In Flutter, make a HTTP request, then depending on response, update the UI or open a new page

Question

I am developing a Flutter app where it acts as a client, connecting to a server via an API. The app makes requests and depending on the response it progresses the state.

My question is the following: Can I make a request, and then depending on the response, either update the UI or open a new page?

I have used FutureBuilder as shown below. The problem is that the FutureBuilder requires to return a UI. In my case, if the response is OK I want to open a new page (see //todo line). I tried using Navigator.pushReplacement but it does not really work. Any ideas?

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'dart:async';
import 'package:flutter/rendering.dart';
import 'model.dart';

class Start extends StatefulWidget {
  final String title;

  Start({Key key, @required this.title}) : super(key: key);

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

class StartState extends State<Start> {

  Future<StartReply> _startReply;

  _makeRequest() {
    setState(() {
      _startReply = ...; // actual API request
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: widget.title,
        home: Scaffold(
            appBar: AppBar(
              title: Text(widget.title),
              leading: IconButton(
                  icon: Icon(Icons.arrow_back),
                  onPressed: () => Navigator.of(context).pop(false)
              ),
            ),
            body: Center(
                child: FutureBuilder(
                  future: _startReply,
                  builder: (context, snapshot) {
                    if(snapshot.connectionState == ConnectionState.none) {
                      return ElevatedButton(
                          onPressed: _makeRequest,
                          child: Text("Make request")
                      );
                    } else if(snapshot.connectionState == ConnectionState.done && snapshot.hasData) {
                      // todo open page here
                      return Text('Started!', style: TextStyle(color: Colors.green, fontStyle: FontStyle.italic));
                    } else if(snapshot.hasError) {
                      debugPrint('StartReply: ${snapshot.data}');
                      return Text('Error (${snapshot.error})', style: TextStyle(color: Colors.red, fontStyle: FontStyle.italic));
                    } else {
                      return CircularProgressIndicator();
                    }
                  }
                )
            )
        )
    );
  }
}

Answer 1

Yes, you should not use a FutureBuilder if you want to do anything other than changing the UI depending on the async task. You should manage your own async. Here's some code to get you started:

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  
  bool loaded;
  
  @override
  void initState() {
    super.initState();
    
    asyncInit();
  }
  
  Future<void> asyncInit() async {
    final response =
    await doTheNetworkRequest() //imagine that this was an http request
    if (yes) {
      setState(() {
        loaded = true;
      });
    } else {
      Navigator.of(context).push(...);
    }
  }
  
  @override
  Widget build(BuildContext context) {
    return loaded == true ? Text('Loaded') : Text('Loading');
  }
}