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事件分发源码解析+总结

事件分发源码解析+总结

作者: allsunny | 来源:发表于2021-06-21 17:32 被阅读0次

    1、View 只有消费了 ACTION_DOWN 事件,才能接收到后续的事件(可点击控件会默认消费所有事件),并且会将后续所有事件传递过来,不会再传递给其他 View,除非上层 View 进行了拦截。
    结论:如果在自定义View中重写了onTouchEvent(),且case:MotionEvent.ACTION_DOWN中返回了false,则onClick()无法生效,因为onClick在MotionEvent.ACTION_UP中。
    2、parent.requestDisallowInterceptTouchEvent()决定了是否进入ViewGroup.onInterceptTouchEvent()方法。

    代码解释:

        boolean ViewGroup.dispatchTouchEvent(){ 
            
            // Handle an initial down. 仅对于ACTION_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();  // 重置mGroupFlags对FLAG_DISALLOW_INTERCEPT的状态,可以理解成把disallowIntercept重置为false->intercepted = false;
            }
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;  // disallowIntercept默认false
                if (!disallowIntercept) {
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;  // disallowIntercept = false->intercepted = false,不拦截事件
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.
                intercepted = true;
            }
        
            // 如果拦截了。则mFirstTouchTarget == null,直接进入ViewGroup.onTouchEvent()处理事件阶段
            if (!canceled && !intercepted) {
                // 只有down和move事件
                if (actionMasked == MotionEvent.ACTION_DOWN
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE){
                    
                    /**
                    *   如果View不消费 ACTION_DOWN 事件(onTouchEvent()的ACTION_DOWN返回false),则dispatchTransformedTouchEvent()返回false,
                    *   则mFirstTouchTarget = null,后续事件的dispatchTransformedTouchEvent()方法的child参数为null,不会将事件传给子View      
                    **/
                    if (dispatchTransformedTouchEvent()){
                        // addTouchTarget()中给mFirstTouchTarget赋值
                        newTouchTarget = addTouchTarget(child, idBitsToAssign);
                    }   
                }
            }
            // 如果拦截了。则mFirstTouchTarget == null,直接进入ViewGroup.onTouchEvent()处理事件阶段
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                // child参数为null,相当于ViewGroup.onTouchEvent()
                handled = dispatchTransformedTouchEvent(ev, canceled, child:null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
                if (dispatchTransformedTouchEvent(ev, cancelChild,target.child, target.pointerIdBits)) {
                    handled = true;
                }
            }
            return handled;
        }
    
    ---------------------ViewGroup.dispatchTransformedTouchEvent---------------------------------------
    
        boolean ViewGroup.dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, child, int desiredPointerIdBits) {
            // 没有 child,将当前 ViewGroup 当作普通的 View 处理。
            if (child == null) {
                handled = super.dispatchTouchEvent(transformedEvent); //即ViewGroup.onTouchEvent() -> View.onTouchEvent()
            }
            return handled;
        }
    
    ------------------View.dispatchTouchEvent----------------------------
        public boolean dispatchTouchEvent(MotionEvent event) {
            if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED  // 注意ENABLED标志
                && mOnTouchListener.onTouch(this, event)){      //处理
                return true;
            }
            return onTouchEvent(event));  //处理
        }
    
    -------------源码补充ViewGroup.requestDisallowInterceptTouchEvent-------
        @Override
        public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
    
            if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
                // We're already in this state, assume our ancestors are too
                return;
            }
    
            if (disallowIntercept) {
                // 0|0=0  0|1=1
                // true,则mGroupFlags被FLAG_DISALLOW_INTERCEPT置位。
                mGroupFlags |= FLAG_DISALLOW_INTERCEPT;  
            } else {
                // 0&0=0  0&1=0  1&0=0  1&1=1
                // 先取反(优先级高),再与。
                // 重置mGroupFlags对FLAG_DISALLOW_INTERCEPT的状态,可以理解成把disallowIntercept重置为false
                // 即resetTouchState()
                mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT; 
            }
    
            // Pass it up to our parent
            if (mParent != null) {
                mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
            }
        }
    

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