How can I wait for data before it is available - flutter

问题

@override
  void didChangeDependencies() async {
    if (_isInit) {
      print('First stat');
      final restaurant = await Provider.of<Restaurants>(context, listen: false)
          .fetchAndSetRestaurants();
      print('test');
    }
    _isInit = false;

    super.didChangeDependencies();
  }

  bool showFavourites = false;

  @override
  Widget build(BuildContext context) {
   
    return Scaffold(
        //bottomNavigationBar: BottomNavBar(),
        appBar: homeAppBar(context),
        body: SingleChildScrollView(
          child: showFavourites
              ? Padding(
                  padding: const EdgeInsets.only(top: 20),
                  child:
                      RestaurantList(), //RestaurantList(restaurants: restaurants)
                )
              : Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                      SearchBox(
                        onChanged: (value) {},
                      ),
                      CategoryList(),
                      RestaurantList(), //RestaurantList(restaurants: restaurants)
                    ]),
        ));
  }
}

class RestaurantList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final List<Restaurant> restaurants1 =
        Provider.of<Restaurants>(context).getRestaurants1;
    print('List: ${restaurants1[0].name}');
    
    return ListView.builder(
      scrollDirection: Axis.vertical,
      shrinkWrap: true,
      itemBuilder: (ctx, i) =>
          restaurants1[i].isAvailable || restaurants1[i].totalQuantity != 0
              ? Column(
                  children: [
                    ChangeNotifierProvider.value(
                        value: restaurants1[i], child: RestaurantCard()),
                    Divider(),
                  ],
                )
              : Container(),
      itemCount: restaurants1.length,
    );
  }

The data (restaurants1) inside the 'RestaurantList' widget is being accessed before it is available. Even putting 'await' while fetching it in didChangeDependencies doesn't seem to work.

This is the output I am getting:

flutter: First stat

flutter: null

flutter: SUCCESS :KFC

flutter: test

[VERBOSE-2:profiler_metrics_ios.mm(184)] Error retrieving thread information: (ipc/send) invalid destination port

null is called by the getter 'getRestaurants1'

回答1:

You can use FutureBuilder widget and retrieve data inside it's future parameter. Official Flutter Documentation has simple example that describe how to declare and work with FutureBuilder.

In your case, try something like this:

FutureBuilder<List<Restaurant>>(
  future: Provider.of<Restaurants>(context, listen: false).fetchAndSetRestaurants(),
  builder: (context, snapshot) {
    // Items are not available and you need to handle this situation, simple solution is to show a progress indicator
    if (!snapshot.hasData) {
      return CircularProgressIndicator();
    }
    return ListView.builder(
      scrollDirection: Axis.vertical,
      shrinkWrap: true,
      itemBuilder: (ctx, i) =>
          restaurants1[i].isAvailable || restaurants1[i].totalQuantity != 0
              ? Column(
                  children: [
                    ChangeNotifierProvider.value(
                        value: restaurants1[i], child: RestaurantCard()),
                    Divider(),
                  ],
                )
              : Container(),
      itemCount: restaurants1.length,
    );
  },
);

来源：https://stackoverflow.com/questions/65967347/how-can-i-wait-for-data-before-it-is-available-flutter