private void drawDrawableWithAlpha(Canvas canvas, @Nullable Drawable drawable, int alpha) {
    if (drawable != null && alpha > 0) {
      mPreventInvalidateCount++;
      if (mMutateDrawables) {
        drawable.mutate();
      }
      drawable.setAlpha(alpha);
      mPreventInvalidateCount--;
      drawable.draw(canvas);
    }
  }
Updates the current alphas based on the ratio of the elapsed time and duration.

@param ratio
@return whether the all layers have reached their target opacity
drawee/src/main/java/com/facebook/drawee/drawable/FadeDrawable.java