Update View 事件分发.md

blogs/Android/Framework/深入理解 Android 内核设计思想/View 事件分发.md

@@ -52,3 +52,58 @@ public boolean dispatchTouchEvent (MotionEvent event) {
 
 #### ViewGroup 中 TouchEvent 的投递流程
 
+ViewGroup 因为涉及对子 View 的处理，其派发流程没有 View 那么简单直接，它重写了 dispatchTouchEvent 方法：
+
+```java
+public boolean dispatchTouchEvent(MotionEvent ev) {
+    boolean handled = false;
+    if (onFilterTouchEventForSecurity(ev)) {
+        final int action = ev.getAction();
+        final int actionMasked = action & MotionEvent.ACTION_MASK;
+        if (actionMasked == MotionEvent.ACTION_DOWN) {
+            cancelAndClearTouchTargets(ev);
+            resetTouchState();
+        }
+
+        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); 
+            } else {
+                intercepted = false;
+            }
+        } else {
+            intercepted = true;
+        }
+        // 不拦截的情况
+        if (actionMasked == MotionEvent.ACTION_DOWN
+            || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
+            || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
+            final int actionIndex = ev.getActionIndex();
+            final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
+                : TouchTarget.ALL_POINTER_IDS;
+            final int childrenCount = mChildrenCount;
+            if (newTouchTarget == null && childrenCount != 0) {
+                final float x = ev.getX(actionIndex);
+                final float y = ev.getY(actionIndex);
+                final View[] children = mChildren;
+                for (int i = childrenCount - 1; i >= 0; i--) {
+                    if (!child.canReceivePointerEvents()
+                        || !isTransformedTouchPointInView(x, y, child, null)) {
+                        continue;
+                        newTouchTarget = getTouchTarget(child);
+                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
+                            break;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+如果 ViewGroup 允许拦截，就调用其 onInterceptTouchEvent 来判断是否要真正执行拦截了，这样做了是方便扩展。如果不拦截，就从后遍历子 View 处理。它有两个函数可以过滤子 View，一个是判断这个子 View 能否接收 Pointer Events 事件，另一个是判断落点有没有落在子 View 范围内。如果都满足，则调用其 dispatchTouchEvent 处理。如果该子 View 是一个 ViewGroup 就继续调用其 dispatchTouchEvent，否则就是 View 的 dispatchTouchEvent。如此循环往复，直到事件真正被处理。