继续分析DrawerLayout的手势分发部分

谈到手势分发，这本身就是个好话题，DrawerLayout作为继承自ViewGroup得布局他可以拦截手势也可以分发给子view，也就是在onInterceptTouchEvent中做的操作，但是他的下面还有一个onTouchEvent方法，先看哪个呢？追溯代码我们可以知道ViewGroup继承自View，而onTouchEvent是View的方法

我们还是先花点时间把两者的关系先确认再继续。

onInterceptTouchEvent和onTouchEvent---鸡和蛋？

定位到ViewGroup，可以发现onInterceptTouchEvent分定义如下，从它前面一段非常长的注释就可以看出其重要性和复杂，默认的返回是false

/**

 * Implement this method to intercept all touch screen motion events.  This

 * allows you to watch events as they are dispatched to your children, and

 * take ownership of the current gesture at any point.

 *

 * <p>Using this function takes some care, as it has a fairly complicated

 * interaction with {@link View#onTouchEvent(MotionEvent)

 * View.onTouchEvent(MotionEvent)}, and using it requires implementing

 * that method as well as this one in the correct way.  Events will be

 * received in the following order:

 *

 * <ol>

 * <li> You will receive the down event here.

 * <li> The down event will be handled either by a child of this view

 * group, or given to your own onTouchEvent() method to handle; this means

 * you should implement onTouchEvent() to return true, so you will

 * continue to see the rest of the gesture (instead of looking for

 * a parent view to handle it).  Also, by returning true from

 * onTouchEvent(), you will not receive any following

 * events in onInterceptTouchEvent() and all touch processing must

 * happen in onTouchEvent() like normal.

 * <li> For as long as you return false from this function, each following

 * event (up to and including the final up) will be delivered first here

 * and then to the target's onTouchEvent().

 * <li> If you return true from here, you will not receive any

 * following events: the target view will receive the same event but

 * with the action {@link MotionEvent#ACTION_CANCEL}, and all further

 * events will be delivered to your onTouchEvent() method and no longer

 * appear here.

 * </ol>

 *

 * @param ev The motion event being dispatched down the hierarchy.

 * @return Return true to steal motion events from the children and have

 * them dispatched to this ViewGroup through onTouchEvent().

 * The current target will receive an ACTION_CANCEL event, and no further

 * messages will be delivered here.

 */

public boolean onInterceptTouchEvent(MotionEvent ev) {

return false;

}

前两段告诉我们，复写onInterceptTouchEvent方法，可以实现监听所有的动作事件MotionEvent，在向子view传递事件前做我们需要的操作，当然这指的是和这个viewgroup相关的事件；同时我们需要慎重处理该函数，因为他和onTouchEvent关系非常紧密，下面是事件接收的顺序：

首先接收的的事按下事件，down事件，他可以被view处理也可以在自身的onTouchEvent里处理，所以实现onTouchEvent并且返回true，这样onTouchEvent继续才能收到down之后的其他事件，同时onInterceptTouchEvent不会在收到后续事件，因为已经转移到onTouchEvent处理了。

那么什么时候onInterceptTouchEvent会把后续事件转移到他的onTouchEvent呢？这取决于onInterceptTouchEvent的返回值，如果返回false，所有事件都会先分发到这里，然后再到目标view的onTouchEvent；相反如果返回true，那么onInterceptTouchEvent将不再收到后续事件，并且目标view会收到cancel事件，接着自身的onTouchEvent几首后续的事件。

这其实从名字来看是比较好理解的onInterceptTouchEvent表示在截取触摸事件的被调用的方法，既然是截取就可以直接吧事件截下来后不再往后传递，这是就是上面的第二种情况，返回true，即我们自己消耗了触摸事件，子view将没有机会得到唤醒。

大致意思就是如果希望自身消耗掉改事件就可以直接返回true，这一点和onTouchEvent的返回类似目的。

博客园有篇文章对这些事件分发做了很好的分析：http://www.cnblogs.com/sunzn/archive/2013/05/10/3064129.html

详细的阐述了了dispatchTouchEvent，onInterceptTouchEvent以及onTouchEvent之间的关系

现在我们回过头来看DrawerLayout里的分发是如何写的：

重写了后面两个方法，先看onInterceptTouchEvent:

@Override

public boolean onInterceptTouchEvent(MotionEvent ev) {

final int action = MotionEventCompat.getActionMasked(ev);

// "|" used deliberately here; both methods should be invoked.

final boolean interceptForDrag = mLeftDragger.shouldInterceptTouchEvent(ev) |

mRightDragger.shouldInterceptTouchEvent(ev);

boolean interceptForTap = false;

switch (action) {

case MotionEvent.ACTION_DOWN: {

final float x = ev.getX();

final float y = ev.getY();

mInitialMotionX = x;

mInitialMotionY = y;

if (mScrimOpacity > 0 &&

isContentView(mLeftDragger.findTopChildUnder((int) x, (int) y))) {

interceptForTap = true;

}

mDisallowInterceptRequested = false;

mChildrenCanceledTouch = false;

break;

}

case MotionEvent.ACTION_MOVE: {

// If we cross the touch slop, don't perform the delayed peek for an edge touch.

if (mLeftDragger.checkTouchSlop(ViewDragHelper.DIRECTION_ALL)) {

mLeftCallback.removeCallbacks();

mRightCallback.removeCallbacks();

}

break;

}

case MotionEvent.ACTION_CANCEL:

case MotionEvent.ACTION_UP: {

closeDrawers(true);

mDisallowInterceptRequested = false;

mChildrenCanceledTouch = false;

}

}

return interceptForDrag || interceptForTap || hasPeekingDrawer() || mChildrenCanceledTouch;

}

1.首先从touch event里面获取当前具体的action动作，MotionEventCompat.getActionMasked(ev)，内部实际上做了一次按位于操作event.getAction() & ACTION_MASK;

2.检查当前是否满足截取drag状态，用于决定onInterceptTouchEvent返回值，这里有个注解说是故意用了|或，而不是||或，两者区别在于||只要第一个条件满足就不在执行第二个检查，二|不同，无论如何都会将两个条件检查一遍；

3.接下来是几个case，根据当前的action做处理；

ACTION_DOWN，当按下时记录按下点的x，y坐标值，根据条件设置当前是否满足tap状态，具体条件有两个，一是mScrimOpacity，表示子view中在屏幕上占据的最大宽度（0-1），二时根据坐标点的位置取得改点对应的最上层view对象，如果是预定义的content view即DrawerLayout里的主内容展示view，也就是同时满足view在屏幕上且点击的位置直接落在了content view上。

ACTION_MOVE,当手按下后开始在屏幕上移动时，如果垂直和水平上的位移差量达到了drag helper的阀值则一处左右两边的回调接口

ACTION_CANCLE和ACTION_UP，手势结束后，关闭菜单

最后结合几个状态来那个来决定onInterceptTouchEvent返回true还是false，

未完待续