ViewGroup 的事件分发机制

Posted 2023-02-23 Young_xiaoT

ViewGroup 的事件分发机制

public boolean dispatchTouchEvent(MotionEvent ev)

用来进行事件的分发，如果能传递给当前的View,那么此方法一定会被调用，返回结果受当前View 的 onTouchEvent 和下级View 的 dispatchTouchEvent 方法的影响，表示是否消耗当前事件。

public boolean onInterceptTouchEvent(MotionEvent event)

在上述的方法中调用，用来判断是否拦截某个事件，如果当前View拦截了某个事件，那么在同一个事件序列中，此方法不会被再次调用，返回结果表示是否拦截当前事件。

public boolean onTouchEvent(MotionEvent event)

在 dispatchTouchEvent 方法中调用，用来处理点击事件，返回结果表示是否消耗当前事件，如果不消耗，则在同一个事件序列中，当前View无法再次接收到事件。

上述三者的关系可以用下面的伪代码来表示：

    public boolean dispatchTouchEvent(MotionEvent event)
        boolean consume  = false;

        if (onInterceptTouchEvent(event))
            consume = onTouchEvent(event);
        else 
            consume = child.dispatchTouchEvent(event);
        
        return consume;
    

通过上面的伪代码，可以大致了解点击事件的传递规则：对于一个根ViewGroup来说，点击事件产生后，首先会传递给它，它的 dispatchTouchEvent 就会被调用，如果这个ViewGroup的 onInterceptTouchEvent方法返回true，表示要拦截当前事件，接着这个事件就会交给这个 ViewGroup 处理，即它的 onTouchEvent 方法就会被调用；如果这个ViewGroup的 onInterceptTouchEvent方法返回false就表示它不拦截当前事件，这时事件就会继续传递给它的子元素，接着子元素的dispatchTouchEvent方法就会被调用，如此反复直到事件被最终处理。

看一下ViewGroup的dispatchTouchEvent方法：

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;