I'd like to animate a gap between two items in a list. I thought of using an AminatedContainer with a height initially at zero but I'm not familiar with how to make this work. My code at the moment is:

```
    new AnimatedContainer(
      duration: const Duration(milliseconds: 200),
      height: App.itemSelected==id ? 50.0 : 0.0,
      curve: Curves.fastOutSlowIn,
    ),

```

That does change the height of the Container but not in an animated way as I had hoped. Any help would be gratefully received!