@Override
  public void setAlpha(int alpha) {
    if (mAlpha != alpha) {
      mAlpha = alpha;
      invalidateSelf();
    }
  }
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