Android TouchEvent事件传递机制
本文从http://blog.csdn.net/morgan_xww/article/details/9372285和http://www.cnblogs.com/linjzong/p/4191891.html整理而来
跟touch事件相关的3个方法： public boolean dispatchTouchEvent(MotionEvent ev); //用来分派event public boolean onInterceptTouchEvent(MotionEvent ev); //用来拦截event public boolean onTouchEvent(MotionEvent ev); //用来处理event
三个方法的用法： dispatchTouchEvent() 用来分派事件。 其中调用了onInterceptTouchEvent()和onTouchEvent()，一般不重写该方法 onInterceptTouchEvent() 用来拦截事件。 ViewGroup类中的源码实现就是{return false;}表示不拦截该事件， 事件将向下传递（传递给其子View）； 若手动重写该方法，使其返回true则表示拦截，事件将终止向下传递， 事件由当前ViewGroup类来处理，就是调用该类的onTouchEvent()方法 onTouchEvent() 用来处理事件。 返回true则表示该View能处理该事件，事件将终止向上传递（传递给其父View）； 返回false表示不能处理，则把事件传递给其父View的onTouchEvent()方法来处理
先上一段代码演示： TestScrollView.java
package com.example.dispatchevent;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.ScrollView;
public class TestScrollView extends ScrollView {
public TestScrollView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
public TestScrollView(Context context, AttributeSet attrs) {
super(context, attrs);
}
public TestScrollView(Context context) {
super(context);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
Log.i("dispatchTouchEvent", "TestScrollView");
return super.dispatchTouchEvent(ev);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
Log.i("onInterceptTouchEvent", "TestScrollView");
return super.onInterceptTouchEvent(ev);
//return true;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
Log.i("onTouchEvent", "TestScrollView");
return super.onTouchEvent(ev);
}
}MyButton.java
package com.example.dispatchevent;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.Button;
public class MyButton extends Button {
public MyButton(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
public MyButton(Context context, AttributeSet attrs) {
super(context, attrs);
}
public MyButton(Context context) {
super(context);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
Log.i("dispatchTouchEvent", "MyButton");
return super.dispatchTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
Log.i("onTouchEvent", "MyButton");
return super.onTouchEvent(ev);
}
}TestActivity.java
package com.example.dispatchevent;
import android.app.Activity;
import android.os.Bundle;
public class TestActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.test);
}
}test.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<com.example.dispatchevent.TestScrollView
android:id="@+id/testScrollView1"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<com.example.dispatchevent.MyButton
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
</com.example.dispatchevent.TestScrollView>
</LinearLayout>当TestScrollView 的onInterceptTouchEvent返回true时，表示被拦截，那么调用自身的onTouchEvent
当返回默认值（false）时，表示不拦截，那么往下传到button
演示完毕。接下来要解释两个地方
1.onInterceptTouchEvent返回默认值（false）？ 跟踪源码
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
/*
* This method JUST determines whether we want to intercept the motion.
* If we return true, onMotionEvent will be called and we do the actual
* scrolling there.
*/
/*
* Shortcut the most recurring case: the user is in the dragging
* state and he is moving his finger. We want to intercept this
* motion.
*/
final int action = ev.getAction();
if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) {
return true;
}
/*
* Don't try to intercept touch if we can't scroll anyway.
*/
if (getScrollY() == 0 && !canScrollVertically(1)) {
return false;
}
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_MOVE: {
/*
* mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
* whether the user has moved far enough from his original down touch.
*/
/*
* Locally do absolute value. mLastMotionY is set to the y value
* of the down event.
*/
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on content.
break;
}
final int pointerIndex = ev.findPointerIndex(activePointerId);
if (pointerIndex == -1) {
Log.e(TAG, "Invalid pointerId=" + activePointerId
+ " in onInterceptTouchEvent");
break;
}
final int y = (int) ev.getY(pointerIndex);
final int yDiff = Math.abs(y - mLastMotionY);
if (yDiff > mTouchSlop) {
mIsBeingDragged = true;
mLastMotionY = y;
initVelocityTrackerIfNotExists();
mVelocityTracker.addMovement(ev);
if (mScrollStrictSpan == null) {
mScrollStrictSpan = StrictMode.enterCriticalSpan("ScrollView-scroll");
}
final ViewParent parent = getParent();
if (parent != null) {
parent.requestDisallowInterceptTouchEvent(true);
}
}
break;
}
case MotionEvent.ACTION_DOWN: {
final int y = (int) ev.getY();
if (!inChild((int) ev.getX(), (int) y)) {
mIsBeingDragged = false;
recycleVelocityTracker();
break;
}
/*
* Remember location of down touch.
* ACTION_DOWN always refers to pointer index 0.
*/
mLastMotionY = y;
mActivePointerId = ev.getPointerId(0);
initOrResetVelocityTracker();
mVelocityTracker.addMovement(ev);
/*
* If being flinged and user touches the screen, initiate drag;
* otherwise don't. mScroller.isFinished should be false when
* being flinged.
*/
mIsBeingDragged = !mScroller.isFinished();
if (mIsBeingDragged && mScrollStrictSpan == null) {
mScrollStrictSpan = StrictMode.enterCriticalSpan("ScrollView-scroll");
}
break;
}
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
/* Release the drag */
mIsBeingDragged = false;//AAAAAAAAAAAAAAAA
mActivePointerId = INVALID_POINTER;
recycleVelocityTracker();
if (mScroller.springBack(mScrollX, mScrollY, 0, 0, 0, getScrollRange())) {
postInvalidateOnAnimation();
}
break;
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
}
/*
* The only time we want to intercept motion events is if we are in the
* drag mode.
*/
return mIsBeingDragged;
}MotionEvent一共有多种值，但是最后的落脚点一定是ACTION_UP，（看标注了AAAAAA的那一行），里面mIsBeingDragged = false;最后return mIsBeingDragged表示默认值是false
2.演示的图片里面为什么有两份调用 因为我点击了一次，依次点击包含两种ACTION：ACTION_DOWN和ACTION_UP，所以有两份。
3.演示图片中第二份为什么会少了一个onInterceptTouchEvent调用 dispatchTouchEvent()方法中还有“记忆”的功能，如果第一次事件向下传递到某View，它把事件继续传递交给它的子View，它会记录该事件是否被它下面的View给处理成功了，（怎么能知道呢？如果该事件由我的onTouchEvent()来处理，那就说明被拦截）；当第二次事件向下传递到该View，该View的dispatchTouchEvent()方法机会判断，若上次的事件由下面的view成功处理了，那么这次的事件就继续交给下面的来处理，若上次的事件没有被下面的处理成功，那么这次的事件就不会向下传递了，该View直接调用自己的onTouchEvent()方法来处理该事件。 当然，“记忆”功能的信息只在一系列事件完成之前有效，如从ACTION_DOWN事件开始，直到后续事件ACTION_MOVE，ACTION_UP结束后，“记忆”的信息就会清除。也就是说如果某View处理ACTION_DOWN事件失败了（onTouchEvent()返回false），那么后续的ACTION_MOVE,ACTION_UP等事件就不会再传递到该View了，由其父View自己来处理。在下一次发生ACTION_DOWN事件的时候，还是会传递到该View的。