一、什么是事件分发
1、事件
用户通过屏幕与手机交互的时候，每一次点击、长按、移动等都是一个事件。
2、事件分发机制
某一个事件从屏幕传递到各个View，由View来使用这一事件（消费事件）或者忽略这一事件（不消费事件），对这整个过程的控制就是事件分发机制。
二、事件分发的对象是谁
系统把事件封装为MotionEvent对象，事件分发的过程就是MotionEvent分发的过程。
下面对MotionEvent进行简单介绍
1、事件的类型
- 按下（ACTION_DOWN）
- 移动（ACTION_MOVE）
- 抬起（ACTION_UP）
- 取消（ACTION_CANCEL）
2、事件序列
从手指按下屏幕开始，到手指离开屏幕所产生的一系列事件。
三、传递层级
Activity -> Window -> DecorView -> ViewGroup -> View
1、涉及到的主要对象及顺序
- Activity
- ViewGroup
- View
四、Activity中的事件分发流程
1、流程图
Activity中的事件分发流程图
2、源码分析：
Activity.java
public boolean dispatchTouchEvent(MotionEvent ev) {
//第一步：判断并调用onUserInteraction()。
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
//第二步：事件派发，返回值表示是否被消费
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
//第三步：没被消费，Activity中的onTouchEvent(ev)方法，进行处理。
return onTouchEvent(ev);
}
public void onUserInteraction() {
}
/**
* 当事件派发后，还没有被消费掉的话，就调用Activity#onTouchEvent方法进行处理。
**/
public boolean onTouchEvent(MotionEvent event) {
//系统默认的处理是：
//如果shouldCloseOnTouch返回true，就finish掉当前的Activity，并返回true，表示事件消费掉了
//不然，就返回false，往下走
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}
//如果shouldCloseOnTouch返回false，返回false
return false;
}
Window.java
private boolean mCloseOnTouchOutside = false;
private boolean mSetCloseOnTouchOutside = false;
public void setCloseOnTouchOutside(boolean close) {
mCloseOnTouchOutside = close;
mSetCloseOnTouchOutside = true;
}
public void setCloseOnTouchOutsideIfNotSet(boolean close) {
if (!mSetCloseOnTouchOutside) {
mCloseOnTouchOutside = close;
mSetCloseOnTouchOutside = true;
}
}
public boolean shouldCloseOnTouch(Context context, MotionEvent event) {
final boolean isOutside =
event.getAction() == MotionEvent.ACTION_DOWN && isOutOfBounds(context, event)
|| event.getAction() == MotionEvent.ACTION_OUTSIDE;
if (mCloseOnTouchOutside && peekDecorView() != null && isOutside) {
return true;
}
return false;
}
在这做一个标注：Activity中的getWindow、onUserInteraction、onTouchEvent，以及setCloseOnTouchOutside方法都是public的。简单来说，这意味着在事件分发的过程中，我们是可以参与、写入自己的逻辑的。
3、事件派发过程分析（从Activity到DecorView）：
在Activity#dispatchTouchEvent中的第二步，调用了下面的一行代码，对事件进行了分发：
getWindow().superDispatchTouchEvent(ev)
整个调用流程如下：
Activity.java
getWindow().superDispatchTouchEvent(ev)
Window.java
public abstract boolean superDispatchTouchEvent(MotionEvent event);
PhoneWindow.java (它是Window的唯一实现类)
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
到这，可以看到事件已经被交给DecorView了。
DecorView.java
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
可以看到，DecorView是调用父类的dispatchTouchEvent方法，对事件进行分发。
那它的父类是谁呢？
DecorView extends FrameLayout
FrameLayout extends ViewGroup
最终调用的是ViewGroup#dispatchTouchEvent
到此，这一步的分发完成了：
事件从Activity传到Window，再从Window传到DecorView中。在DecorView中调用了父类ViewGroup#dispatchTouchEvent方法进行下一步的分发。
四、ViewGroup中的事件分发流程
首先看一下ViewGroup事件分发中涉及到的三个主要方法：
- public boolean dispatchTouchEvent(MotionEvent ev) {...}
事件进入ViewGroup中。 - public boolean onInterceptTouchEvent(MotionEvent ev) {...}
事件分发过程中调用：
如果返回true，表示当前的ViewGroup会拦截掉这个事件，也就是说不会再往下进行传递了；
如果返回false，表示当前的ViewGroup不会拦截掉这个事件。 - public boolean onTouchEvent(MotionEvent event) {...}
这个方法在ViewGroup的父类View中。
1、流程图
ViewGroup事件分发流程图
2、ViewGroup#dispatchTouchEvent方法做了什么
1）去判断是否需要拦截事件
2）在当前ViewGroup中找到用户真正点击的View
3）分发事件到View上
3、源码分析
public boolean dispatchTouchEvent(MotionEvent ev) {
//与事件分发无关，无需关注，debug用的
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
//辅助功能选项，暂无需关注
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
//事件分发逻辑开始
//handled作为此方法的返回值，表示事件是否被消费
boolean handled = false;
//第一步：判断是否符合安全策略
//安全策略：如果当前的View被其他视图遮挡了，并且当前的View被设置了——不在顶部时不响应事件，那么该方法就会返回false，也就不会向其分发事件了。这个方法的源码在后面分析
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
//处理最初的DOWN事件，这个处理并不会影响事件的分发，本次分析不作为重点
if (actionMasked == MotionEvent.ACTION_DOWN) {
//取消并且清除一个触摸事件的目标链表
cancelAndClearTouchTargets(ev);
//重置触摸状态
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 {
intercepted = true;
}
//暂无需关注
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// Check for cancelation.
//true：表示当前事件为取消事件
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
//如果为true，表示当前的事件分发给多个子视图，因为多个子视图可能重叠在一起
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
//如果不是取消事件，并且当前不去拦截事件，那么进入到事件分发的逻辑中
if (!canceled && !intercepted) {
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<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;
}
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;
}
}
}
// 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;
}
安全策略源码：
public boolean onFilterTouchEventForSecurity(MotionEvent event) {
//如果当前View设置了：被遮挡时需要过滤此触摸事件。并且当前触摸事件处于被遮挡状态。
if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0
&& (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {
// Window is obscured, drop this touch.
//过滤掉这个触摸事件，也就是不分发了
return false;
}
return true;
}
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