Android事件分发机制

作者: 又尔enter | 来源:发表于2018-04-24 20:37 被阅读0次

这一篇文章主要讲解Android分发机制

为了能正确的理解，我们自定义一个ViewGroup，一个View

public class TestViewGroup extends LinearLayout {
    public static final String TAG = "TestViewGroup";

    public TestViewGroup(Context context) {
        super(context, null);
    }

    public TestViewGroup(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public TestViewGroup(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d(TAG, "dispatchTouchEvent");
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG, "onTouchEvent");
        return super.onTouchEvent(event);
    }
}

public class TestView extends TextView {
    public static final String TAG = "TestView";

    public TestView(Context context) {
        this(context, null);
    }

    public TestView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public TestView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.d(TAG, "dispatchTouchEvent");
        //getParent().requestDisallowInterceptTouchEvent(true);
        return super.dispatchTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG, "onTouchEvent");
        return super.onTouchEvent(event);
    }

}
//activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<com.dy.touchevent.TestViewGroup
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity"
    android:gravity="center">

    <com.dy.touchevent.TestView
        android:id="@+id/testView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TestView"/>

</com.dy.touchevent.TestViewGroup>

public class MainActivity extends AppCompatActivity {
    public static final String TAG = "MainActivity";
    TestView mTestView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTestView = findViewById(R.id.testView);

        //如果返回true，则不会调用onClick,onLongClick
        mTestView.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                Log.d(TestView.TAG, "onTouchListener");
                return false;
            }
        });
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d(TAG, "dispatchTouchEvent");
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG, "onTouchEvent");
        return super.onTouchEvent(event);
    }
}

我们分别重写Activity，ViewGroup，View的dispatchTouchEvent，onTouchEvent方法，加上Log,当我们在屏幕上点击TestView时，Log日志如下：

//action_down
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/MainActivity: dispatchTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestViewGroup: dispatchTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestView: dispatchTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestView: onTouchListener
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestView: onTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestViewGroup: onTouchEvent
04-24 07:47:19.610 8168-8168/com.dy.touchevent D/MainActivity: onTouchEvent
//action_up
04-24 07:47:25.289 8168-8168/com.dy.touchevent D/MainActivity: dispatchTouchEvent
04-24 07:47:25.290 8168-8168/com.dy.touchevent D/MainActivity: onTouchEvent

从上到下传递

当屏幕上按下，产生一个ACTION_DOWN事件，这时会回调Activity的dispatchTouchEvent方法

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

onUserInteraction()方法是个空方法，不用管，它主要是用来检测分发事件用的，继续看Window

Window.java
public abstract boolean superDispatchTouchEvent(MotionEvent event);

Window是一个抽象类，它的唯一实现类是PhoneWindow

PhoneWindow.java
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
    return mDecor.superDispatchTouchEvent(event);
}

这个mDecor是一个DecorView，是布局文件的顶级View,它继承自FrameLayout

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

它会调用父类，即ViewGroup的dispatchTouchEvent();

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    .....
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        cancelAndClearTouchTargets(ev);
        resetTouchState();
    }
    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;
    }
    .....
    //父ViewGroup不拦截，传给子View
    if (!canceled && !intercepted) {
        .....
         //找到正确的子View
        for (int i = childrenCount - 1; i >= 0; i--) {
                .....
                //如果View在播放动画，或者点击事件的坐标x，y不在子View的区域内，则寻找下一个
                if (!canViewReceivePointerEvents(child)
                        || !isTransformedTouchPointInView(x, y, child, null)) {
                    ev.setTargetAccessibilityFocus(false);
                    continue;
                }
                //找到了

                if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                    .....
                    newTouchTarget = addTouchTarget(child, idBitsToAssign);
                    .....
                }
        }
    }
}

dispatchTransformedTouchEvent里面作分发事件的操作，这个child不为空，所以分发给子view

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
    final boolean handled;
    if (child == null) {
        handled = super.dispatchTouchEvent(event);
    } else {
        handled = child.dispatchTouchEvent(event);
    }
    return handled;
}

addTouchTarget()方法主要是把child赋给mFirstTouchTarget这个变量，之后会讲到。
接下来进入View的dispatchTouchEvent

public boolean dispatchTouchEvent(MotionEvent event) {
    .....
    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;
        }
    }
    .....
}

首先View会判断自身是否设置onTouchListener，如果设置了并且onTouch返回true，则直接返回true，消费了事件，则后续的onTouchEvent方法不会被调用，可见onTouchListener比onTouchEvent优先级高。接下来我们进入onTouchEvent看下

public boolean onTouchEvent(MotionEvent event) {
    final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
    if ((viewFlags & ENABLED_MASK) == DISABLED) {
        if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
            setPressed(false);
        }
        mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
        // A disabled view that is clickable still consumes the touch
        // events, it just doesn't respond to them.
        return clickable;
    }
    if (mTouchDelegate != null) {
        if (mTouchDelegate.onTouchEvent(event)) {
            return true;
        }
    }

    if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
        switch (action) {
            case MotionEvent.ACTION_UP:
                .....
                performClick();
                .....
        }
    }
}

它会判断view是否可点击，只要View的clickable和longclickable有一个为true，它就会消耗这个时间，并触发performClick()方法。

public boolean performClick() {
    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;
}

如果设置了OnClickListener，则调用它的onClick方法。到这里，ACTION_DOWN事件就从最顶级的Activity，到DecorView，到ViewGroup，最后到达子View，从上至下的过程，我们可以从Log日志看出来，也可正式onTouchListener比onTouchEvent优先级高

04-24 07:47:19.609 8168-8168/com.dy.touchevent D/MainActivity: dispatchTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestViewGroup: dispatchTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestView: dispatchTouchEvent
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestView: onTouchListener
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestView: onTouchEvent

从下到上回传

接下来我们看看，子View没消费事件，则事件会从下往上回传给ViewGroup,即View的onTouchEvent返回false，则dispatchTouchEvent返回false，那么在ViewGroup的dispatchTransformedTouchEvent方法中handled为false

View.java

public boolean dispatchTouchEvent(MotionEvent event) {
    boolean result = false;
    .....
    if (!result && onTouchEvent(event)) {
        result = true;
    }
    .....
    //返回false
    return result;
}

ViewGroup.java
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;
    .....
    handled = child.dispatchTouchEvent(event);
    .....
    return handled;
}

那么如果dispatchTransformedTouchEvent返回false，那么将不会执行newTouchTarget = addTouchTarget(child, idBitsToAssign);即mFirstTouchTarget为null，则会执行以下代码

if (mFirstTouchTarget == null) {
        // No touch targets so treat this as an ordinary view.
        handled = dispatchTransformedTouchEvent(ev, canceled, null,
                TouchTarget.ALL_POINTER_IDS);
}

child为空，则回执行super.dispatchTouchEvent(event);即View的dispatchTouchEvent(event)，所以这时会执行ViewGroup里面的onTouchEvent，由于都返回false，所以导致ViewGroup的dispatchTouchEvent返回false，最后回到Activity中，所以回调Activity的onTouchEvent,从Log可以验证我们的设想。至此，时间从下往上传递也完成了。

//返回false，执行onTouchEvent
if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
}
return onTouchEvent(ev);
//Log
04-24 07:47:19.609 8168-8168/com.dy.touchevent D/TestViewGroup: onTouchEvent
04-24 07:47:19.610 8168-8168/com.dy.touchevent D/MainActivity: onTouchEvent

ACTION_UP

04-24 07:47:25.289 8168-8168/com.dy.touchevent D/MainActivity: dispatchTouchEvent
04-24 07:47:25.290 8168-8168/com.dy.touchevent D/MainActivity: onTouchEvent

为什么ACTION_UP事件只回调了MainActivity的方法呢，ViewGroup，View的相关方法为什么为回调呢？其实回调了ViewGroup的相关方法，只不过之前ACTION_DOWN方法子View没消费，那么之后的一系列事件都不会传给子View了，除非在遇到ACTION_DOWN,因为一个完成整的屏幕触摸操作是由一个ACTION_DOWN开始，中间有或没有ACTION_MOVE,以ACTION_UP结束。

action.png

其实可以断点进去查看事件的分发流程，断点查看是学习Android Framework源码的最好途径。 由之前ACTION_DOWN事件往上传递，得知mFirstTouchTarget为null，我们继续进入ViewGroup的dispatchTouchEvent

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;
}

可以看到，由于是ACTION_UP事件并且mFirstTouchTarget为null，则直接令intercepted为true，事件不再往下传递，由自身拦截。

if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null,
        TouchTarget.ALL_POINTER_IDS);
}

后面的流程跟ACTION_DOWN事件往上传递是一样的，直接回调ViewGroup的onTouchEvent()方法，由于都返回false，所以最后会回调Activity的onTouchEvent方法。
这里还有个标志位FLAG_DISALLOW_INTERCEPT，这个是干什么用的呢，这个是子View干预父View的拦截操作，即子View通知父ViewGroup不要拦截它的事件。当子View设置FLAG_DISALLOW_INTERCEPT为true时，则disallowIntercept 为true，直接就令intercepted为false了，这样父ViewGroup就不会拦截事件，不会调用onInterceptTouchEvent，可见，onInterceptTouchEvent不是每次都会被调用的。

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;
}

//子view调用
getParent().requestDisallowInterceptTouchEvent(true);

那是不是之后的所有事件都不拦截了呢，错了，当新的ACTION_DOWN事件来到是，会重置FLAG_DISALLOW_INTERCEPT为false

/**
     * Resets all touch state in preparation for a new cycle.
     */
private void resetTouchState() {
    clearTouchTargets();
    resetCancelNextUpFlag(this);
    //重置
    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    mNestedScrollAxes = SCROLL_AXIS_NONE;
}

这也比较好理解，因为完整的事件序列以ACTION_DOWN开头。

附图

action_dispatch.png

Android事件分发机制

这一篇文章主要讲解Android分发机制

从上到下传递

从下到上回传

ACTION_UP

附图

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