#ViewGroup#dispatchTouchEvent摘要

// 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();    //每一次down重置mGroupFlags
}

// Check for interception.
final boolean intercepted;
//mFirstTouchTarget是child.dispatchTouchEvent返回ture时赋值为child。
if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null) {
   //可以利用child.getParent.requestDisallowInterceptTouchEvent设置 
   final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
   if (!disallowIntercept) {
       intercepted = onInterceptTouchEvent(ev); //重写此方法可以自行判断是否拦截
       ev.setAction(action); // restore action in case it was changed
   } else {
       //设置之后ViewGroup不再拦截事件。
       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;
 }
            

从代码中可以看到TouchEvent拦截的一些规则：

1 VIew没有办法通过requestDisallowIntercepTouchEvent阻止View'Group拦截Down事件，因为Donw事件会重置mGroupFlags

2 假如在Down事件，所有child.dispatchTouchEvent均返回false。那么接下来的所有事件，ViewGroup将会直接拦截。

3 只有在Down事件，VIewGroup.onInterceptTouchEvent返回false，child.dispatchTouchEvent返回true。接下来的事件才会继续判断哪一个拦截，child可以通过requestDisallowInterceptTouchEvent阻止ViewGroup拦截事件，优先级比VIewGroup.onInterceptTouchEvent高