@Override
    public void setAlpha(int alpha) {
        mAlpha = alpha;
        invalidateSelf();
    }
Set the alpha level for this drawable [0..255]. Note that this drawable
also has a color in its paint, which has an alpha as well. These two
values are automatically combined during drawing. Thus if the color's
alpha is 75% (i.e. 192) and the drawable's alpha is 50% (i.e. 128), then
the combined alpha that will be used during drawing will be 37.5% (i.e.
96).
caprice/ui/src/main/java/net/qiujuer/genius/ui/drawable/StateColorDrawable.java