如何为裁剪的圆形图像添加阴影？

Question

我使用以下方法将图像转换为圆形。 （目的是使用圆形圆圈图像作为Android应用程序中的按钮。）

我试图添加阴影，但我无法让它发挥作用。有人可以帮忙吗？

public static class CircleTransform extends BitmapTransformation {
    public CircleTransform(Context context) {
        super(context);
    }

    @Override
    protected Bitmap transform(BitmapPool pool, Bitmap toTransform, int outWidth, int outHeight) {
        return circleCrop(pool, toTransform);
    }

    private static Bitmap circleCrop(BitmapPool pool, Bitmap source) {
        if (source == null) return null;
        int size = Math.min(source.getWidth(), source.getHeight());
        int x = (source.getWidth() - size) / 2;
        int y = (source.getHeight() - size) / 2;
        Bitmap squared = Bitmap.createBitmap(source, x, y, size, size);
        Bitmap result = pool.get(size, size, Bitmap.Config.ARGB_8888);
        if (result == null) {
            result = Bitmap.createBitmap(size, size, Bitmap.Config.ARGB_8888);
        }
        Canvas canvas = new Canvas(result);
        Paint paint = new Paint();
        paint.setShader(new BitmapShader(squared, BitmapShader.TileMode.CLAMP, BitmapShader.TileMode.CLAMP));
        paint.setAntiAlias(true);
        float r = size / 2f;
        canvas.drawCircle(r, r, r, paint);
        return result;
    }

    @Override
    public String getId() {
        return getClass().getName();
    }
}