Android事件分发拦截机制(图解)

转载请注明出处：http://blog.csdn.net/kiddTeb/article/details/52562567

前言

• 祝中秋佳节愉快！本文为了自己复习留下一个脚印，有错误望指正，望轻喷~~

分析

• 在我们平常开发过程中，有时候有出现View与ViewGroup的嵌套，也就是view放在一个viewGroup里面，而这个viewGroup又放在这个另外一个viewGroup当中,那么这个时候的触摸事件应该分配给谁呢？这就涉及了事件分发拦截机制。
• 首先分析一下，ViewGroup中关键的几个函数

public boolean dispatchTouchEvent(MotionEvent ev){...}

/**     * 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.     */public boolean onInterceptTouchEvent(MotionEvent ev) {        return false;    }

• 再来看看View中的几个关键函数

/**     * Checks a touch event.     * @param event The event.     * @param nestingLevel The nesting level: 0 if called from the base class,     * or 1 from a subclass.  If the event was already checked by this consistency verifier     * at a higher nesting level, it will not be checked again.  Used to handle the situation     * where a subclass dispatching method delegates to its superclass's dispatching method     * and both dispatching methods call into the consistency verifier.     */public boolean onTouchEvent(MotionEvent event){    。。}

/**     * 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){    。。}

• 上面的几个函数在后面都会讲到，现在可以看看官方注释留个大概印象。
  

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小案例

mRelativeLayout1.setOnTouchListener(new OnTouchListener() {    @Override    public boolean onTouch(View v, MotionEvent event) {        Log.d("TAG", "mRelativeLayout1 on touch");        return false;    }});mRelativeLayout2.setOnTouchListener(new OnTouchListener() {    @Override    public boolean onTouch(View v, MotionEvent event) {        Log.d("TAG", "mRelativeLayout2 on touch");        return false;    }});mButton.setOnClickListener(new OnClickListener() {    @Override    public void onClick(View v) {        Log.d("TAG", "clicked button");    }});

• 假如一个屏幕内有两个Viewgroup和一个view进行嵌套，那么如果我们点击button的时候会出现什么情况呢？可以打印log出来观察一下会发现 “clicked button” 。这应该很熟悉，那么这个事件是如何进行分发的呢？

/**     * {@inheritDoc}     */    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);        }        // If the event targets the accessibility focused view and this is it, start        // normal event dispatch. Maybe a descendant is what will handle the click.        if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {            ev.setTargetAccessibilityFocus(false);        }        boolean handled = false;        if (onFilterTouchEventForSecurity(ev)) {            final int action = ev.getAction();            final int actionMasked = action & MotionEvent.ACTION_MASK;            // 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);                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.                intercepted = true;            }            // If intercepted, start normal event dispatch. Also if there is already            // a view that is handling the gesture, do normal event dispatch.            if (intercepted || mFirstTouchTarget != null) {                ev.setTargetAccessibilityFocus(false);            }            // Check for cancelation.            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) {                // If the event is targeting accessiiblity focus we give it to the                // view that has accessibility focus and if it does not handle it                // we clear the flag and dispatch the event to all children as usual.                // We are looking up the accessibility focused host to avoid keeping                // state since these events are very rare.                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();                        final boolean customOrder = preorderedList == null                                && isChildrenDrawingOrderEnabled();                        final View[] children = mChildren;                        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 (preorderedList != null) preorderedList.clear();                    }                    if (newTouchTarget == null && mFirstTouchTarget != null) {                        // Did not find a child to receive the event.                        // Assign the pointer to the least recently added target.                        newTouchTarget = mFirstTouchTarget;                        while (newTouchTarget.next != null) {                            newTouchTarget = newTouchTarget.next;                        }                        newTouchTarget.pointerIdBits |= idBitsToAssign;                    }                }            }            // 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);            } else {                // Dispatch to touch targets, excluding the new touch target if we already                // dispatched to it.  Cancel touch targets if necessary.                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;                }            }            // Update list of touch targets for pointer up or cancel, if needed.            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) {            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);        }        return handled;    }

/**     * Transforms a motion event into the coordinate space of a particular child view,     * filters out irrelevant pointer ids, and overrides its action if necessary.     * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.     */    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;    }

• 当我们点击按钮的时候，会去调用这个按钮所在布局的dispatchTouchEvent方法，在这个方法第92行中会遍历这个布局中的所有子View，在168和182行都有调用dispatchTransformedTouchEvent方法，那么我们进入这个方法看看，在这个方法中的15-19行就有个判断，如果孩子是否为空来决定调用父类的还是调用孩子的dispatchTouchEvent方法
  
• 事件的大概方向就是如此，从Viewgroup往View下进行一个传递，最后再逐级往上面ViewGroup回传结果。其中ViewGroup中的onInterceptTouchEvent方法，返回一个boolean值，默认是返回false。看看官方的注释就知道，如果返回false的时候，是不会对事件进行一个拦截，如果我们去重写这个方法返回true的话，那么这个事件就会在这里被拦截掉，从而不会再往下进行一个分发。
• 另外还有一些要注意的

/**     * 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;    }

• 在子View的dispatchTouchEvent方法中，第34-38行中有个判断，第一个值在getListenerInfo()方法中new出一个mListenerInfo，只要有添加监听就不会为空，至于第二个值只要注册setOnTouchListener(OnTouchListener l)就不会为空，第三个值是判断当前点击的控件是否是enable的，按钮默认都是enable的，因此这个条件恒定为true，第四个值呢，比较关键，只要这里返回true那么整个方法就会返回true，这就取决去在注册setOnTouchListener的时候你返回了什么值，如果你返回了true那么事件在这里就会被消费掉，不会再执行按钮的onClick，因此如果你在setOnTouchListener中返回true，你又在按钮注册了setOnClickListener的话，你试着在onClick中打印下日志就可以知道，是不会打印出来的。这就说明事件被消费掉了。这就是在View中的事件分发。
• 但是如果在setOnTouchListener返回来的是false，那么显然onClick是没有问题的，是可以打印出来的，那就说明了按钮的onClick是在第40行中的onTouchEvent(event)方法中的，这就不列源码出来了，其实在这个方法中就是进行一个switch的判断当前事件是抬起手指还是手指移动还是手指抬起，对应进入case中，MotionEvent.ACTION_UP这个case当中有一个performClick()方法，进入这个方法就很明确能找到mOnClickListener.onClick(this);这一句关键代码，就是在这里进行按钮的onClick的。值得留意的是，ACTION_DOWN，ACTION_MOVE，ACTION_UP等事件中只有前一个事件返回true，后一个事件才能被触发。
  

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总结

• 最后画一张总结图来总结下知识点