Android View Touch的事件分发机制

开发一段时间的Android，或多或少对Android的事件有一些了解，对诸如dispatchTouchEvent、onTouchEvent方法有些了解。但真正在面试中被问起，整个机制，或者具体的分析ViewGroup+ViewGroup+View的具体回调顺序，就懵了。百度出的第一位博客讲解的很到位：



当一个Touch事件(触摸事件为例)到达根节点，即Acitivty的ViewGroup时，它会依次下发，下发的过程是调用子View(ViewGroup)的dispatchTouchEvent方法实现的。简单来说，就是ViewGroup遍历它包含着的子View，调用每个View的dispatchTouchEvent方法，而当子View为ViewGroup时，又会通过调用ViwGroup的dispatchTouchEvent方法继续调用其内部的View的dispatchTouchEvent方法。上述例子中的消息下发顺序是这样的：①-②-⑤-⑥-⑦-③-④。dispatchTouchEvent方法只负责事件的分发，它拥有boolean类型的返回值，当返回为true时，顺序下发会中断。在上述例子中如果⑤的dispatchTouchEvent返回结果为true，那么⑥-⑦-③-④将都接收不到本次Touch事件。

ViewGroup+ViewGroup+View事件传递的具体分析

题目：比如最上面有一个ViewGroup，下面有一个ViewGroup，最下面有一个Button，那么点击Button，请问事件是如何传递的，这期间会回调哪些方法，顺序如何？

看了上面的讲解，其实心中已有一定的了解。但不能止于此。于是我使用代码验证并查看Android的源代码来看看到底是怎么回事，来找到最后的答案。

新建Activity，layout的结构为LinearLayout - RelativeLayout - Button，刚好是题目所示的结构。为了验证回调函数，我将这三个View都继承了，同时也在Activity中覆盖dispatchTouchEvent和onTouchEvent：

layout布局如下：

<?xml version="1.0" encoding="utf-8"?>
<com.idengpan.life100.viewtouch.TouchLinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/white"
android:orientation="vertical" >

<com.idengpan.life100.viewtouch.TouchRelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent" >

<com.idengpan.life100.viewtouch.TouchButton
android:id="@+id/btn_test"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="测试" />
</com.idengpan.life100.viewtouch.TouchRelativeLayout>

</com.idengpan.life100.viewtouch.TouchLinearLayout>

三个自定义的View/ViewGroup如下：

public class TouchLinearLayout extends LinearLayout {

public TouchLinearLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
System.out.println("TouchLinearLayout onInterceptTouchEvent");
boolean result = super.onInterceptTouchEvent(ev);
System.out.println("TouchLinearLayout onInterceptTouchEvent return " + result);
return result;
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("TouchLinearLayout dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("TouchLinearLayout dispatchTouchEvent return " + result);
return result;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
System.out.println("TouchLinearLayout onTouchEvent");
boolean result = super.onTouchEvent(event);
System.out.println("TouchLinearLayout onTouchEvent return " + result);
return result;
}
}

public class TouchRelativeLayout extends RelativeLayout{

public TouchRelativeLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
System.out.println("TouchRelativeLayout onInterceptTouchEvent");
boolean result = super.onInterceptTouchEvent(ev);
System.out.println("TouchRelativeLayout onInterceptTouchEvent return " + result);
return result;
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("TouchRelativeLayout dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("TouchRelativeLayout dispatchTouchEvent return " + result);
return result;
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
System.out.println("TouchRelativeLayout onTouchEvent");
boolean result = super.onTouchEvent(ev);
System.out.println("TouchRelativeLayout onTouchEvent return " + result);
return result;
}
}

public class TouchButton extends Button {

public TouchButton(Context context, AttributeSet attrs) {
super(context, attrs);
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("TouchButton dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("TouchButton dispatchTouchEvent return " + result);
return result;
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
System.out.println("TouchButton onTouchEvent");
boolean result = super.onTouchEvent(ev);
System.out.println("TouchButton onTouchEvent return " + result);
return result;
}

}

//Activity中覆盖：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("activity dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("activity dispatchTouchEvent return " + result);
return result;
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
System.out.println("activity onTouchEvent");
boolean result = super.onTouchEvent(ev);
System.out.println("activity onTouchEvent return " + result);
return result;
}

点击Button，可以看到回调的执行顺序是：

I/System.out(7034): activity dispatchTouchEvent
I/System.out(7034): TouchLinearLayout dispatchTouchEvent
I/System.out(7034): TouchLinearLayout onInterceptTouchEvent
I/System.out(7034): TouchLinearLayout onInterceptTouchEvent return false
I/System.out(7034): TouchRelativeLayout dispatchTouchEvent
I/System.out(7034): TouchRelativeLayout onInterceptTouchEvent
I/System.out(7034): TouchRelativeLayout onInterceptTouchEvent return false
I/System.out(7034): TouchButton dispatchTouchEvent
I/System.out(7034): TouchButton onTouchEvent
I/System.out(7034): TouchButton onTouchEvent return true
I/System.out(7034): TouchButton dispatchTouchEvent return true
I/System.out(7034): TouchRelativeLayout dispatchTouchEvent return true
I/System.out(7034): TouchLinearLayout dispatchTouchEvent return true
I/System.out(7034): activity dispatchTouchEvent return true

看到日志输出，更是一目了然了。去查看了下ViewGroup的源码，它的onInterceptTouchEvent方法未做任何处理，总是返回false的。

public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}

再看看ViewGroup中的dispatchTouchEvent方法：

public boolean dispatchTouchEvent(MotionEvent ev) {
....
// 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;
}

...
//dispatchTransformedTouchEvent方法中
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;

// Canceling motions is a special case.  We don't need to perform any transformations
// or filtering.  The important part is the action, not the contents.
final int oldAction = event.getAction();
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;
}
...
}

}

那View里的dispatchTouchEvent方法执行的很明显，直接实现onTouch监听或执行onTouchEvent方法：

public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}

if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}

if (onTouchEvent(event)) {
return true;
}
}

if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}

而ViewGroup中没有覆盖onTouchEvent方法，都是执行View中的onTouchEvent方法。

在View的onTouchEvent方法中，会在ACTION_UP事件中调用performCLick方法，从而响应Button.onClick事件:

public boolean onTouchEvent(MotionEvent event) {
...
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;
if ((mPrivateFlags & PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}

if (prepressed) {
// The button is being released before we actually
// showed it as pressed.  Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true);
}

if (!mHasPerformedLongPress) {
// 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();
}
}
}
....
}

所以这个题目的答案就是，点击事件从Activity传递到ViewGroup，最上层的ViewGroup调用dispatchTouchEvent进行事件分发，接着会调用下面一层ViewGroup的dispatchTouchEvent方法，然后就是Button的dispatchTouchEvent方法，而Button的dispatchTouchEvent方法会调用Button的onTouchEvent方法，在这个方法里，点击事件被消费，返回了true，接着Button的dispatchTouchEvent方法也返回true，然后中间ViewGroup的dispatchTouchEvent方法也返回true，然后是最上层的dispatchTouchEvent方法也返回true，最后是Activity的dispatchTouchEvent方法也返回true。事件一步一步向下传递，然后一步一步向上冒泡反馈。

参考文章：/article/5242433.html