@Override
    public void setAlpha(int alpha) {
        int currentAlpha = state.paint.getAlpha();
        if (currentAlpha != alpha) {
            state.setAlpha(alpha);
            invalidateSelf();
        }
    }
A static {@link GlideDrawable} for displaying a single image.
core/src/main/java/com/bumptech/glide/load/resource/bitmap/GlideBitmapDrawable.java