深入理解Android事件分发机制之源码分析

首先从Activity开始分发（一直看着这个ev，看看会往哪里传）

public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}

Activity传给Window

public abstract boolean superDispatchTouchEvent(MotionEvent event);

找到Window的实现类PhoneWindow

@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}

所以Window会传给DecorView，它其实是Activity真正的根布局，继承自FrameLayout，里面包裹着你在xml中所设置的布局，所以他会向下分发。

ViewGroup核心代码

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);
} else {
intercepted = false;
}
} else {
intercepted = true;
}

如果是down事件或者mFirstTouchTarget不为null。在后文，如果down事件被成功处理了，那么这个mFirstTouchTarget就会被赋值，那么当move，up事件来临的时候，这个条件就不会被满足，所以intercepted被置为true，代表了已拦截，这直接导致了onInterceptTouchEvent不会被调用，意思就是不会再启用拦截机制了，因为你已经决定消费它了。这验证了上文的猜测。

此外上文还提及了requestDisallowInceptTouchEvent方法。这是子View用来使得父ViewGroup的拦截机制无效化的东西！（除了down事件）

@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {

if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
return;
}

if (disallowIntercept) {
mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
} else {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}

if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
}
}

子View可以通过这个方法来给父类设置FLAG_DISALLOW_INTERCEPT这个标记位。可是为什么down事件不行？

// 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();
}

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;
}
}

可以看到，如果是down事件的话，就会调用重置标记位的方法，这是解决滑动冲突的一个很重要的方法。不仅如此，mFirstTouchTarget也会被置为空，避免在下一次事件传来时干扰了他。

当ViewGroup的dispatchTouchEvent的返回值是super的时候，那么他将不会拦截事件，而是分发。所以可以看一下分发的代码。

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();
}

首先遍历，每个子元素需要满足2点条件才能接收到事件。

1.子元素是否在播放动画

2.点击事件的坐标是否落在子元素的区域内

这里的dispatchTransformedTouchEvent方法实际上就是就是调用了子元素的dispatchTouchEvent方法。所以我们看一下这个if语句。

假设子元素的dispatchTouchEvent返回的是true，那么会执行这个if里的这3行代码，并且break跳出循环。

newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;

private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}

在这个addTouchTarget方法里，就完成了对mFirstTouchTarget的赋值（这个值就不为空了，验证了我们上面的猜测，move和up不会进入上面的那个if）。
假设子元素的dispatchTouchEvent返回的是false，ViewGroup就会继续遍历下一个子元素。

再细化一下dispatchTransformedTouchEvent方法，上面说了他调用了子元素的dispatchTouchEvent方法

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;
}

事件分发我们知道是一层层深入的。如果child非空，就会继续调用孩子的dispatchTouchEvent，以实现这个循环递归遍历的过程。从上面的分析到这里，算是一轮事件分发。

但是遍历完了，却找不到合适的子元素，有两种情况：

1.ViewGroup中没有子元素

2.子元素的dispatchTouchEvent返回了false或onTouchEvent中返回了false

这种情况下，ViewGroup会自己处理点击事件。

接着遍历子元素下面的代码看

// 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);
}

dispatchTransformedTouchEvent这个方法似曾相识，就是上面出现过的，调用孩子的dispatchTouchEvent的那个方法。但是注意，这一次第三个参数是null，本来是应该放孩子的。所以从前面这个方法内的if、else语句来看，如果它的孩子是空值，那么就意味着你传进来的这个child已经是view级别了，再也不是容器了，所以这个时候就要调用View的dispatchTouchEvent方法来着手处理事件了。

view的处理

view不是嵌套的结构，所以处理起来要简单的多了

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;
}
}

这一段就是View对点击事件的处理过程了。首先会判断有没有onTouchListener，根据li.mOnTouchListener != null。其次li.mOnTouchListener.onTouch(this, event)代表假如onTouch方法返回的是true，那么onTouchEvent便不会被调用。这验证了我们onTouch更优先的结论。

进入onTouchEvent

if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}

这里又印证了一个结论。(viewFlags & ENABLED_MASK) == DISABLED即使一个View是disable的，他依然会消费事件。

先看看up事件

if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
...

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();
}
}
}

if (((viewFlags & CLICKABLE) == CLICKABLE ||

        (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||

        (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {

代表只要能长按，点击，甚至可点击状态，都可以消费事件。

然后关注这个方法performClick()。

public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}

sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
return result;
}

当up事件发生，就会触发这个方法。如果这个View设置了onClickListener，li.mOnClickListener != null这句话，那么就会调用onClick方法。

setOnClickListener会 自动把View的CLICKABLE设为true，LONGCLICKABLE也是一样，所以不要好奇为什么设置的不可点击，结果还是能点击

public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}

所以你想让点击事件失效，必须setclickable（false）在注册点击事件之后才行。而设置为enable为false就不会有这个情况了。