View的事件分发机制
什么是事件
从用户手指点击屏幕到抬起之间的一系列动作就是事件。一般包含：
ACTION_DOWN
ACTION_UP
ACTION_MOVE
ACTION_CANCEL
具体查看MotionEvent类。事件分发就是MotionEvent分发的过程，当系统产生一个MotionEvent后，系统需要把这个事件传递到具体的View，这就是事件分发过程。
分发过程
事件分发过程由三个重要的方法完成。
public boolean dispatchTouchEvent(MotionEvent ev)
如果一个MotionEvent传递给了View，那么dispatchTouchEvent方法一定会被调用！
返回值：表示是否消费了当前事件。可能是View本身的onTouchEvent方法消费，也可能是子View的dispatchTouchEvent方法中消费。返回true表示事件被消费，本次的事件终止。返回false表示View以及子View均没有消费事件，将调用父View的onTouchEvent方法。
public boolean onInterceptTouchEvent(MotionEvent ev)
事件拦截，当一个ViewGroup在接到MotionEvent事件序列时候，首先会调用此方法判断是否需要拦截。特别注意，这是ViewGroup特有的方法，View并没有拦截方法
返回值：是否拦截事件传递，返回true表示拦截了事件，那么事件将不再向下分发而是调用View本身的onTouchEvent方法。返回false表示不做拦截，事件将向下分发到子View的dispatchTouchEvent方法。如果当前view拦截了该某个事件，那么同一个事件序列当中，该方法不会被再次调用。在dispatchTouchEvent中调用。
public boolean onTouchEvent(MotionEvent ev)
真正对MotionEvent进行处理或者说消费的方法。在dispatchTouchEvent进行调用。
返回值：返回true表示事件被消费，本次的事件终止。返回false表示事件没有被消费，将调用父View的onTouchEvent方法。如果不消耗，那么同一事件序列中，当前view无法再次收到事件。
三者关系
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean consume = false;
if (onInterceptTouchEvent(ev)) {
consume = onTouchEvent(ev);
} else {
consume = child.dispatchTouchEvent(MotionEvent ev);
}
return consume;
}
传递流程
还需注意：
- 子View可以通过
requestDisallowInterceptTouchEvent方法干预父View的事件分发过程（ACTION_DOWN事件除外）。阻止父view拦截事件。 - 对于View（注意！ViewGroup也是View）而言，如果设置了
onTouchListener，那么OnTouchListener方法中的onTouch方法会被回调。onTouch方法返回true，则onTouchEvent方法不会被调用（onClick事件是在onTouchEvent中调用）所以三者优先级是onTouch->onTouchEvent->onClick View的onTouchEvent方法默认都会消费掉事件（返回true），除非它是不可点击的（clickable和longClickable同时为false），View的longClickable默认为false，clickable需要区分情况，如Button的clickable默认为true，而TextView的clickable默认为false。
源码分析下
系统产生事件最先传递到的是当前的Activity, 由其dispatchTouchEvent进行事件分发。
/**
* Called to process touch screen events. You can override this to
* intercept all touch screen events before they are dispatched to the
* window. Be sure to call this implementation for touch screen events
* that should be handled normally.
*
* @param ev The touch screen event.
*
* @return boolean Return true if this event was consumed.
*/
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) { //用户点击屏幕交互
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
// 传递到window, 如果消费掉事件，直接返回
return true;
}
// 没有消费，Activity自己消费
return onTouchEvent(ev);
}
getWindow其实就是获取android.view.PhoneWindow这是Window的唯一实现。
/**
* Abstract base class for a top-level window look and behavior policy. An
* instance of this class should be used as the top-level view added to the
* window manager. It provides standard UI policies such as a background, title
* area, default key processing, etc.
*
* The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/
public abstract class Window {
}
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
//调用的是decorview的superDispatchTouchEvent.
// DecorView就是Window的顶级View，我们通过setContentView设置的View是它的子View （Activity的setContentView，最终是调用PhoneWindow的setContentView
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
DecorView是viewGroup，因此进入View的事件分发流程。
//DecorView
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
//viewGroup
public boolean dispatchTouchEvent(MotionEvent ev) {
// Handle an initial down.
// 并且重置mFirstTouchTarget = null
// mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
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.
// 只有actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null 采取判断是否拦截事件，
// mFirstTouchTarget表示成功处理事件的子元素，也就是收如果mFirstTouchTarget == null表示该事件由自己处理
// 后续事件自己拦截处理。
// 对于FLAG_DISALLOW_INTERCEP通过requestDisallowInterceptTouchEvent方法设置，一旦设置后可以看到intercepted == true，
//父view就不会拦截事件，但是不会对ACTION_DOWN生效，因为actionMasked == MotionEvent.ACTION_DOWN会重置mGroupFlags状态。
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;
}
//看viewGroup分发给子view
// 遍历所有子view
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;
}
// 如果点击坐标没在这个子view内或者子view在播放动画那么找下一个子view
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);
// 同时满足上面两个条件传递给子view处理
// 如果返回true也就是被当前子view处理了，那么mFirstTouchTarget在addTouchTarget(child, idBitsToAssign);被赋值。跳出循环。
// 如果返回false那么循环下一个子view
//
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);
}
// Dispatch to touch targets.
// 没有找到自己处理进入view的分发流程
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
}
View的分发流程
public boolean dispatchTouchEvent(MotionEvent event) {
boolean result = false;
...
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
// 首先判断是否设置了OnTouchListener, 如果设置了就调用其onTouch方法，如果返回true, 那么
// onTouchEvent就不会被调用， 否则调用onTouchEvent
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;
}
public boolean onTouchEvent(MotionEvent event) {
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
// 不可用的view也会消耗事件
if ((viewFlags & ENABLED_MASK) == DISABLED
&& (mPrivateFlags4 & PFLAG4_ALLOW_CLICK_WHEN_DISABLED) == 0) {
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;
}
//只要是可点击就消耗事件
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
.....
{
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// 触发点击事件
// 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)) {
performClickInternal();
}
}
}
...
return true
}
reture false
}
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);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
}
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}