Fastfile

# This file contains the fastlane.tools configuration
# You can find the documentation at https://docs.fastlane.tools
#
# For a list of all available actions, check out
#
#     https://docs.fastlane.tools/actions
#
# For a list of all available plugins, check out
#
#     https://docs.fastlane.tools/plugins/available-plugins
#

# Uncomment the line if you want fastlane to automatically update itself
# update_fastlane

default_platform(:android)

platform :android do
  desc "Runs all the tests"
  lane :test do
    gradle(task: "test")
  end

  desc "Submit a new Beta Build to Google Play (beta track)"
  lane :beta do
    gradle(task: "clean assembleRelease")
    upload_to_play_store(track: 'beta',
                         apk: '../build/app/outputs/apk/release/app-release.apk',
                         json_key_data: Base64.decode64(ENV['GOOGLE_PLAY_JSON']))

    # sh "your_script.sh"
    # You can also use other beta testing services here
  end

  desc "Generate new icons"
  lane :icons do
    android_appicon(
      appicon_image_file: 'master-launcher-icon.jpg',
      appicon_icon_types: [:launcher],
      appicon_path: 'app/src/main/res/mipmap'
    )
  end
end

main.dart

import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:convert';

void main() => runApp(new MyApp());

class Complex {
  String name;
  int id;

  Complex(String name, int id) {
    this.name = name;
    this.id = id;
  }

  factory Complex.fromJson(Map<String, dynamic> json) {
    return new Complex(
      json['title'],
      json['id']
    );
  }
}


Future<List<Complex>> getAllComplexes(String language) async {
  var list = new List<Complex>();

  final response = """
    {
      "complexes": [
          {
            "title": "A",
            "id": 21
          },
          {
            "title": "B",
            "id": 2
          }
      ]
    }
  """;

  final responseJson = json.decode(response);

  for (var element in responseJson['complexes']) {
    list.add(Complex.fromJson(element));
  }

  return list;
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Example',
      theme: new ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or press Run > Flutter Hot Reload in IntelliJ). Notice that the
        // counter didn't reset back to zero; the application is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Example'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return new Scaffold(
      appBar: new AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: new Text(widget.title),
      ),
      body: new Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: new FutureBuilder<List<Complex>>(
          future: getAllComplexes('nl'),
          builder: (context, snapshot) {
            if (snapshot.hasData) {
              return new ListView(
                children: new List<Widget>.from(snapshot.data.map<Widget>((Complex c) {
                  return new ListTile(
                      title: new Text(c.name)
                  );
                })),
              );
            } else if (snapshot.hasError) {
              return new Text("${snapshot.error}");
            }

            // By default, show a loading spinner
            return new CircularProgressIndicator();
          },
        ),
      ),
    );
  }
}