Android View 与 ViewGroup 事件分发总结

1.View 点击事件分发

当一个控件被点击时，该控件的dispatchTouchEvent方法就会被调用，由于View的子类没有重写该方法（ViewGroup等会再讨论），故该控件的类会不断往基类查找，最终点击事件的分发由View里的 dispatchTouchEvent方法开始进行。

public boolean dispatchTouchEvent(MotionEvent event) {        ...            //noinspection SimplifiableIfStatement            ListenerInfo li = mListenerInfo;            if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) {                return true;            }            if (onTouchEvent(event)) {                return true;            }        ...        return false;    }

以上为View的dispatchTouchEvent方法的代码，dispatchTouchEvent传入一个带有点击信息的event参数，返回一个布尔值，表示这个点击事件是否已经被消费处理。代码中比较关键的是第9行的if语句：其中判断了mOnTouchListener是否为null，而mOnTouchListener顾名思义就是我们平时经常注册的touch事件监听器了

public void setOnTouchListener(OnTouchListener l) {        getListenerInfo().mOnTouchListener = l;    }

以上为View中的注册touch事件监听器代码。

发现touch事件监听器不为null时，View就会调用listener的onTouch方法了，如果onTouch方法返回了true，那么该方法就不会往下继续执行，dispatchTouchEvent就会返回true代表事件被消费掉了。

但如果我们没有注册touch事件监听器，或者在onTouch方法中返回了false，方法就会继续执行进入到onTouchEvent中

public boolean onTouchEvent(MotionEvent event) {        final int viewFlags = mViewFlags;        if ((viewFlags & ENABLED_MASK) == DISABLED) {            if (event.getAction() == 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));        }        if (mTouchDelegate != null) {            if (mTouchDelegate.onTouchEvent(event)) {                return true;            }        }        if (((viewFlags & CLICKABLE) == CLICKABLE ||                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {            switch (event.getAction()) {                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);                       }                        if (!mHasPerformedLongPress) {                            // 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();                    }                    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();                        }                        postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());                    } else {                        // Not inside a scrolling container, so show the feedback right away                        setPressed(true);                        checkForLongClick(0);                    }                    break;                case MotionEvent.ACTION_CANCEL:                    setPressed(false);                    removeTapCallback();                    removeLongPressCallback();                    break;                case MotionEvent.ACTION_MOVE:                    final int x = (int) event.getX();                    final int y = (int) event.getY();                    // 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;    }

以上是View的onTouchEvent的代码。由于代码很长，我们只看关键的部分，从第9行的注释我们可以得知，如果被点击的控件是disable的，那么该方法不会执行任何动作，但如果该控件是clickable的，dispatchTouchEvent方法依然会返回true，该点击事件依然会被消费掉。

接着看到第23行，如果该控件是可点击的，那么代码就进入if语句里面，执行第60行的performClick方法

public boolean performClick() {        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);        ListenerInfo li = mListenerInfo;        if (li != null && li.mOnClickListener != null) {            playSoundEffect(SoundEffectConstants.CLICK);            li.mOnClickListener.onClick(this);            return true;        }        return false;    }

以上就是View中performClick方法的代码，第7行就是我们调用了再熟悉不过的onClickListener的onClick方法了。

综上所述，View的事件分发如下图所示：

2.ViewGroup点击事件分发

讲完了View的事件分发机制，我们再来讲讲ViewGroup的事件分发机制。ViewGroup继承自View，然而ViewGroup重写了View中的dispatchTouchEvent方法，代码如下：

@Override    public boolean dispatchTouchEvent(MotionEvent ev) {        ...        boolean handled = false;        if (onFilterTouchEventForSecurity(ev)) {            ...            // Check for interception.            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;            }            ...            // Update list of touch targets for pointer down, if needed.            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;            TouchTarget newTouchTarget = null;            boolean alreadyDispatchedToNewTouchTarget = false;            if (!canceled && !intercepted) {                ...                        final boolean customOrder = isChildrenDrawingOrderEnabled();                        for (int i = childrenCount - 1; i >= 0; i--) {                            final int childIndex = customOrder ?                                    getChildDrawingOrder(childrenCount, i) : i;                            final View child = children[childIndex];                            if (!canViewReceivePointerEvents(child)                                    || !isTransformedTouchPointInView(x, y, child, null)) {                                continue;                            }                            newTouchTarget = getTouchTarget(child);                            if (newTouchTarget != null) {                                // Child is already receiving touch within its bounds.                                // Give it the new pointer in addition to the ones it is handling.                                newTouchTarget.pointerIdBits |= idBitsToAssign;                                break;                            }                            resetCancelNextUpFlag(child);                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {                                // Child wants to receive touch within its bounds.                                mLastTouchDownTime = ev.getDownTime();                                mLastTouchDownIndex = childIndex;                                mLastTouchDownX = ev.getX();                                mLastTouchDownY = ev.getY();                                newTouchTarget = addTouchTarget(child, idBitsToAssign);                                alreadyDispatchedToNewTouchTarget = true;                                break;                            }                        }                    ...            }            // Dispatch to touch targets.            if (mFirstTouchTarget == null) {                // No touch targets so treat this as an ordinary view.                handled = dispatchTransformedTouchEvent(ev, canceled, null,                        TouchTarget.ALL_POINTER_IDS);            }...        }        if (!handled && mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);        }        return handled;    }

我们依然是找关键代码来看，在第15行有一个非常关键的方法：onInterceptTouchEvent，该方法的返回值赋值给了布尔变量intercepted，而在第32行intercepted的取值决定了是否进入该if语句，而该if语句就是ViewGroup分发touchEvent给他的子View的关键所在。

ViewGroup中的onInterceptTouchEvent方法如下：

/**     * Implement this method to intercept all touch screen motion events.  This     * allows you to watch events as they are dispatched to your children, and     * take ownership of the current gesture at any point.     *     * <p>Using this function takes some care, as it has a fairly complicated     * interaction with {@link View#onTouchEvent(MotionEvent)     * View.onTouchEvent(MotionEvent)}, and using it requires implementing     * that method as well as this one in the correct way.  Events will be     * received in the following order:     *     * <ol>     * <li> You will receive the down event here.     * <li> The down event will be handled either by a child of this view     * group, or given to your own onTouchEvent() method to handle; this means     * you should implement onTouchEvent() to return true, so you will     * continue to see the rest of the gesture (instead of looking for     * a parent view to handle it).  Also, by returning true from     * onTouchEvent(), you will not receive any following     * events in onInterceptTouchEvent() and all touch processing must     * happen in onTouchEvent() like normal.     * <li> For as long as you return false from this function, each following     * event (up to and including the final up) will be delivered first here     * and then to the target's onTouchEvent().     * <li> If you return true from here, you will not receive any     * following events: the target view will receive the same event but     * with the action {@link MotionEvent#ACTION_CANCEL}, and all further     * events will be delivered to your onTouchEvent() method and no longer     * appear here.     * </ol>     *     * @param ev The motion event being dispatched down the hierarchy.     * @return Return true to steal motion events from the children and have     * them dispatched to this ViewGroup through onTouchEvent().     * The current target will receive an ACTION_CANCEL event, and no further     * messages will be delivered here.     */    public boolean onInterceptTouchEvent(MotionEvent ev) {        return false;    }

注释非常长，而代码仅仅只是返回了一个false。由注释可知，该方法的作用是返回当前ViewGroup是否拦截点击的touchEvent，如果该方法返回false，则dispatchTouchEvent方法将会进入第32行的if语句，执行第54行的dispatchTransformedTouchEvent方法：

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,            View child, int desiredPointerIdBits) {        final boolean handled;        // Canceling motions is a special case.  We don't need to perform any transformations        // or filtering.  The important part is the action, not the contents.        final int oldAction = event.getAction();        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {            event.setAction(MotionEvent.ACTION_CANCEL);            if (child == null) {                handled = super.dispatchTouchEvent(event);            } else {                handled = child.dispatchTouchEvent(event);            }            event.setAction(oldAction);            return handled;        }        ...    }

该方法为ViewGroup的一个private方法，他的返回值表示该点击事件是否被消费掉，方法的第1个参数即是我们的点击事件，我们重点注意方法的第3个参数child，在第10行的if语句中，如果child为null，则把touchEvent交给基类处理（就是把当前ViewGroup当成一个普通的View进行事件分发处理），如果child不为null，则把touchEvent交给child（子View） 来处理。

在dispatchTouchEvent的第54行我们传入了child的参数，故我们在尝试把touchEvent交给ViewGroup的子View处理。然而当onInterceptTouchEvent返回了true或子View处理点击事件失败时，我们就会执行dispatchTouchEvent方法的第71行，此时我们在此调用了dispatchTransformedTouchEvent方法，但是这次我们传入的child参数为null，故该方法会把点击事件交给ViewGroup的基类处理。

综上所述，ViewGroup的点击事件分发流程如下：