ImageMagnifier 如何添加边框

有ImageMagnifier，它是ImageView的“放大镜”：

public class ImageMagnifier extends android.support.v7.widget.AppCompatImageView {

    private PointF zoomPos;
    private boolean zooming = false;
    private Matrix matrix;
    private Paint paint;
    private Bitmap bitmap;
    private BitmapShader shader;
    private int sizeOfMagnifier = 200;

    public ImageMagnifier(Context context) {
        super(context);
        init();
    }

    public ImageMagnifier(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

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

    private void init() {
        zoomPos = new PointF(0, 0);
        matrix = new Matrix();
        paint = new Paint();

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();

        zoomPos.x = event.getX();
        zoomPos.y = event.getY();

        switch (action) {
            case MotionEvent.ACTION_DOWN:
            case MotionEvent.ACTION_MOVE:
                zooming = true;
                this.invalidate();
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                zooming = false;
                this.invalidate();
                break;

            default:
                break;
        }

        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (!zooming) {
            buildDrawingCache();
        } else {

            bitmap = getDrawingCache();
            shader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);

            paint = new Paint();
            paint.setShader(shader);

            matrix.reset();
            matrix.postScale(2f, 2f, zoomPos.x, zoomPos.y);
            paint.getShader().setLocalMatrix(matrix);
            canvas.drawCircle(zoomPos.x, zoomPos.y - 150, sizeOfMagnifier, paint);
        }
    }
}

一切正常，但我不知道如何为放大镜添加框架。试图做类似的事情：

paint.setStyle(Paint.Style.STROKE);
paint.setColor(Color.BLACK);

但在这种情况下，得到了一个反转：放大的图像显示在一个框架中（应该是黑色的），放大镜本身是空的。