Android ViewGroup事件分发

 @Override
   public boolean dispatchTouchEvent(MotionEvent ev) {
           //...
   		boolean handled = false;
   		//...


   		//1.取消之前的手势
           // 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();
        }

		//2.判断是否拦截
        // Check for interception.
           final boolean intercepted;
           if (actionMasked == MotionEvent.ACTION_DOWN
                   || mFirstTouchTarget != null) { //DOWN
           	//父类是否拦截  getParent().requestDisallowInterceptTouchEvent();来改变值
               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;
           }

    		 //....
		//3.0   如果是不取消不拦截为down，并且dispatchTransformedTouchEvent返回为true的时候会为 mFirstTouchTarget赋值
			// 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;
           //3.1 如果不取消并且不拦截的情况下，
           if (!canceled && !intercepted) {
           	 if (actionMasked == MotionEvent.ACTION_DOWN
                       || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                       || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {// 3.2 DOWN的时候
           	 	//...
				if (newTouchTarget == null && childrenCount != 0) { 
					//...
					final View[] children = mChildren;
                       for (int i = childrenCount - 1; i >= 0; i--) {//3.3 反序for循环，为了先拿到上层的view
                       	//...
                       	//3.4 拿到child
							final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
							//...
							//3.5 根据child给newTouchTarget赋值   DOWN的时候因为 mFirstTouchTarget==null  所以进不去  返回的是null
							newTouchTarget = getTouchTarget(child);
                       }
                      //...
                       //3.6. 执行操作 是执行自己的dispatchTouchEvent还是child的dispatchTouchEvent
                       if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {

                       	//...
                       	//3.7 子View如果返回true添加一个newTouchTarget  并且为mFirstTouchTarget赋值
                       	 newTouchTarget = addTouchTarget(child, idBitsToAssign);
                       	 //....
                         }
           		}	
           	}
           }
//...
			// Dispatch to touch targets.
           if (mFirstTouchTarget == null) {//执行自身的dispatchTouchEvent
               // No touch targets so treat this as an ordinary view.
               handled = dispatchTransformedTouchEvent(ev, canceled, null,
                       TouchTarget.ALL_POINTER_IDS);
           } else {// mFirstTouchTarget已经赋值
               // 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) {//执行完3.7操作的
                       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;
               }
           }



           return handled;
   }

/**
     * Cancels and clears all touch targets.
     */
    private void cancelAndClearTouchTargets(MotionEvent event) {
        if (mFirstTouchTarget != null) {
            boolean syntheticEvent = false;
            if (event == null) {
                final long now = SystemClock.uptimeMillis();
                event = MotionEvent.obtain(now, now,
                        MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
                event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
                syntheticEvent = true;
            }

            for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
                resetCancelNextUpFlag(target.child);
                dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
            }
            clearTouchTargets();

            if (syntheticEvent) {
                event.recycle();
            }
        }
    }

    //清楚所有的TouchTarget
    /**
     * Clears all touch targets.
     */
    private void clearTouchTargets() {
        TouchTarget target = mFirstTouchTarget;
        if (target != null) {
            do {
                TouchTarget next = target.next;
                target.recycle();
                target = next;
            } while (target != null);
            mFirstTouchTarget = null;
        }
    }

	
	//根据childVie得到TouchTarget
	/**
     * Gets the touch target for specified child view.
     * Returns null if not found.
     */
    private TouchTarget getTouchTarget(@NonNull View child) {
    	// DOWN的时候因为 mFirstTouchTarget==null  所以进不去  返回的是null
        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
            if (target.child == child) {
                return target;
            }
        }
        return null;
    }




    /**
     * 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) {
    	//伪代码
       	 final boolean handled;
            if (child == null) {//执行View.dispatchTouchEvent  也就是自己的dispatchTouchEvent
                handled = super.dispatchTouchEvent(event);
            } else {//执行child的dispatchTouchEvent
                handled = child.dispatchTouchEvent(event);
            }
            return handled;
	}



	//添加TouchTarget 并且给mFirstTouchTarget赋值
	/**
     * Adds a touch target for specified child to the beginning of the list.
     * Assumes the target child is not already present.
     */
    private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
        target.next = mFirstTouchTarget;
        mFirstTouchTarget = target;
        return target;
    }