Android Developer Documentation

• MotionEvent

Exercise

In this exercise you are going to use the onTouchEvent() method to respond to motion on the display.

1. In MyCanvasView, override the onTouchEvent() method to cache the x and y coordinates of the passed in event. Then use a when expression to handle motion events for touching down on the screen, moving on the screen, and releasing touch on the screen. These are the events of interest for drawing a line on the screen. For each event type, call a utility method, as shown in the code below. See the MotionEvent class documentation for a full list of touch events.

override fun onTouchEvent(event: MotionEvent): Boolean {
   motionTouchEventX = event.x
   motionTouchEventY = event.y

   when (event.action) {
       MotionEvent.ACTION_DOWN -> touchStart()
       MotionEvent.ACTION_MOVE -> touchMove()
       MotionEvent.ACTION_UP -> touchUp()
   }
   return true
}

2. At the class level, add the missing motionTouchEventX and motionTouchEventY variables for caching the x and y coordinates of the current touch event (the MotionEvent coordinates). Initialize them to 0f.

private var motionTouchEventX = 0f
private var motionTouchEventY = 0f

3. Create stubs for the three functions touchStart(), touchMove(), and touchUp().

private fun touchStart() {}

private fun touchMove() {}

private fun touchUp() {}

4. Your code should build and run, but you won't see any different from the colored background yet.