Android TouchEvent事件传递机制

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。

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