在标签栏控制器项目中，两个视图控制器之间的通信。

I'm using a Tab Bar Controller in my project where the FirstViewController has a Mapbox map view and the SecondViewController has buttons that when pressed add a tile layer to the map view. Here's what I've tried. It creates error ***Use of undeclared identifier '_mapView' in the SecondViewController.mI'm using a Tab Bar Controller in my project wh