View事件分发机制

点击事件的传递规则

点击事件产生后，传递过程：Activity->Window->顶层View->分发到具体的View。前两个传递比较简单，不用说。顶层View一般为ViewGroup，ViewGroup会首先根据onInterceptTouchEvent判断是否拦截，如果拦截，那么就会调用自己的onTouchEvent方法进行处理，如果不拦截，就会分发给子View，子View再调用自己的dispatchTouchEvent做进一步分发。如果所有的View都不消费掉这个事件，那么会调用Activity的onTouchEvent方法来处理。
对于一个View而言，OnTouchListener的优先级高于自己的onTouchEvent方法，在onTouchEvent方法中，TouchDelegate的优先级最高，其次是OnLongClickListener（如果是长按的话），最后才是OnClickListener。

源码解析

当触摸事件发生时，Activity的dispatchTouchEvent首先被调用。

// Activity$dispatchTouchEventpublic boolean dispatchTouchEvent(MotionEvent ev) {    if (ev.getAction() == MotionEvent.ACTION_DOWN) {        onUserInteraction();    }    // 首先分发给Window去处理，如果该事件没有被消费掉，那么    // 会调用Activity的ouTouchEvent方法    if (getWindow().superDispatchTouchEvent(ev)) {        return true;    }    return onTouchEvent(ev);}

// PhoneWindow$superDispatchTouchEvent@Overridepublic boolean superDispatchTouchEvent(MotionEvent event) {    return mDecor.superDispatchTouchEvent(event);}

// DecorView$superDispatchTouchEventpublic boolean superDispatchTouchEvent(MotionEvent event) {    return super.dispatchTouchEvent(event);}

// ViewGroup$dispatchTouchEvent@Overridepublic boolean dispatchTouchEvent(MotionEvent ev) {    ...    // 出于安全考虑，如果设置了窗口被遮挡后丢弃事件的属性，那么窗口被    // 遮挡时，事件直接被丢弃，而不会分发出去。也就是说，当出现了Toast、    // 对话框等的时候，触摸事件就不会被响应了。该属性一般不会被设置。    if (onFilterTouchEventForSecurity(ev)) {        ...        // Handle an initial down.        if (actionMasked == MotionEvent.ACTION_DOWN) {            // Throw away all previous state when starting a new touch gesture.            // The framework may have dropped the up or cancel event for the previous gesture            // due to an app switch, ANR, or some other state change.            cancelAndClearTouchTargets(ev);            // 这个方法中会重置FLAG_DISALLOW_INTERCEPT标识，该标识表示当前            // ViewGroup不能拦截触摸事件，也就是说，无论如何该标识都不能控制            // ACTION_DOWN事件的拦截            resetTouchState();        }        // 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.            // 也就是说如果ViewGorup拦截了某触摸手势的ACTION_DOWN事件后，            // 该触摸手势的后续事件都交给这个ViewGroup进行处理，不再需要用            // onInterceptTouchEvent判断是否拦截            intercepted = true;        }        ...    }}...

// 还是ViewGroup$dispatchTouchEvent@Overridepublic boolean dispatchTouchEvent(MotionEvent ev) {    ...    // ViewGroup不拦截时的事件分发    for (int i = childrenCount - 1; i >= 0; i--) {        final int childIndex = customOrder                ? getChildDrawingOrder(childrenCount, i) : i;        final View child = (preorderedList == null)                ? children[childIndex] : preorderedList.get(childIndex);        // If there is a view that has accessibility focus we want it        // to get the event first and if not handled we will perform a        // normal dispatch. We may do a double iteration but this is        // safer given the timeframe.        if (childWithAccessibilityFocus != null) {            if (childWithAccessibilityFocus != child) {                continue;            }            childWithAccessibilityFocus = null;            i = childrenCount - 1;        }        if (!canViewReceivePointerEvents(child)                || !isTransformedTouchPointInView(x, y, child, null)) {            ev.setTargetAccessibilityFocus(false);            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();            if (preorderedList != null) {                // childIndex points into presorted list, find original index                for (int j = 0; j < childrenCount; j++) {                    if (children[childIndex] == mChildren[j]) {                        mLastTouchDownIndex = j;                        break;                    }                }            } else {                mLastTouchDownIndex = childIndex;            }            mLastTouchDownX = ev.getX();            mLastTouchDownY = ev.getY();            newTouchTarget = addTouchTarget(child, idBitsToAssign);            alreadyDispatchedToNewTouchTarget = true;            break;        }        // The accessibility focus didn't handle the event, so clear        // the flag and do a normal dispatch to all children.        ev.setTargetAccessibilityFocus(false);    }    ...    if (mFirstTouchTarget == null) {        // ViewGroup进行拦截或者在子View中没有处理事件时，ViewGroup        // 调用父类View的dispatchTouchEvent进行处理。        // No touch targets so treat this as an ordinary view.        handled = dispatchTransformedTouchEvent(ev, canceled, null,                TouchTarget.ALL_POINTER_IDS);    }    ...}

明显，在正常的事件（通常是ACTION_DOWN）分发时，ViewGroup会依次查询每个子View，如果子View能够接收到触摸事件，就会把事件交给该子View去处理。判断是否能接收到触摸事件有两点：view没有在播放动画，也没有准备播放动画；触摸事件的坐标处于view的范围内。子View收到事件后，会调用自己的dispatchTouchEvent来进一步分发。如果子View处理了该事件，那么mFirstTouchTarget就会在addTouchTarget方法中被赋值，并且终止事件的分发。在之前的代码中，可以看到，如果mFirstTouchTarget为null，也就是没有任何子View处理该事件（通常是ACTION_DOWN），那么接下来的所有事件都会被当前的ViewGroup拦截。

// ViewGroup$dispatchTransformedTouchEventprivate boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,        View child, int desiredPointerIdBits) {    ...    // Perform any necessary transformations and dispatch.    if (child == null) {        handled = super.dispatchTouchEvent(transformedEvent);    } else {        final float offsetX = mScrollX - child.mLeft;        final float offsetY = mScrollY - child.mTop;        transformedEvent.offsetLocation(offsetX, offsetY);        if (! child.hasIdentityMatrix()) {            transformedEvent.transform(child.getInverseMatrix());        }        handled = child.dispatchTouchEvent(transformedEvent);    }    ...}

// View$dispatchTouchEventpublic boolean dispatchTouchEvent(MotionEvent event) {    ...    boolean result = false;    ...    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;        }    }    ...    return result;}

View在处理触摸事件时，会先判断是否设置了OnTouchListener，如果设置了而且onTouch方法处理了该事件，就不再调用onTouchEvent进行处理；否则才会调用onTouchEvent。明显，onTouch优先级高于onTouchEvent。

// View$onTouchEventpublic boolean onTouchEvent(MotionEvent event) {    ...    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:                // 如果在ListView等可以滚动的容器中ACTION_DOWN时，                // 会有一个延迟之后，才会置位PFLAF_PRESSED，此前都是                // PFLAG_PREPRESSED被置位。                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();                            }                        }                    }                }                break;                ...        }        return true;    }    return false;}

明显，只要是CLICKABLE或者LONG_CLICKABLE或者CONTEXT_CLICKABLE的，不管View是否是enabled状态，都会消费点击事件。如果View是enabled的，那么会看是否设置了代理，如果有代理，那么先调用代理的onTouchEvent方法看是否被消费掉，如果没有消费，那么才会真正由View来做处理。在ACTION_UP时如果点击事件没有被OnLongClickListener.onLongClick方法消费掉，那么就会调用OnClickListener的onClick方法。

// View$performLongClickpublic boolean performLongClick() {    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);    boolean handled = false;    ListenerInfo li = mListenerInfo;    if (li != null && li.mOnLongClickListener != null) {        handled = li.mOnLongClickListener.onLongClick(View.this);    }    if (!handled) {        handled = showContextMenu();    }    if (handled) {        performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);    }    return handled;}

// View$performClickpublic 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);    return result;}