事件分发源码解析+总结

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);
        }
    }