Android事件分发流程

概述

最近在复习androidUI的相关知识点，在网上看了一些关于事件分发的文章与视频，因此在此记录一下自己的理解过程。

首先思考2个问题：

什么是事件?什么是事件分发机制?

用户与手机的交互会产生一系列的事件 ACTION_DOWN，ACTION_MOVE，ACTION_UP，ACTION_CANCEL。每一个事件都会封装成一个ACTIONEVENT.

事件分发主要解决由谁来处理事件的问题，当你点击Actiivty上的某一个控件时实际点击了不止一个控件，按照下图的显示。 当用户点击了view1时，实际上用户也点击到了ViewGroupA、RootView、DecorView，那事件到底交由哪个控件去处理？另外事件是由一系列的事件序列组成的，从DOWN–>MOVE–>UP，无法割裂开来，要么被忽略，要么都被某一个view所消费。

事件分发流程

如果在屏幕点击了一个控件View，最终这个控件View消费了这个事件的话，那大概的分发流程应该是Activity–>ViewGroup–>View。

针对分发流程我们需要关注下面几个方法： dispatchTouchEvent() //分发触摸事件 （dispath 就是分发的意思） onInterceptTouchEvent() // 拦截触摸事件 （Intercept 拦截的意思） onTouchEvent() // 执行触摸事件 （on 可以理解为执行）

无论是Activity，View，ViewGroup都会调用dispatchTouchEvent 与 onTouchEvent。而onInterceptTouchEvent比较特殊，只有在ViewGroup中有。

先看前面一段事件是如何从Activity传递到ViewGroup的。 当我们查看Activity的dispatchTouchEvent()方法，向上追溯可以看到如下的调用路径： Activity–>PhoneWindow–>DecorView–>ViewGroup Activity

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

PhoneWindow

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

DecorView

public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event); }

ViewGroup的dispatchTouchEvent()比较长，但总体执行逻辑可以按照下面伪代码表示

public boolean dispatchTouchEvent(MotionEvent ev) { boolean consume = false; if(onInterceptTouchEvent(ev)){ //拦截 将事件交给onTouchEvent()处理 consume = onTouchEvent(ev); }else{ //不拦截 将事件分发给子view consuem = child.dispatchTouchEvent(ev); } return consume; }

这里分发的流程就从ViewGrop分发到了View的DispatchTouchEvent.

看下View的DispatchTouchEvent 方法：

public boolean dispatchTouchEvent(MotionEvent event) { //一系列操作.. 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; } } //又是一系列操作 return result; }

上述代码只需要关注2个方法 一个onTouch方法，一个onTouchEvent()方法。了解onTouch方法是在onTouchEvent()之前执行，假如设置了View的mOnTouchListener，那onTouch()方法就会执行。

下面看下View的onTouchEvent()方法，看下它是如何来执行触摸事件的。

public boolean onTouchEvent(MotionEvent event) { //一系列操作1 final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE; //一系列操作2 if (clickable) { switch (action) { case MotionEvent.ACTION_UP: //一系列操作3 break; case MotionEvent.ACTION_DOWN: //一系列操作4 break; case MotionEvent.ACTION_CANCEL: //一系列操作5 break; case MotionEvent.ACTION_MOVE: //一系列操作6 break; } return true; } return false; }

事件最终分发到View（View消费掉事件）

在onTouchEvent方法中，假如view是可以点击或者长按，无论是DOWN，UP ，MOVE，还是CANCEL 方法都会返回true，那就表示这个事件在view中被消费掉。那view的dispatchTouchEvent() 也会返回ture，继续往下追溯，ViewGroup的DispatchTouchEvent()返回true，Activity的dispatchTouchEvent() 返回true。

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事件最终分发到View（View没有消费掉事件）

假如View的没有消费掉事件(返回false)，往前回溯，那View的dispatchTouchEvent() 也会返回false，ViewGroup的dispatchTouchEvent()返回false，事件最终会通过Activity的dispatchTouchEvent()方法交由onTouchEvent()方法来处理。

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事件被VIewGroup拦截(ViewGroup消费掉事件)：

当然上述的图默认表示的是ViewGroup的onInterceptTouchEvent()返回false的逻辑。假如ViewGroup的onInterceptTouchEvent()返回true。那代码很明显就要走ViewGroup的onTouchEvent()方法，此时如果子类重写了onTouchEvent()方法并消费了事件，方法调用关系如下图:

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小结一下

假如ViewGroup拦截了View的事件，那么最终事件就被分发到ViewGroup的onTouchEvent() 方法中。 如果没有拦截，事件就被分发到了View的onTouchEvent()方法中。 此时的View默认是可以点击的(比如Button)，那子View就默认消费了事件。如果子View不能点击(比如TextView),那子View就不会消费事件。

还剩下一个问题，之前说View要么不处理事件，要么就是整个事件序列一起处理，这部分是如何实现的？ 这部分源码只看了个大概(没看太懂…)。 大致的流程： 在ViewGroup的dispatchTouchEvent() 方法中。如果接收到的事件是ACTION_DOWN时，会将处理DOWN事件的子View 保存到mFirstTouchTarget变量中。后面的MOVE，UP事件来的时候，通过调用dispatchTransformedTouchEvent(child)方法来执行child的dispatchTouchEvent()方法。

Click与LongClick事件

Click与LongClick事件是如何进行判断的呢？

Click事件对应较短时间内(<0.5s)的一次DOWN和UP事件。 LongClick事件对应较长事件(>0.5s)的一次DOWN事件和UP事件。

而DOWN跟UP事件都是发生在View的onTouchEvent()方法中。因此可以初步判断Click事件与LongClick事件都可以在View的onTouchEvent()事件中找到调用链。

public boolean onTouchEvent(MotionEvent event) { final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE; if (clickable ) { switch (action) { case MotionEvent.ACTION_UP: //一系列操作 if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { //一系列操作 if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) { //一系列操作 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)) { performClickInternal(); } } } //一系列操作 break; case MotionEvent.ACTION_DOWN: //一系列操作 if (!clickable) { checkForLongClick(0, x, y); break; } //一系列操作 break; case MotionEvent.ACTION_CANCEL: //一系列操作 break; case MotionEvent.ACTION_MOVE: //一系列操作 break; } return true; } return false; }

通过上面的代码可以看到在UP事件中有一个mPerformClick对象；在DOWN事件中有个checkForLongClick()方法。通过名字我们似乎可以看出UP事件处理的Click相关的回调。而DOWN事件处理的LongClick相关的回调。

继续往下跟代码发现确实如此。在View的performClickInternal()方法

private boolean performClickInternal() { // Must notify autofill manager before performing the click actions to avoid scenarios where // the app has a click listener that changes the state of views the autofill service might // be interested on. notifyAutofillManagerOnClick(); return performClick(); }

performClick() 方法

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; } sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED); notifyEnterOrExitForAutoFillIfNeeded(true); return result; }

我们看到了li.mOnClickListener.onClick(this);

同样在也是在View的checkForLongClick()方法：

private void checkForLongClick(int delayOffset, float x, float y) { if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE || (mViewFlags & TOOLTIP) == TOOLTIP) { mHasPerformedLongPress = false; if (mPendingCheckForLongPress == null) { mPendingCheckForLongPress = new CheckForLongPress(); } mPendingCheckForLongPress.setAnchor(x, y); mPendingCheckForLongPress.rememberWindowAttachCount(); mPendingCheckForLongPress.rememberPressedState(); postDelayed(mPendingCheckForLongPress, ViewConfiguration.getLongPressTimeout() - delayOffset); } }

CheckForLongPress() 对象中

private final class CheckForLongPress implements Runnable { private int mOriginalWindowAttachCount; private float mX; private float mY; private boolean mOriginalPressedState; @Override public void run() { if ((mOriginalPressedState == isPressed()) && (mParent != null) && mOriginalWindowAttachCount == mWindowAttachCount) { if (performLongClick(mX, mY)) { mHasPerformedLongPress = true; } } } //... }

performLongClick()方法。继续往下多跟几个方法

private boolean performLongClickInternal(float x, float y) { sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED); boolean handled = false; final ListenerInfo li = mListenerInfo; if (li != null && li.mOnLongClickListener != null) { handled = li.mOnLongClickListener.onLongClick(View.this); } //... }

最终可以看到li.mOnLongClickListener.onLongClick() 这一行代码。 基本可以验证我们上面的猜想。 在View的onTouchEvent() 方法的UP事件中调用了Click事件，在DOWN事件中调用了LongClick事件。

结论

一个时间序列从手指接触屏幕到手指离开屏幕，在这个过程产生的一系列事件，从DOWM事件开始，一系列的MOVE 事件，以UP事件结束正常情况下，一个事件序列由一个view处理某个ViewGroup一旦决定拦截，那么所有的事件序列都会由它的onTouchEvent()处理，并且它的onInterceptTouchEvent()不会再调用某个View一旦开始处理事件，假如它没有消耗DOWN事件，那么同一事件序列的其它事件都不会再交给它处理。会交由它的父元素处理（父元素的onTouchEvent()被调用）事件的传递过程是由外向内的，即事件总是先传递给父元素，然后再由父元素分发给子ViewViewGroup默认不拦截事件，即onInterceptTouchEvent默认返回false.View没有onInterceptTouchEvent方法，一旦有事件被拦截，那么它的onTouchEvent方法会被调用View的onTouchEvent默认会消耗事件(返回true)，除非它是不可点击的（clickable与longClickable同时为false）。View的longClickable默认都是false,clickable要分情况，比如button的clickable默认是true，Textview的clickable默认是falseView的enable属性不影响onTouchEvent()的默认返回值。哪怕一个View是disable状态，onTouchEvent()的返回值只跟clickable 与 longClickable 相关onClick的响应的前提是view是可点击的，并且收到了DOWN与UP事件，并且受长按事件的影响，当长按事件返回true,onClick不会响应onLongClick再DOWN事件中判断，要想执行长按事件该view必须是longClickable的并且设置了onLongClickListener.

引用：

https://blog.csdn.net/weixin_41101173/article/details/79685305?utm_source=blogxgwz1 https://www.gcssloop.com/customview/dispatch-touchevent-theory https://zhuanlan.zhihu.com/p/27608989 https://blog.csdn.net/guolin_blog/article/details/9097463 https://blog.csdn.net/guolin_blog/article/details/9153747