如何在imageview上的触摸位置绘制圆圈？

Question

我使用UIL将我的apps目录中存储的图像加载到imageView中。我想要做的是根据用户的触摸在屏幕上的图像上绘制一个小圆圈。所以我决定使用表面视图。我可以根据onTouch事件绘制圆圈，但我面临的问题是整个屏幕都有白色背景。我想要在图像视图上绘制圆圈。

public class SingleViewActivity extends Activity {
int position;
String[] imageUrls;
ImageView imageView;
ImageLoader imageloader;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.single_view);
    File Path = getExternalFilesDir(null);
    Constants cons = new Constants();
    imageUrls = cons.getImagePath(Path.toString());
    // Get intent data
    Intent i = getIntent();

    // Selected image id
    position = i.getExtras().getInt("position", -1);
    imageView = (ImageView) findViewById(R.id.SingleView);
    imageloader = ImageLoader.getInstance();
    imageloader.displayImage("file://" + imageUrls[position], imageView);
    setContentView(new DrawingView(this));

}

class DrawingView extends SurfaceView {

    private final SurfaceHolder surfaceHolder;
    private final Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

    public DrawingView(Context context) {
        super(context);
        setZOrderOnTop(true);
        surfaceHolder = getHolder();
        surfaceHolder.setFormat(PixelFormat.TRANSLUCENT);
        paint.setColor(Color.RED);
        paint.setStyle(android.graphics.Paint.Style.FILL);

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_DOWN) {
            if (surfaceHolder.getSurface().isValid()) {
                Canvas canvas = surfaceHolder.lockCanvas();
                canvas.drawCircle(event.getX(), event.getY(), 50, paint);
                surfaceHolder.unlockCanvasAndPost(canvas);
            }
        }
        return false;
    }
}

}