Flutter应用程序启动时如何设置数据库？

Question

我想在我的应用程序启动时尽快在屏幕上显示一个小部件。在向用户显示小部件之后，我想通过按下另一个屏幕在离开启动屏幕之前进行一些应用初始化（例如，设置数据库）。我无法弄清楚我的应用初始化代码放在哪里。类似于iOS上的viewDidAppear的东西。

这是我基于initState尝试的。

class Launch extends StatefulWidget {
  @override
  _LaunchState createState() {
    return _LaunchState();
  }
}

class _LaunchState extends State<Launch> {
  @override
  Widget build(final BuildContext context) {
    print('LaunchState build start');
    final Widget w = Center(
      child: Text('Launching...'),
    );
    print('LaunchState build end');
    return w;
  }

  @override
  void initState() {
    print('LaunchState initState start');
    super.initState();
    print('LaunchState initState middle');
    _appInitialization();
    print('LaunchState initState end');
  }

  void _appInitialization() {
    print('LaunchState _appInitialization');
  }

}

输出是

flutter: LaunchState initState start
flutter: LaunchState initState middle
flutter: LaunchState _appInitialization
flutter: LaunchState initState end
flutter: LaunchState build start
flutter: LaunchState build end

我想我想看到的输出是

flutter: LaunchState build start
flutter: LaunchState build end
flutter: LaunchState initState start
flutter: LaunchState initState middle
flutter: LaunchState _appInitialization
flutter: LaunchState initState end

也许我会以一种非常轻松的方式解决这个问题。如果有一种完全不同的方法，我会全力以赴。

解

感谢Mangaldeep Pannu的回答，看来我缺少async。

class Launch extends StatefulWidget {
  @override
  _LaunchState createState() {
    return _LaunchState();
  }
}

class _LaunchState extends State<Launch> {
  @override
  Widget build(final BuildContext context) {
    print('LaunchState build start');
    final Widget w = Center(
      child: Text('Launching...'),
    );
    print('LaunchState build end');
    return w;
  }

  @override
  void initState() {
    print('LaunchState initState start');
    super.initState();
    print('LaunchState initState middle');
    _appInitialization(); // no await
    print('LaunchState initState end');
  }

  void _appInitialization() async {
    print('LaunchState _appInitialization begin');
    // simulate some time consuming initialization task
    await Future.delayed(Duration(seconds: 5));
    print('LaunchState _appInitialization middle');
    Navigator.push(...);
    print('LaunchState _appInitialization end');
  }

}

结果是

flutter: LaunchState initState start
flutter: LaunchState initState middle
flutter: LaunchState _appInitialization begin
flutter: LaunchState initState end
flutter: LaunchState build start
flutter: LaunchState build end
[5 second pause]
flutter: LaunchState _appInitialization middle
flutter: LaunchState _appInitialization end

Answer 1

在应用程序启动时打开的第一个小部件中

@override
void initState() {
  onStart();
}

void onStart() async {
  await loadData();  //load your data here
  Navigator.push();  //push to next screen
}

onStart将异步加载数据。 当数据加载后推送到下一个屏幕。

Answer 2

我们可以用BLoC处理这个问题。

创建一个app_bloc.dart文件，并有如下所示的AppBloc class：

import 'dart:async';

final appBloc =  AppBloc();

enum AppEvent{
  onStart, onAppInitialized, onStop
}

class AppBloc {
  final _appEventController = StreamController<AppEvent>.broadcast();

  Stream<AppEvent> get appEventsStream => _appEventController.stream;

  dispatch(AppEvent event) {
    switch(event) {
      case AppEvent.onStart:
        _initializeApp();
        _sinkEvent(AppEvent.onStart);
        break;
      case AppEvent.onStop:
        _dispose();
        _sinkEvent(AppEvent.onStop);
        break;
      case AppEvent.onAppInitialized:
        _sinkEvent(AppEvent.onAppInitialized);
        break;
    }
  }

  void _sinkEvent(AppEvent appEvent) => _appEventController.sink.add(appEvent);

  _dispose() {
    _appEventController.close();
  }

  void _initializeApp() async {
    await DBProvider.db.initDB();
    dispatch(AppEvent.onAppInitialized); // will execute when all initializations are complete,
  }
}

创建一个database.dart文件，并在其中包含您的DBProvider class：

class DBProvider {
  static final DBProvider _instance = new DBProvider._internal();

  static final db = DBProvider();

  factory DBProvider() {
    return _instance;
  }

  DBProvider._internal();

  initDB() async {
    // todo initialize your database here.
  }
}

您的主文件应该是这样的：

import 'package:flutter/material.dart';

import 'package:st_overflow/app_bloc.dart';

void main() => runApp(App());

class App extends StatefulWidget {

  App() {
    appBloc.dispatch(AppEvent.onStart);
  }

  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {

  @override
  Widget build(BuildContext context) {
    return Launch();
  }

  @override
  void dispose() {
    appBloc.dispatch(AppEvent.onStop);
    super.dispose();
  }
}

并在您的Launch Widget使用StreamBuilder消耗onAppInitialize事件，如下所示：

StreamBuilder(
    stream: appBloc.appEventsStream,
    builder: (context, snapshot){
        switch (snapshot.data) {
            case AppEvent.onAppInitialized:
            // remove your Launch widget and show Screen 1 of your app (may be dashboard or something).
            break;        
        }
    },
);