code: public SwipeBack setSwipeBackTransformer(SwipeBackTransformer transformer) {
  		mSwipeBackTransformer = transformer;
  		if (mSwipeBackView != null) {
  			notifySwipeBackViewCreated(mSwipeBackView);
  		}
  		return this;
  	}
docs: Set the {@link SwipeBackTransformer}
  
  @param transformer