Android View体系（五）从源码解析View的事件分发机制

Android View体系（五）从源码解析View的事件分发机制

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前言

三年前写过事件分发机制的文章但是写的不是很好，所以重新再写一篇，关于事件分发机制的文章已经有很多，但是希望我这篇是最简洁、最易懂的一篇。

1.处理点击事件的方法

View的层级

我们知道View的结构是树形的结构，View可以放在ViewGroup中，这个ViewGroup也可以放到另一个ViewGroup中，这样层层的嵌套就组成了View的层级。

什么是点击事件分发

当我们点击屏幕，就产生了触摸事件，这个事件被封装成了一个类：MotionEvent。而当这个MotionEvent产生后，那么系统就会将这个MotionEvent传递给View的层级，MotionEvent在View的层级传递的过程就是点击事件分发。

点击事件分发的重要方法

点击事件有三个重要的方法它们分别是：

dispatchTouchEvent(MotionEvent ev)：用来进行事件的分发 onInterceptTouchEvent(MotionEvent ev)：用来进行事件的拦截，在dispatchTouchEvent()中调用，需要注意的是View没有提供该方法 onTouchEvent(MotionEvent ev)：用来处理点击事件，在dispatchTouchEvent()方法中进行调用

为了了解这三个方法的关系，我们先来看看ViewGroup的dispatchTouchEvent()方法的部分源码：

 @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
       ...省略
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                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;
            }

           ...省略
        return handled;
    }

很明显在dispatchTouchEvent()方法中调用了onInterceptTouchEvent()方法来判断是否拦截事件，来看看onInterceptTouchEvent()方法：

 public boolean onInterceptTouchEvent(MotionEvent ev) {
        return false;
    }

onInterceptTouchEvent()方法默认返回false，不进行拦截，接着来看看dispatchTouchEvent()方法剩余的部分源码：

 public boolean dispatchTouchEvent(MotionEvent ev) {
 ...省略
              final View[] children = mChildren;
              for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder
                                    ? getChildDrawingOrder(childrenCount, i) : i;
                            final View child = (preorderedList == null)
                                    ? children[childIndex] : preorderedList.get(childIndex);

                            // If there is a view that has accessibility focus we want it
                            // to get the event first and if not handled we will perform a
                            // normal dispatch. We may do a double iteration but this is
                            // safer given the timeframe.
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }

                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }

                            newTouchTarget = getTouchTarget(child);
                            if (newTouchTarget != null) {
                                // Child is already receiving touch within its bounds.
                                // Give it the new pointer in addition to the ones it is handling.
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }

                            resetCancelNextUpFlag(child);
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                // Child wants to receive touch within its bounds.
                                mLastTouchDownTime = ev.getDownTime();
                                if (preorderedList != null) {
                                    // childIndex points into presorted list, find original index
                                    for (int j = 0; j < childrenCount; j++) {
                                        if (children[childIndex] == mChildren[j]) {
                                            mLastTouchDownIndex = j;
                                            break;
                                        }
                                    }
                                } else {
                                    mLastTouchDownIndex = childIndex;
                                }
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }

                            // The accessibility focus didn't handle the event, so clear
                            // the flag and do a normal dispatch to all children.
                            ev.setTargetAccessibilityFocus(false);
                        }

 ...省略

}

我们看到了for循环，首先遍历ViewGroup的子元素，判断子元素是否能够接收到点击事件，如果子元素能够接收到则交由子元素来处理。接下来看看37行的dispatchTransformedTouchEvent()方法中实现了什么：

 private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
        final boolean handled;

        // Canceling motions is a special case.  We don't need to perform any transformations
        // or filtering.  The important part is the action, not the contents.
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }
  ...省略      
 }       

如果有子View则调用子View的dispatchTouchEvent(event)方法。如果ViewGroup没有子View则调用super.dispatchTouchEvent(event)，ViewGroup是继承View的，我们再来看看View的dispatchTouchEvent(event)：

 public boolean dispatchTouchEvent(MotionEvent event) {
       ...省略
        boolean result = false;
        if (onFilterTouchEventForSecurity(event)) {
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null
                    && (m