如何等待 Class 初始化？

Question

我有一个 class、calendar，它异步读取文件。

// Copyright 2017 <Abhi Agarwal>
// Refer to LICENSE

// Dart Imports
import 'dart:async';
import 'dart:convert';

// Flutter Imports
import 'package:flutter/services.dart';

// Local Imports
import 'event.dart';

/// Class deals with the school-wide calendar Calendar.
class Calendar {
  /// The days off in the year.
  List<Event> daysOff = new List<Event>();

  /// The half-days, not necessarily days off.
  List<Event> halfDays = new List<Event>();

  /// Vacations, extended days off.
  List<Event> vactations = new List<Event>();

  DateTime schoolStart;
  DateTime schoolEnd;
  DateTime schoolMaxEnd;

  Calendar() {
    _addDaysOff();
    //  _addHalfDays();
    //  _addVacations();
    //  _addTimes();
  }

  static const String filePath = "assets/calendar/";

  /// Reads a JSON File specified by the Arguments and returns a Decoded Object.
  Future<List<Map<String, String>>> _readJson(String fileName) async =>
      await JSON.decode(await rootBundle.loadString(filePath + fileName));

  Future _addDaysOff() async {
    const String fileName = "days_off.json";
    final List<Map<String, String>> parsed = await _readJson(fileName);

    for (final Map<String, String> item in parsed) {
      Event event = new Event(item["name"], DateTime.parse(item["date"]));
      daysOff.add(event);
    }
  }
}

一切正常，但是当我想运行一个简单的测试时，问题就出现了。

// Copyright 2017 <Abhi Agarwal>
// Refer to LICENSE

import 'package:flutter/material.dart';

import 'definitions/calendar/calendar.dart';

void main() {
  Calendar myCalendar = new Calendar();
  runApp(new Center(child: new Text(myCalendar.daysOff[0].name)));
}

使用 flutter run，我得到 RangeError (index): Invalid value: Valid value range is empty: 0。这是有道理的，List只有在初始化的时候才会有元素。问题是，在数据加载完成之前，我该如何放置占位符？

Answer 1

您可以重构您的 Calendar class 以公开 Future 以在 Calendar 准备好使用时完成：

// Private constructor, use create() to get an instance
Calendar._();

// Future that completes when the new Calendar is ready to use
static Future<Calendar> create() async {
  Calendar calendar = Calendar._();
  await calendar._addDaysOff();
  //    await calendar._addHalfDays();
  //    await calendar._addVacations();
  //    await calendar._addTimes();
  return calendar;
}

然后你可以await Future，例如：

main() async {
  Calendar myCalendar = await Calendar.create();
  runApp(new Center(child: new Text(myCalendar.daysOff[0].name)));
}

如果您希望 Calendar 在您的 Flutter 小部件树中更深的某个地方被 StatefulWidget 实例化而不是让它实例化，您可以等待 Calendar 使用 FutureBuilder 进行初始化活在根本上。

Answer 2

来自Collin Jackson helped me immensely, but it took some time for me to understand the context enough to implement it for my needs. Below is a more generic example of this which uses FutureBuilder, as Collin suggested, and can be run directly in DartPad的答案。

import 'package:flutter/material.dart';

class Delay {
  Delay.init();

  static Future<Delay> create() async {
    Delay delay = Delay.init();
    await delay.delay4();
    return delay;
  }

  Future delay4() {
    return Future.delayed(Duration(seconds: 4), () => null);
  }

  Future<String> delay5() {
    return Future.delayed(Duration(seconds: 5), () => "end of 5");
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark(),
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

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

class _MyWidgetState extends State<MyWidget> {
  Future<Delay> d = initDelay();

  static Future<Delay> initDelay() async {
    return await Delay.create();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<Delay>(
      future: d,
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return FutureBuilder<String>(
            future: snapshot.data!.delay5(),
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                return Text("result: ${snapshot.data}");
              } else if (snapshot.hasError) {
                return Text("result: error5");
              } else {
                return Text("result: loading5 ...");
             }
          });
        } else if (snapshot.hasError) {
          return Text("result: error4");
        } else {
          return Text("result: loading4 ...");
        }
      }
    );
  }
}

如何等待 Class 初始化？

How to wait for a Class to initialize?

flutter