Flutter ListView not updating

Turns out the problem was that

List<Manga> _listContent = new List<Manga>();

was defined on top of widget.build and not on top of the State class. Defining it like this works fine:

class _MyHomePageState extends State<MyHomePage> {
  List<Manga> _listContent = new List<Manga>();

  @override
  Widget build(BuildContext context) {
    ...
  }