@Nullable
  public Drawable getDrawable(int index) {
    Preconditions.checkArgument(index >= 0);
    Preconditions.checkArgument(index < mLayers.length);
    return mLayers[index];
  }
Gets the drawable at the specified index.

@param index index of drawable to get
@return drawable at the specified index
drawee/src/main/java/com/facebook/drawee/drawable/ArrayDrawable.java