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。