View事件分发源码详解

用户触摸屏幕后,会生成touch事件,之后应用可以在Looper中获得当前应用需要处理的touch事件,接下来交给ViewRootImpl去处理,ViewRootImpl确定该事件是touch事件后,传递给当前activity的phoneWindow里的DecorView的dispatchTouchEvent方法

本文主要回答以下问题:
1. dispatchTouchEvent过程,怎么找到目标View来处理事件的?
2. onTouch超出View的范围时,会发生什么?
3. View的onClick方法能否和onTouch方法共用?
4. View是按照什么顺序获得touch事件的?如果有View不处理时,又如何?

如果已经知道具体答案的童鞋,那么本篇文章就可以跳过了

源码解析

先看decorView的

        @Override        public boolean dispatchTouchEvent(MotionEvent ev) {            final Callback cb = getCallback();//cb即为Activity,也就是下面的代码会调用Activity的dispatchTouchEvent            return cb != null && !isDestroyed() && mFeatureId < 0 ? cb.dispatchTouchEvent(ev)                    : super.dispatchTouchEvent(ev);        }

解释:decorView的getCallBack()为何为Activity,原因是Actyvity的attach方法中调用了以下两行代码,attach则会在Activity创建过程中被调用:

        mWindow = new PhoneWindow(this);        mWindow.setCallback(this);

Activity的dispatchTouchEvent方法:

    public boolean dispatchTouchEvent(MotionEvent ev) {        if (ev.getAction() == MotionEvent.ACTION_DOWN) {            onUserInteraction();//默认空方法,可重写来查看事件        }        if (getWindow().superDispatchTouchEvent(ev)) {//调用PhoneWindow的dispatchTouchEvent方法            return true;        }        //如果所有View都不处理touch事件,那么调用Activity的onTouchEvent        return onTouchEvent(ev);    }

PhoneWindow的dispatchTouchEvent方法实际是ViewGroup的dispatchTouchEvent方法,源码及解释:

@Override    public boolean dispatchTouchEvent(MotionEvent ev) {        if (mInputEventConsistencyVerifier != null) {//检查事件是否一致            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);        }        if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {            ev.setTargetAccessibilityFocus(false);        }        boolean handled = false;        if (onFilterTouchEventForSecurity(ev)) {//如果View被遮挡,返回false,不继续分发(遮挡可能的方式:)            final int action = ev.getAction();            final int actionMasked = action & MotionEvent.ACTION_MASK;            if (actionMasked == MotionEvent.ACTION_DOWN) {                //当开始一个ACTION_DOWN时,清除原先的状态,因为当遇到ANR,切换APP等变化时,事件可能被丢弃或取消                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);//先看自己是否拦截该事件                    ev.setAction(action); // restore action in case it was changed                } else {                    intercepted = false;                }            } else {                //事件不是touch_down事件(之前已经处理touch_down事件,现在还有其他事件,说明之前进行了拦截和处理) 或者 没有处理这个事件的view(包含在mFirstTouchTarget中),那么继续拦截事件                intercepted = true;            }            //如果被打断了,或已经有View处理这个事件了,就将这个事件作为普通事件处理            if (intercepted || mFirstTouchTarget != null) {                ev.setTargetAccessibilityFocus(false);            }            // 检查是否取消事件            final boolean canceled = resetCancelNextUpFlag(this)                    || actionMasked == MotionEvent.ACTION_CANCEL;            // 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) {//不是取消事件 并且当前view没拦截                View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()                        ? findChildWithAccessibilityFocus() : null;                if (actionMasked == MotionEvent.ACTION_DOWN                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {                    final int actionIndex = ev.getActionIndex(); // always 0 for down                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)                            : TouchTarget.ALL_POINTER_IDS;                    // Clean up earlier touch targets for this pointer id in case they                    // have become out of sync.                    removePointersFromTouchTargets(idBitsToAssign);                    final int childrenCount = mChildrenCount;                    if (newTouchTarget == null && childrenCount != 0) {                        final float x = ev.getX(actionIndex);                        final float y = ev.getY(actionIndex);                        // Find a child that can receive the event.                        // Scan children from front to back.                        final ArrayList<View> preorderedList = buildOrderedChildList();//按照Z向量值由小到大                        final boolean customOrder = preorderedList == null                                && isChildrenDrawingOrderEnabled();                        final View[] children = mChildren;                        for (int i = childrenCount - 1; i >= 0; i--) {//这里是倒序                        //所以以下基本上是先看获得焦点的view(被点击的view),如果被点击的view,不处理,那么处理child的顺序 : Z值大 -> 小 (android5.0后) 或 view重叠顺序上层 -> 下层                            final int childIndex = customOrder                                    ? getChildDrawingOrder(childrenCount, i) : i;                            final View child = (preorderedList == null)                                    ? children[childIndex] : preorderedList.get(childIndex);                            //优先让焦点View处理这个事件                            if (childWithAccessibilityFocus != null) {                                if (childWithAccessibilityFocus != child) {                                    continue;                                }                                childWithAccessibilityFocus = null;                                i = childrenCount - 1;                            }                            //canViewReceivePointerEvents判断view的状态是可以接受点击事件的                            //isTransformedTouchPointInView判断点击事件在View的范围内                            if (!canViewReceivePointerEvents(child)                                    || !isTransformedTouchPointInView(x, y, child, null)) {                                ev.setTargetAccessibilityFocus(false);                                continue;                            }                            newTouchTarget = getTouchTarget(child);                            if (newTouchTarget != null) {//已经处理过这个touch事件了,避免重复操作                                newTouchTarget.pointerIdBits |= idBitsToAssign;                                break;                            }                            resetCancelNextUpFlag(child);                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {                            //这个方法会对Event进行一些转换,再调用child.dispatchTouchEvent方法,如果希望处理                                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 (preorderedList != null) preorderedList.clear();                    }                    if (newTouchTarget == null && mFirstTouchTarget != null) {                        // 没有找到View去处理这个event                        // 把指针交给最久添加的target                        newTouchTarget = mFirstTouchTarget;                        while (newTouchTarget.next != null) {                            newTouchTarget = newTouchTarget.next;                        }                        newTouchTarget.pointerIdBits |= idBitsToAssign;                    }                }            }            // Dispatch to touch targets.            if (mFirstTouchTarget == null) {                // child没有在点击范围时,或者都没处理时,让该View自己来处理                handled = dispatchTransformedTouchEvent(ev, canceled, null,                        TouchTarget.ALL_POINTER_IDS);            } else {                // 分发event给目标view,除了之前那些分发过的view                TouchTarget predecessor = null;                TouchTarget target = mFirstTouchTarget;                while (target != null) {                    final TouchTarget next = target.next;                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {                        handled = true;                    } else {                        final boolean cancelChild = resetCancelNextUpFlag(target.child)                                || intercepted;                        if (dispatchTransformedTouchEvent(ev, cancelChild,                                target.child, target.pointerIdBits)) {                            handled = true;                        }                        if (cancelChild) {                            if (predecessor == null) {                                mFirstTouchTarget = next;                            } else {                                predecessor.next = next;                            }                            target.recycle();                            target = next;                            continue;                        }                    }                    predecessor = target;                    target = next;                }            }            // 做一些重置操作            if (canceled                    || actionMasked == MotionEvent.ACTION_UP                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {                resetTouchState();            } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {                final int actionIndex = ev.getActionIndex();                final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);                removePointersFromTouchTargets(idBitsToRemove);            }        }        if (!handled && mInputEventConsistencyVerifier != null) {//事件未处理时,作一些忽略操作            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);        }        return handled;    }

该方法如果child不为null,就要把Event进行一些转换,转成适合child处理的Event,在让child.dispatchTouchEvent,否则交给父类super.dispatchTouchEvent处理

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;        }        // Calculate the number of pointers to deliver.        final int oldPointerIdBits = event.getPointerIdBits();        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;        // If for some reason we ended up in an inconsistent state where it looks like we        // might produce a motion event with no pointers in it, then drop the event.        if (newPointerIdBits == 0) {            return false;        }        // If the number of pointers is the same and we don't need to perform any fancy        // irreversible transformations, then we can reuse the motion event for this        // dispatch as long as we are careful to revert any changes we make.        // Otherwise we need to make a copy.        final MotionEvent transformedEvent;        if (newPointerIdBits == oldPointerIdBits) {            if (child == null || child.hasIdentityMatrix()) {                if (child == null) {                    handled = super.dispatchTouchEvent(event);                } else {                    final float offsetX = mScrollX - child.mLeft;                    final float offsetY = mScrollY - child.mTop;                    event.offsetLocation(offsetX, offsetY);                    handled = child.dispatchTouchEvent(event);                    event.offsetLocation(-offsetX, -offsetY);                }                return handled;            }            transformedEvent = MotionEvent.obtain(event);        } else {            transformedEvent = event.split(newPointerIdBits);        }        // 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);        }        // Done.        transformedEvent.recycle();        return handled;    }

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);            }            //一个disable的view但是是clickable时,也是可以接收事件的,只是不会做出xiang'ying响应            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) {//如果有Clickable,longClickable、ContextClickable后续直接返回true            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) {                            // 调用setPressed方法,该方法可重写                            setPressed(true, x, y);                       }                        if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {                            // This is a tap, so remove the longpress check                            removeLongPressCallback();                            // 只有在点击状态是调用onClick方法                            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;                    }                    // 查看是否在可scroll的容器里                    boolean isInScrollingContainer = isInScrollingContainer();                    if (isInScrollingContainer) {//如果在可scroll的容器里时,延迟事件分发,以确定是scroll事件,还是其他事件                        mPrivateFlags |= PFLAG_PREPRESSED;                        if (mPendingCheckForTap == null) {                            mPendingCheckForTap = new CheckForTap();                        }                        mPendingCheckForTap.x = event.getX();                        mPendingCheckForTap.y = event.getY();                        postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());                    } else {                        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)) {//如果移动到View外面,则不为点击状态                        // Outside button                        removeTapCallback();                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {                            // Remove any future long press/tap checks                            removeLongPressCallback();                            setPressed(false);                        }                    }                    break;            }            return true;        }        return false;    }

结论

• dispatchTouchEvent过程,怎么找到目标View的?
  目标View为第一个符合以下条件的View: 1 一个在touch范围内的 2 希望处理事件的 view
• onTouch超出View的范围时,会发生什么?
  由下面代码可知:touch事件仍然继续传递,但是onTouchEvent方法会取消click、longClick等状态,即不会调用onClick、onLongClick方法,如果touch始终在一个View的内部,那么click事件会被调用,即使手指移动了很长距离

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;

• View的onClick方法能否和onTouch方法共用?
  View的onClick方法调用在onTouch方法内部,所以如果自定义View希望继续支持onClick,需要调用super.onTouch,至于自定义View的返回值是true or false都不影响,如下面示例代码:

    @Override    public boolean onTouchEvent(MotionEvent event) {        Log.e(this.getClass().getSimpleName(),"onTouchEvent" + EventUtil.getEventType(event));        super.onTouchEvent(event);        return true;    }

• View是按照什么顺序获得touch事件的?如果有View不处理时,又如何?

  本View的onInterceptTouchEvent()方法 > 容器中获得焦点的View,一般就是被点击的View > 覆盖在上层的View/Z轴值更大的View > 覆盖在下层的View/Z轴值更小的View > 本View的super.dispatchTouchEvent
  如果有View不处理时,按照以上顺序不断传递,如果最后仍没有View处理,那么将调用Activity的onTouchEvent方法处理