事件分发机制
1、事件分发、拦截与消费
事件分发用到三个相关的方法，dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent。
对于java层，事件是从Activity的dispatchTouchEvent开始的，再往上属于C和C++，这里不做记录。
以下表格只是展示一下这三个方法所在的位置，方便后面讲解查看。
| 类型 | 相关方法 | Activity | ViewGroup | View |
|---|---|---|---|---|
| 事件分发 | dispatchTouchEvent | √ | √ | √ |
| 事件拦截 | onInterceptTouchEvent | × | √ | × |
| 事件消费 | onTouchEvent | √ | × | √ |
2、事件处理
事件处理相对比较简单，所以先说一下事件处理逻辑
在activity中为一个view设置点击事件和触摸事件
view.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.e(TAG,"onClick");
}
});
view.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
Log.e(TAG,"onTouch"+event.getAction());
return false;
}
});
当onTuoch返回false时，onClick方法会执行，返回为true时，onClick不执行。
这里只分析事件处理，所以回到View中的dispatchTouchEvent(),以下只贴出关键代码：
public boolean dispatchTouchEvent(MotionEvent event) {
...
boolean result = false;
...
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;
}
}
...
return result;
}
代码中可以看到共有四个判断条件：
- li != null
- li.mOnTouchListener!=null
- (mViewFlags & ENABLED_MASK) == ENABLED
- li.mOnTouchListener.onTouch(this, event)
li=mListenerInfo，在activity中view调用了setOnTouchEvent方法，代码如下
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}
ListenerInfo getListenerInfo() {
if (mListenerInfo != null) {
return mListenerInfo;
}
mListenerInfo = new ListenerInfo();
return mListenerInfo;
}
由上代码可以看出mListenerInfo是个单例，不为null,所以li!=null成立;
li.mOnTouchListener就是在activity中调用setTouchListener()设置的OnTouchListener对象,所以li.mOnTouchListener!=null成立;
(mViewFlags & ENABLED_MASK) == ENABLED,view可以按,所以也成立;
前三个判断条件都为true,所以接下来执行mOnTouchListener中的onTouch()方法,所以onTouch决定了result的返回值
if (!result && onTouchEvent(event)) {
result = true;
}
所以，当result为false(result默认为false)，即onTouch返回值为false时,执行onTouchEvent()方法，由最开始测试结果：当onTuoch返回false时，onClick方法会执行，返回为true时，onClick不执行，猜测onClick可能会在onTouchEvent()中执行，接下来看一下onTouchEvent()方法做了些什么，按照惯例还是贴出关键代码
public boolean onTouchEvent(MotionEvent event) {
...
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
...
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)) {
//1
performClickInternal();
}
}
}
...
mIgnoreNextUpEvent = false;
break;
...
}
return true;
}
return false;
}
private boolean performClickInternal() {
...
//2
return performClick();
}
public boolean performClick() {
// We still need to call this method to handle the cases where performClick() was called
// externally, instead of through performClickInternal()
notifyAutofillManagerOnClick();
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
//3
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
上面注释中，
- //1、onTouchEvent中当事件为ACTION_UP时，会调用performClickInternal()，
- //2、然后调用perfromClick()
- //3、如果在activity中调用了setOnClickListener，则li.mOnClickListener不为null，所以最终会执行mOnClickListener的onClick()方法
到这里事件处理执行完毕，应该明白了为什么当onTuoch返回false时，onClick方法会执行，返回为true时，onClick不执行。
总结：如果view同时设置了OnTouchListener和OnClickListener，当onTouch返回为false时，事件的执行顺序是：OnTouchListener.onTouch ---> View.onTouchEvent ---> View.performClickInternal() ---> View.performClick() --- > OnClickListener.onClick();
2、事件分发
事件分发总流程
- Activity#dispatchTouchEvent()
- PhoneWindow#superDispatchTouchEvent()
- DecorView#superDispatchTouchEvent()
- ViewGroup#dispatchTouchEvent()
- View#dispatchTouchEvent()
- View#onTouchEvent()
在Java层面，事件分发从Activity的dispatchTouchEvent开始，再之前数据C、C++的范畴，不做深究。
首先看下Activity中的dispatchTouchEvent做了些什么：
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;
}
Window是一个抽象类，从Window中的注释可以看到Window只有一个实现类为PhoneWindow,
/ * <p>The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/
从Activity的attach方法也能看出，mWindow就是PhoneDow的实例：
final void attach(Context context, ActivityThread aThread,
...
mWindow = new PhoneWindow(this, window, activityConfigCallback);
...
}
所以，Activity的dispatchTouchEvent其实是调用得PhoneWindow的superDispatchTouchEvent:
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
PhoneWindow的构造函数中可以知道mDecor为DecorView的实例：
public PhoneWindow(Context context, Window preservedWindow,
ActivityConfigCallback activityConfigCallback) {
...
if (preservedWindow != null) {
mDecor = (DecorView) preservedWindow.getDecorView();
...
}
所以，调用的是DecorView的superDispatchTouchEvent方法：
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
DecorView是ViewGroup的子类，所以继续调用ViewGroup的dispatchTouchEvent,这里完成了从Activity到ViewGroup的事件分发，ViewGroup的dispatchTouchEvent方法比较复杂，分段来看：
public boolean dispatchTouchEvent(MotionEvent ev) {
...
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;
}
...
}
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
private void clearTouchTargets() {
TouchTarget target = mFirstTouchTarget;
if (target != null) {
do {
TouchTarget next = target.next;
target.recycle();
target = next;
} while (target != null);
mFirstTouchTarget = null;
}
}
- 首先判断事件如果为ACTION_DOWN，则调用resetTouchState，resetTouchState中又调用clearTouchTargets,里面对mFistouchTarget置空。
- 事件由ACTION_DOWN开始，ACTION_UP结束，上一步中明确了mFirstToucharget = null，所以只有当事件开始即ACTION_DOWN时，可能会执行方法onInterceptTouchEvent，ACTION_MOVE和ACTION_UP都会走默认执行拦截了的方法。
- 标记位FLAG_DISALLOW_INTERCEPT，它一般是由子View的requestDisallowInterceptTouchEvent方法设置的，表示ViewGroup无法拦截除了ACTION_DOWN以外的其他动作，我们看到源码第一个判断就会明白，只要是ACTION_DOWN动作，这个标记位都会被重置，并且ViewGroup会调用自己onInterceptTouchEvent方法表达是否需要拦截这新一轮的点击事件。
接下来看后续代码：
public boolean dispatchTouchEvent(MotionEvent ev) {
...
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 (!child.canReceivePointerEvents()
|| !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;
}
}
}
...
}
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
...
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
...
handled = child.dispatchTouchEvent(transformedEvent);
}
}
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
-
isTransformedTouchPointInView点击位置是否包含在子View，如果不在范围内直接continue，在范围内则调用dispatchTransformedTouchEvent，里面调用了child.dispatchTouchEvent，即View的disPatchTouchEvent，这里完成了ViewGroup到View的事件分发。
-
如果子View消费了事件即dispatchTransformedTouchEvent返回true,则调用addTouchTarget方法，方法中对mFirstTouchTarget做了赋值，即mFirstTouchTarget = target。所以当ACTION_MOVE和ACTION_UP时，mFirstTouchTarget对事件拦截起到了关键性作用。
-
如果子View没有消费事件即onTouch方法返回false，则mFirstTouchTarget不经过赋值依然为null，
当mFirstTouchTarget == null成立时，执行：if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); } private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { ... if (child == null) { handled = super.dispatchTouchEvent(transformedEvent); } else { ... handled = child.dispatchTouchEvent(transformedEvent); }}
可见child为null，调用super.dispatchTouchEvent，ViewGroup继承View，所以调用View的dispatchTouchEvent，完成了ViewGroup到View的事件分发。
handled即为View的dispatchTouchEvent的返回结果。
返回Activity中的dispatchTouchEvent，如果handled为false，继续执行Activity中的onTouchEvent方法。
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