Flutter ListView not updating

Question

I wanted to create a ListView in Flutter that update when new data arrives. I am using a RefreshIndicator that triggers the list loading for testing. For my list I am using a ListBuilder to map my objects to view objects.

From what I understand, the setState() should trigger an update to my list, however it does not.

Debugging showed that my _listContent is actually filled with 72 items after when the setState() callback is called. Why does my list not auto update? Am I missing something?

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    List<Manga> _listContent = new List<Manga>();

    void _addMangas(List<Manga> mangas) {
      setState(() {
        _listContent.addAll(mangas);
        print('Cleared list');
        });
    }

    Future<Null> _onRefresh() {
      Completer<Null> completer = new Completer<Null>();

      loadMangaItems().then((mangas) => _addMangas(mangas));
      completer.complete();

      return completer.future;
    }

    return new Scaffold(
      appBar: new AppBar(

        title: new Text(widget.title),
      ),
      body: new RefreshIndicator(
          child: new ListView.builder(
              itemCount: _listContent.length,
              itemBuilder: (BuildContext ctx, int index) {
                Manga manga = _listContent[index];
                return new MangaItem(
                  name: manga.writtenName,
                  number: manga.currentNr,
                  state: false,
                );
              }),
          onRefresh: _onRefresh),
       );
    }
}

Answer 1

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) {
    ...
  }

Answer 2

I solved a similiar issue by calling the setState method (notify the framework that the internal state of this object has changed).

Example here: https://googleflutter.com/flutter-add-item-to-listview-dynamically/