颤动：在启动时初始化变量

我正在尝试使用SharedPreferences中保存的值来初始化我的应用程序中的几个变量 . 在Flutter中，SharedPreferences是异步的，因此它导致变量稍后在代码中初始化，这会导致我的应用程序出现问题，因为在调用构建方法时，某些变量为null .

这是我写的一个小测试Flutter应用程序，用于演示此问题：

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import 'package:path_provider/path_provider.dart';
import 'dart:io';
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';

class TestingApp extends StatefulWidget {

  TestingApp() {}

  @override
  State<StatefulWidget> createState() {
// TODO: implement createState
    return new _CupertinoNavigationState();
  }
}

class _CupertinoNavigationState extends State<TestingApp> {

  int itemNo;

  @override
  void initState() {
    super.initState();
//    SharedPreferences.getInstance().then((sp) {
//      sp.setInt("itemNo", 3);
//    });
    SharedPreferences.getInstance().then((sp)  {
      print("sp " + sp.getInt("itemNo").toString());
      setState(() {
        itemNo = sp.getInt("itemNo");
      });
    });
    print("This is the item number " + itemNo.toString());
  }

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    print("item number on build " + itemNo.toString());
    return new Text("Hello");
  }
}

这是控制台中的结果：

Performing full restart...
flutter: This is the item number null
flutter: item number on build null // build method being called and variable is null
Restarted app in 1 993ms.
flutter: sp 3
flutter: item number on build 3

您可以看到，当我尝试在启动时从SharedPreferences获取变量时，由于SharedPreference是异步的，因此itemNo为null . 然后app运行构建方法并在itemNo = null上运行构建方法，这会导致应用程序崩溃 .

一旦从SharedPreferences获取值，我调用setState，然后再使用正确的值再次调用构建方法 . 但是，不应该发生使用itemNo = null进行构建的初始调用 .

我希望SharedPreferences有一个synch方法，但它似乎不存在 . 如何运行应用程序以便在启动时在Flutter中正确初始化变量？

我尝试通过使用同步方法来解决这个问题，通过写入json文件来初始化我的变量，然后使用以下简短的Flutter测试应用程序从中读取 - 对我来说，这似乎是过度的，因为保存变量以初始化但我仍然尝试了一下：

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import 'package:path_provider/path_provider.dart';
import 'dart:io';
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';

class TestingApp extends StatefulWidget {

  TestingApp() {}

  @override
  State<StatefulWidget> createState() {
// TODO: implement createState
    return new _CupertinoNavigationState();
  }
}

class _CupertinoNavigationState extends State<TestingApp> {

  int itemNo;
  File  jsonFile;
  String fileName = "items.json";
  Directory dir;
  bool fileExists = false;

void createFile(Map content, Directory dir, String fileName) {
//  print("Creating file for category " + dir.path);
  File file = new File(dir.path + "/" + fileName);
  file.createSync();
  fileExists = true;
  file.writeAsStringSync(json.encode(content));
}

void writeToFile(int itemNo) {
//  print("Writing to category file");
  Map itemMap = new Map();
  itemMap['item'] = itemNo;
  if (fileExists) {
    print("category file exists");
    Map jsonFileContent = json.decode(jsonFile.readAsStringSync());
    jsonFileContent.addAll(itemMap);
    jsonFile.writeAsStringSync(json.encode(itemMap));
  } else {
    print("category File does not exists");
    getApplicationDocumentsDirectory().then((Directory directory) {
      dir = directory;
      createFile(itemMap, dir, fileName);
    });

  }
}

fetchSavedItemNo() {
  //load the currency from the saved json file.
  getApplicationDocumentsDirectory().then((Directory directory) {
    dir = directory;
    jsonFile = new File(dir.path+ "/" + fileName);
    fileExists = jsonFile.existsSync();
    setState(() {
      if (fileExists)
        itemNo = json.decode(jsonFile.readAsStringSync())['item'];
      print("fetching saved itemNo " +itemNo.toString());
      if (itemNo == null) {
        itemNo = 0;
      }
    });


    return itemNo;
    //else the itemNo will just be 0
  });
}

  @override
  void initState() {
    super.initState();
    writeToFile(3);
    setState(() {
      itemNo = fetchSavedItemNo();
    });

  }

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    print("item number on build " + itemNo.toString());
    return new Text("Hello");
  }
}

我仍然有结果，在变量完全初始化之前调用构建方法，导致应用程序崩溃 .

Performing full restart...
flutter: category File does not exists
flutter: item number on build null   // the build method is called here
Restarted app in 1 894ms.
flutter: fetching saved itemNo 3
flutter: item number on build 3

如何在应用启动时初始化Flutter中的变量？