Android View的事件分发机制（一）：View

来源：互联网发布：php 数组函数编辑：程序博客网时间：2024/06/05 04:34

参考资料：http://blog.csdn.net/guolin_blog/article/details/9097463

官方View的树状图：
这里写图片描述
View的子类：ImageView、TextView、Button …… 很多

解析View的onClick和OnTouch的处理机制

以一个可点击的控件为例：button
在Activity中给button注册一个点击事件：

 button.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.e("button:", "onClick");            }        });

再给button注册一个触摸的事件，即touch事件

 button.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                Log.e("button:", "onTouch" + event.getAction());                return false;            }        });

onTouch事件会执行手指按下、手指移动、手指抬起这三个事件。
如果onClick和onTouch事件都注册了，那先执行那个呢？我们编个demo，运行一下就知道了，运行结果如下：
这里写图片描述
onTouch后面的数字含义：0–ACTION_DOWN；1–ACTION_UP；2–ACTION_MOVE。
由此可见，先执行onTouch，然后传递给onClick。（onTouch执行了多次是因为手指可能抖动了）

我们可以看到在onTouch方法里有返回值，且为false，那如果设置为true，会是什么样的结果呢，测试结果如下：
这里写图片描述
没有执行onClick方法。

总结：onTouch返回false，事件可以传递给onClick方法；onTouch返回true，事件不会传递给onClick方法。

那这是为什么呢？接下来，我们就用源码说话。
首先，我们触摸到任何一个控件，都会调用该控件的dispatchTouchEvent()方法，我们找到这个方法看一下：
这里写图片描述
如图所示，我们在View中找到了这个方法（button继承TextView，TextView继承View）,dispatchTouchEvent()方法的源码如下：

/**     * Pass the touch screen motion event down to the target view, or this     * view if it is the target.     *     * @param event The motion event to be dispatched.     * @return True if the event was handled by the view, false otherwise.     */    public boolean dispatchTouchEvent(MotionEvent event) {        // If the event should be handled by accessibility focus first.        if (event.isTargetAccessibilityFocus()) {            // We don't have focus or no virtual descendant has it, do not handle the event.            if (!isAccessibilityFocusedViewOrHost()) {                return false;            }            // We have focus and got the event, then use normal event dispatch.            event.setTargetAccessibilityFocus(false);        }        boolean result = false;        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(event, 0);        }        final int actionMasked = event.getActionMasked();        if (actionMasked == MotionEvent.ACTION_DOWN) {            // Defensive cleanup for new gesture            stopNestedScroll();        }        if (onFilterTouchEventForSecurity(event)) {            //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;            }        }        if (!result && mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);        }        // Clean up after nested scrolls if this is the end of a gesture;        // also cancel it if we tried an ACTION_DOWN but we didn't want the rest        // of the gesture.        if (actionMasked == MotionEvent.ACTION_UP ||                actionMasked == MotionEvent.ACTION_CANCEL ||                (actionMasked == MotionEvent.ACTION_DOWN && !result)) {            stopNestedScroll();        }        return result;    }

我们主要看31–43行这段代码。我们看下这句：
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event))
这里面总共有四个条件，我们看后面三个:
①、li.mOnTouchListener!=null，这个只要们给控件设置了触摸监听器，即button.setOnTouchListener(new OnTouchListener()…),这就不可能是null；

②、(mViewFlags&ENABLED_MASK)==ENABLED，这个意思应该就是控件是enabled的，即可点击的，如：button是可点击的，TextView就是不可点击的（不过可以在布局里面设置 android:clickable=”true”就成可点击的了）

③li.mOnTouchListener.onTouch(this,event)，这个方法就是我们自己实现的onTouch()方法，我们知道先执行onTouch，然后传递给onClick。所以这个条件的真假我们是可以控制的，我们先看源码中的19行：boolean result = false，即result的初始值为false；onTouch的返回值分析如下：

若是false，则result = true,不会执行，这时result的值还是false，则第40行 if (!result && onTouchEvent(event))中的！result就是true，然后onTouchEvent(event)就可以执行了，然后事件就传递给onClick了，即onClick方法在onTouchEvent()方法中。

若是true，则三个条件成立，result = true就会执行，这时result的值为ture，则第40行if (!result && onTouchEvent(event))中的！result就是false，那onTouchEvent(event)就不会执行了，所以onClick方法没有执行。

由上述可知，onClick方法在onTouchEvent方法中，接下来我们就看下onTouchEvent()方法的源码：

/**     * Implement this method to handle touch screen motion events.     * <p>     * If this method is used to detect click actions, it is recommended that     * the actions be performed by implementing and calling     * {@link #performClick()}. This will ensure consistent system behavior,     * including:     * <ul>     * <li>obeying click sound preferences     * <li>dispatching OnClickListener calls     * <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when     * accessibility features are enabled     * </ul>     *     * @param event The motion event.     * @return True if the event was handled, false otherwise.     */    public boolean onTouchEvent(MotionEvent event) {        final float x = event.getX();        final float y = event.getY();        final int viewFlags = mViewFlags;        final int action = event.getAction();        if ((viewFlags & ENABLED_MASK) == DISABLED) {            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {                setPressed(false);            }            // A disabled view that is clickable still consumes the touch            // events, it just doesn't respond to them.            return (((viewFlags & CLICKABLE) == CLICKABLE                    || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)                    || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);        }        if (mTouchDelegate != null) {            if (mTouchDelegate.onTouchEvent(event)) {                return true;            }        }        if (((viewFlags & CLICKABLE) == CLICKABLE ||                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||                (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {            switch (action) {                case MotionEvent.ACTION_UP:                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;                    if ((mPrivateFlags & PFLAG_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, x, y);                       }                        if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {                            // 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();                                }                            }                        }                        if (mUnsetPressedState == null) {                            mUnsetPressedState = new UnsetPressedState();                        }                        if (prepressed) {                            postDelayed(mUnsetPressedState,                                    ViewConfiguration.getPressedStateDuration());                        } else if (!post(mUnsetPressedState)) {                            // If the post failed, unpress right now                            mUnsetPressedState.run();                        }                        removeTapCallback();                    }                    mIgnoreNextUpEvent = false;                    break;                case MotionEvent.ACTION_DOWN:                    mHasPerformedLongPress = false;                    if (performButtonActionOnTouchDown(event)) {                        break;                    }                    // Walk up the hierarchy to determine if we're inside a scrolling container.                    boolean isInScrollingContainer = isInScrollingContainer();                    // For views inside a scrolling container, delay the pressed feedback for                    // a short period in case this is a scroll.                    if (isInScrollingContainer) {                        mPrivateFlags |= PFLAG_PREPRESSED;                        if (mPendingCheckForTap == null) {                            mPendingCheckForTap = new CheckForTap();                        }                        mPendingCheckForTap.x = event.getX();                        mPendingCheckForTap.y = event.getY();                        postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());                    } else {                        // Not inside a scrolling container, so show the feedback right away                        setPressed(true, x, y);                        checkForLongClick(0);                    }                    break;                case MotionEvent.ACTION_CANCEL:                    setPressed(false);                    removeTapCallback();                    removeLongPressCallback();                    mInContextButtonPress = false;                    mHasPerformedLongPress = false;                    mIgnoreNextUpEvent = false;                    break;                case MotionEvent.ACTION_MOVE:                    drawableHotspotChanged(x, y);                    // Be lenient about moving outside of buttons                    if (!pointInView(x, y, mTouchSlop)) {                        // Outside button                        removeTapCallback();                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {                            // Remove any future long press/tap checks                            removeLongPressCallback();                            setPressed(false);                        }                    }                    break;            }            return true;        }        return false;    }

只看重点：在41行，我们就可以看出，如果该控件可以点击，就会进入switch()语句，当用户抬起手指时，就会进入 case MotionEvent.ACTION_UP:这个语句，经过各种判断，会执行到76行performClick()方法，performClick()的源码如下：

 /**     * Call this view's OnClickListener, if it is defined.  Performs all normal     * actions associated with clicking: reporting accessibility event, playing     * a sound, etc.     *     * @return True there was an assigned OnClickListener that was called, false     *         otherwise is returned.     */    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;        }        return result;    }

我们看12行，只要li != null && li.mOnClickListener != null，我们就会执行onClick方法，对于一个要点击的控件，我们肯定会给他设置onClickListener的，即button.setOnClickListener(new OnClickListener()…)。这样的话，li != null && li.mOnClickListener != null这个条件肯定成立，即onClick方法能够执行。

注意：
1、如果是不可点击的控件（如：TextView）注册了onTouch事件，如下：

textView.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                Log.e("TextView:", "onTouch" + event.getAction());                return false;            }        });

测试结果，如下：
这里写图片描述
只有一条数据，且onTouch后面的数字是0，表示是ACTION_DOWN；那为什么没有ACTION_MOVE、ACTION_UP呢？这是因为TextView是不可点击的，到onTouchEvent源码的41行时，就进不去了，直接跳到154行返回false了，即后面的ACTION_MOVE、ACTION_UP就不能执行了。

解决方法：
① 只要在TextView的布局文件中加：android:clickable=”true”就可以了，ACTION_DOWN、ACTION_MOVE、ACTION_UP都可以执行了。或者重写onTouchEvent方法
② 在onTouch方法中，返回true就可以了

2、为什么给ListView引入了一个滑动菜单的功能，ListView就不能滚动了？
你应该会知道滑动菜单的功能是通过给ListView注册了一个touch事件来实现的。如果你在onTouch方法里处理完了滑动逻辑后返回true，那么ListView本身的滚动事件就被屏蔽了，自然也就无法滑动(原理同前面例子中按钮不能点击)，因此解决办法就是在onTouch方法里返回false。

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