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I believe many inexperienced people are at a loss about how to understand the distribution and consumption mechanism of Touch events under Android programming. Therefore, this paper summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

The methods related to Touch events in Android include: dispatchTouchEvent (MotionEvent ev), onInterceptTouchEvent (MotionEvent ev), onTouchEvent (MotionEvent ev); controls that can respond to these methods include: ViewGroup and its subclasses, Activity. The correspondence between the method and the control is shown in the following table:

Touch event related method function

ViewGroup

Activity

Public boolean dispatchTouchEvent (MotionEvent ev) event distribution

YesYespublic boolean onInterceptTouchEvent (MotionEvent ev)

Event interception

YesNopublic boolean onTouchEvent (MotionEvent ev) event response

YesYes

From this table, we can see that ViewGroup and its subclasses respond to all three methods related to Touch events, while Activity does not respond to onInterceptTouchEvent (MotionEvent ev), that is, event interception. It should also be noted that the premise of View's response to dispatchTouchEvent (MotionEvent ev) and onInterceptTouchEvent (MotionEvent ev) is that a sub-View can be added to the View. If the current View is already a smallest unit View (such as TextView), then it cannot add a sub-View to the minimum View, and it cannot distribute and intercept events to the sub-View, so it has no dispatchTouchEvent (MotionEvent ev) and onInterceptTouchEvent (MotionEvent ev), only onTouchEvent (MotionEvent ev).

I. Touch event analysis

Event distribution: public boolean dispatchTouchEvent (MotionEvent ev)

When a Touch event occurs, the dispatchTouchEvent (MotionEvent ev) method of Activity tunnels the event down from the root element to the innermost child element or stops delivery in the middle element due to some condition) to the dispatchTouchEvent (MotionEvent ev) method of the outermost View, and the event is distributed by the dispatchTouchEvent (MotionEvent ev) method of that View. The event distribution logic for dispatchTouchEvent is as follows:

If return true, the event is distributed to the current View and consumed by the dispatchTouchEvent method, and the event stops passing downwards

If return false, event distribution is divided into two situations:

If the event obtained by the current View comes directly from Activity, the event will be returned to the onTouchEvent of Activity for consumption

If the event obtained by the current View is from the outer parent control, the event is returned to the onTouchEvent of the parent View for consumption.

If the system default super.dispatchTouchEvent (ev) is returned, the event is automatically distributed to the onInterceptTouchEvent method of the current View.

Event interception: public boolean onInterceptTouchEvent (MotionEvent ev)

When the dispatchTouchEvent (MotionEvent ev) method of the outer View returns the system default super.dispatchTouchEvent (ev), the event is automatically distributed to the onInterceptTouchEvent method of the current View. The event interception logic of onInterceptTouchEvent is as follows:

If onInterceptTouchEvent returns true, it means that the event is intercepted and the intercepted event is handed over to the onTouchEvent of the current View for processing

If onInterceptTouchEvent returns false, the event will be released, and the event on the current View will be passed to the child View, and the distribution of the event will be started by the dispatchTouchEvent of the child View

If onInterceptTouchEvent returns super.onInterceptTouchEvent (ev), the event is intercepted by default, and the intercepted event is handed over to the onTouchEvent of the current View for processing.

Event response: public boolean onTouchEvent (MotionEvent ev)

OnTouchEvent is called when dispatchTouchEvent returns super.dispatchTouchEvent (ev) and onInterceptTouchEvent returns true or super.onInterceptTouchEvent (ev). The event response logic for onTouchEvent is as follows:

If the event is passed to the onTouchEvent method of the current View, and the method returns false, then the event is passed up from the current View and is received by the onTouchEvent of the upper View. If the onTouchEvent passed above also returns false, the event will "disappear" and will not receive the next event.

If true is returned, the event is received and consumed.

If super.onTouchEvent (ev) is returned, the default logic for handling events is the same as when false is returned.

After reading the above, have you mastered how to understand the distribution and consumption mechanism of Touch events under Android programming? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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