Android：点击按钮，在自定义画布中绘制圆圈

I'm doing a simple drum set app where the users can click the button to make the corresponding sound and draw a circle on the canvas at a random location. For the canvas, I created a custom view called CanvasView, then I implements that CanvasView into the MainActivity, so I can control that View each time the buttons are clicked with OnClickListener in the MainActivity. I'm doing a simple drum set app where the users