How to ensure code is run only once in a widget?

Question

I do have a lot of code that looks like this:

bool _somethingFromApiLoaded = false;
Something _somethingFromApi;

loadSomething() async {
  final something = await ServiceProvider.of(context).apiService.getSomething();
  setState(() => _somethingFromApi = something);
}

@override
Widget build(BuildContext context) {
  if (!_somethingFromApiLoaded) {
      loadSomething();
      _somethingFromApiLoaded = true;
  }
}

Note how I produce a lot of boilerplate code to ensure loadSomething is only called once.

I wonder if there isn't a lifecycle method to do so that I somehow misinterpret. I can't use initState because it does not have context.

there is solution here you can try. https://stackoverflow.com/questions/49457717/flutter-get-context-in-initstate-method — bingcheng45, Apr 04 '20 at 18:53
@gegobyte: It results in an error `dependOnInheritedWidgetOfExactType() or dependOnInheritedElement() was called before _MyClass.initState() completed.` @bingcheng45: Hm, that looks like even more boilerplate. — shredding, Apr 04 '20 at 18:56

score 3 · Accepted Answer · answered Apr 04 '20 at 19:57

I would try to a use a StatefulWidget and use initState() method.

That is the lifecycle you are referring to.

You should try to use a Future inside the initState()

  @override
  void initState() {
    super.initState(); // make sure this is called in the beggining

    // your code here runs only once

    Future.delayed(Duration.zero,() {
      _somethingFromApi = await ServiceProvider.of(context).apiService.getSomething();
    });

  }

As User gegobyte said, Context is available in the initState. But apparently can't be used for everything.

That does not ensure that some things are initialised on build, the Future runs after the first build. — shredding, Apr 05 '20 at 08:07

score 0 · Answer 2 · answered Apr 30 '21 at 06:21

You can use context in initState() by passing it to the widget:

class HomeScreen extends StatefulWidget {
  final BuildContext context;
  HomeScreen(this.context);

  @override
  State<StatefulWidget> createState() => new _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  bool _somethingFromApiLoaded = false;
  Something _somethingFromApi;

  loadSomething() async {
    final something = await ServiceProvider.of(widget.context).apiService.getSomething();
    setState(() => _somethingFromApi = something);
  }

  @override
  void initState() {
    super.initState();
    if (!_somethingFromApiLoaded) {
      loadSomething();
      _somethingFromApiLoaded = true;
    }
  }
}

How to ensure code is run only once in a widget?

2 Answers2