8");
MotionEventView view9 = new MotionEventView(this);
view9.setViewName("view9");
MotionEventView view10 = new MotionEventView(this);
view10.setViewName("view10");
view3.addView(view8);
view8.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view3.addView(view9);
view9.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view3.addView(view10);
view10.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
MotionEventView view11 = new MotionEventView(this);
view11.setViewName("view11");
MotionEventView view12 = new MotionEventView(this);
view12.setViewName("view12");
MotionEventView view13 = new MotionEventView(this);
view13.setViewName("view13");
view4.addView(view11);
view11.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view4.addView(view12);
view12.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
view4.addView(view13);
view13.setLayoutParams(new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
if (EventSwitch.LOG_DOWN_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent ACTION_DOWN");
}
break;
case MotionEvent.ACTION_MOVE:
if (EventSwitch.LOG_MOVE_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent ACTION_MOVE");
}
break;
case MotionEvent.ACTION_UP:
if (EventSwitch.LOG_UP_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent ACTION_UP");
}
break;
case MotionEvent.ACTION_CANCEL:
if (EventSwitch.LOG_CANCEL_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent ACTION_CANCEL");
}
break;
default:
break;
}
boolean dispatch = super.dispatchTouchEvent(ev);
if (EventSwitch.LOG_RESULT) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
if (EventSwitch.LOG_DOWN_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent = " + dispatch);
}
break;
case MotionEvent.ACTION_MOVE:
if (EventSwitch.LOG_MOVE_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent = " + dispatch);
}
break;
case MotionEvent.ACTION_UP:
if (EventSwitch.LOG_UP_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent = " + dispatch);
}
break;
case MotionEvent.ACTION_CANCEL:
if (EventSwitch.LOG_CANCEL_EVENT) {
Log.d("zyl", viewName + " dispatchTouchEvent = " + dispatch);
}
break;
default:
break;
}
}
return dispatch;
}
运行结果：
image.png
和我们分析的过程一模一样，毫无PS痕迹 完美
没有View消费事件时MOVE和UP事件的传递
MOVE和UP事件传递给Activity的第一层ViewGroup,触发ViewGroup#dispatchTouchEvent,这里不是DOWN事件，mFirstTouchTarget也为空，会触发ViewGroup#dispatchTouchEvent第174行逻辑
dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS)
我们通过前面的分析知道如果child传值为null，会触发自身的onTouchEvent，因为onTouchEvent返回值为false，dispatchTransformedTouchEvent肯定也为false,因此Activity#dispatchTouchEvent也为false,继而调用Activity#onTouchEvent
许多博客会说如果没有View去处理事件，该事件最终会被Activity的onTouchEvent消费，这是不准确的，其实事件是尝试向下传递后发现传不下去才交还给Activity#onTouchEvent执行
DOWN事件传递有View消费事件的情况：
DOWN事件的传递
现在我们将V11的onTouchEvent返回值设置为true 现在看下DOWN事件的传递过程
在v1 -> v4 -> v13 -> v12 阶段的传递过程跟前面并无二致，直接跳过
上面我们讲过 View#onTouchEvent View#dispatchTouchEvent ViewGroup#dispatchTransformedTouchEvent的返回值是相关的，所以在DOWN事件传递到V11节点时，因为它的onTouchEvent返回了true， 在ViewGroup#dispatchTouchEvent方法的第
129行处dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign也为true,这里会进入if分支执行， 在ViewGroup#dispatchTouchEvent方法的第146行，这里生产出一个新的TouchTarget:
newTouchTarget = addTouchTarget(child, idBitsToAssign)
看下addTouchTarget的实现：
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
在设置完TouchTarget和alreadyDispatchedToNewTouchTarget后，ViewGroup#dispatchTouchEvent跳出了遍历子View的循环，在183行，因为两个条件都满足，handled 会被置为true，继续执行其他一些逻辑后，ViewGroup#dispatchTouchEvent返回了handled 即true
此时V4的mFirstTouchTarget.child指向了V11
如图：
image.png
因为V4的dispatchTouchEvent是在V1的dispatchTransfromedTouchEvent中执行，其返回值也是强相关，因此，在V1节点重复了上述过程后， V1生成了mFirstTouchTarget并且将mFirstTouchTarget.child指向了V4
这里是我们编程思想中回溯算法的完美提现，在经过不断回溯后，最终形成了一条 DecoreView -> ContentView -> V1 -> V4 -> V11的链条
image.png
MOVE 和 UP 的传递
因为不是DOWN且mFirstTouchTarget不为空，ViewGroup#dispatchTouchEvent会执行到第186行的逻辑处，因为我们没有做任何拦截，因此会执行第189行的dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)接下来的一切又非常熟悉了，这里不做赘述
因此 MOVE和UP的传递链条就是
v1 -> v4 -> v11
验证一下,将V11的onTouchEvent返回值设为ture,打印下结果：
down事件.png
UP事件.png
OK 和我们的分析一模一样 童叟无欺
有View消费事件和没有View消费事件时分发策略的区别
- 当有View消费事件时，会在DOWN事件被消费后形成一条传递链，MOVE和UP事件会沿着传递链传递
- 当有View消费事件时，DOWN事件被消费后传递链条中止，不会再回溯触发父布局的事件传递
- 当没有View消费事件时，DOWN事件会对View树进行深度遍历，MOVE和UP则是尝试向ViewGroup进行事件分发失败后转给Activity的onTouchEvent处理
当上层ViewGroup拦截事件时事件分发的处理
现在我们设想一个场景:V11处理onTouchEvent 但是当滑动的Y值累计大于500时，v1将该事件拦截、
那么这种场景中事件处理是如何分发的呢？
首先看DOWN事件，其实DOWN事件在前面的场景中已经分析过了，这里就不说了
这个场景的重头戏是MOVE事件，当我们在V1中判断累计MOVE的Y值大于500时，v1的onInterceptTouchEvent会返回true,而v1的onInterceptTouchEvent方法调用是在v1的dispatchTouchEvent中：
看下ViewGroup#dispatchTouchEvent的第33行附近，这里的执行有一个前提条件，就是当前时间为DOWN事件或者mFirstTouchTarget != null
原因也比较好理解，DOWN事件是事件传递的起点，如果mFirstTouchTarget == null 则说明该ViewGroup没有后代消费这个事件，所谓拦截也就无从谈起了
当满足要求的MOVE事件传来时，intercepted会接收到true的返回值，接着执行逻辑，在一系列操作后会执行到ViewGroup#dispatchTouchEvent方法的第186行，这cancel是一个true值
当当我们将cancel的值置为ture来调用dispatchTransformedTouchEvent时，会执行ViewGroup#dispatchTransformedTouchEvent方法的第9行区块，
也就是递归执行子类的ViewGroup#dispatchTouchEvent，因为传入event事件类型是Cancel,ViewGroup#dispatchTouchEvent逻辑又会执行到186行区域，最终会调用View的onTouchEvent:
View#onTouchEvent
case MotionEvent.ACTION_CANCEL:
if (clickable) {
setPressed(false);
}
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
break;
这里是一些重置和清理工作
而在执行过dispatchTransformedTouchEvent后，会将mFirstTouchTarget进行重置：
while (target != null) {
final TouchTarget next = target.next;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
// 如果在事件在本View这里被拦截了， 生成Cancel事件并向子View传递
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;
}
因为是递归调用，因此重置也是自下而上的
回到上面的案例中，如果拦截事件触发，Cancel事件的传递顺序应该是
v4.dispatchTouchEvent -> v4.onInterceptTouchEvent -> v11.dispatchTouchEvent -> v11.onTouchEvent
而在Cancel事件传递后，v4及其子View v11便不会再接收事件了
看下demo的运行结果：
image.png
在Cacnel事件发生之前，V4和V11一直在接收事件，但是Cancel后，便只剩下V1了，符合我们的得出的结论
以上就是事件分发流程源码的分析，听懂掌声
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