在手指android画布下画圆圈

public class CircleView extends View {
    private float x, y; // 手指触摸的坐标
    private boolean isTouching = false; // 是否正在触摸

    public CircleView(Context context) {
        super(context);
    }

    public CircleView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        if (isTouching) {
            Paint paint = new Paint();
            paint.setColor(Color.RED);
            paint.setStyle(Paint.Style.STROKE);
            paint.setStrokeWidth(5);

            canvas.drawCircle(x, y, 50, paint);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                isTouching = true;
                x = event.getX();
                y = event.getY();
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                x = event.getX();
                y = event.getY();
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                isTouching = false;
                invalidate();
                break;
        }
        return true;
    }
}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.example.CircleView
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>