/**dispatchTouchEvent用于事件的分发，将屏幕的触摸事件往下传递到目标View，或者当前View就是目标View。dispatchTouchEvent的返回结果受当前View的onTouchEvent和下级View的dispatchTouchEvent方法的影响，返回true表示消耗当前触摸事件。 onInterceptTouchEvent是ViewGroup类中的方法，其方法说明如下。
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event)
/**onInterceptTouchEvent是在dispatchTouchEvent内部调用，用于判断是否拦截触摸事件，返回true表示拦截当前触摸事件，从上述代码中可以看到，该方法默认返回false，即不拦截触摸事件。
* 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;
}
onTouchEvent用于处理触摸事件，其方法说明如下。
/**onTouchEvent在我们自定义View的时候经常遇到，其在dispatchTouchEvent方法中调用，用来处理触摸事件，返回true表示消耗当前触摸事件。在上述说明中提到了一个performClick方法，该方法是用来触发View的点击事件。
* Implement this method to handle touch screen motion events.
* <p>
* If this method is used to detect click actions, it is recommended that
* the actions be performed by implementing and calling
* {@link #performClick()}. This will ensure consistent system behavior,
* including:
* <ul>
* <li>obeying click sound preferences
* <li>dispatching OnClickListener calls
* <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
* accessibility features are enabled
* </ul>
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
public boolean onTouchEvent(MotionEvent event)
上述三个方法的关系用伪代码描述如下。
public boolean dispatchTouchEvent(MotionEvent event) {
boolean consume = false;
if (onInterceptTouchEvent(event)) {
consume = onTouchEvent(event);
} else {
consume = child.dispatchTouchEvent(event);
}
return consume;
} 对于一个跟ViewGroup来说，当触摸事件触发后，首先会传递给自身，接着dispatchTouchEvent方法被调用，然后再判断它是否要拦截当前触摸事件，如果拦截，则调用自身的onTouchEvent方法，消耗触摸事件。如果不拦截，则将事件传递给它的child，接着child的dispatchTouchEvent方法被调用。如果View不想被它的父View或者祖先View拦截事件，那么可以通过调用requestDisallowInterceptTouchEvent方法来获取触摸事件，其说明如下。
/**那么我们会想requestDisallowInterceptTouchEvent和onInterceptTouchEvent哪一个的优先级高呢？这里通过查看ViewGroup类中dispatchTouchEvent方法中有一段代码。
* Called when a child does not want this parent and its ancestors to
* intercept touch events with
* {@link ViewGroup#onInterceptTouchEvent(MotionEvent)}.
*
* <p>This parent should pass this call onto its parents. This parent must obey
* this request for the duration of the touch (that is, only clear the flag
* after this parent has received an up or a cancel.</p>
*
* @param disallowIntercept True if the child does not want the parent to
* intercept touch events.
*/
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept);
// Check for interception.通过源码，我们可以很深刻地体会，View事件分发机制的工作原理，这对我们以后写高级自定义View很有帮助。
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//disallowIntercept的值可以通过requestDisallowInterceptTouchEvent来设定
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
//通过下面的代码，就知道requestDisallowInterceptTouchEvent优先级高于onInterceptTouchEvent
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;
}