Flutter: Why setState(( ) { }) set data again and again

问题

I use setState(() {}) for assigning a value to a variable. But it's printing again and again. Why does it react like this? And how can I fix it?

Here is my code:

class Sample extends StatefulWidget {
  @override
  _SampleState createState() => _SampleState();
}

class _SampleState extends State<Sample> {
  String _message;
  String _appLink;
  Firestore db = Firestore.instance;
  @override
  Widget build(BuildContext context) {
    db.collection('share').document('0').get().then((value) {
      var message = value.data['message'];
      print(message);
      var appLink = value.data['appLink'];
      setState(() {
        _message = message;
        _appLink = appLink;
      });
    });
    return Container(
      child: Text('$_message $_appLink'),
    );
  }
}

Here is my Output:

Here _appLink value is www.facebook.com

回答1:

The purpose of setState is to tell the framework that a variable in the state has changed and the widget needs to be rebuilt to reflect that change. So calling setState calls the build function again, which in your case recalls your Future, which calls setState again, which triggers build and so on.

To fix this you should call the Future in initState, and use a FutureBuilder to display the data when it's ready.

Example:

class _SampleState extends State<Sample> {
  Firestore db = Firestore.instance;
  Future databaseFuture;

  @override
  void initState() {
    databaseFuture = db.collection('share').document('0').get()
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: databaseFuture,
      builder: (context, snapshot) {
        if(!snapshot.hasData) {
          return CircularProgressIndicator();
        }
        var message = snapshot.data.data['message'];
        print(message);
        var appLink = snapshot.data.data['appLink'];
        return Text('$message $appLink');
      }
    ),
  }
}

来源：https://stackoverflow.com/questions/62842729/flutter-why-setstate-set-data-again-and-again

标签

Flutter

dart

setstate