@Override
  protected boolean onLevelChange(int level) {
    boolean levelChanged = false;
    for (int i = 0; i < mLayers.length; i++) {
      Drawable drawable = mLayers[i];
      if (drawable != null && drawable.setLevel(level)) {
        levelChanged = true;
      }
    }
    return levelChanged;
  }
Sets a new drawable at the specified index, and return the previous drawable, if any.
drawee/src/main/java/com/facebook/drawee/drawable/ArrayDrawable.java