颤振: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应用程序中的几个变量。在flutter中,sharedreferences是异步的,因此会导致稍后在代码中初始化变量,这会给我的应用程序带来问题,因为调用build方法时,某些变量为空。

下面是我为演示这个问题而编写的一个小型测试颤振应用程序:

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,当我在启动时尝试从sharedreferences中获取变量时,由于sharedreference是异步的,所以itemno为空。然后,应用程序运行build方法并在itemno=null上运行build方法,这会导致应用程序崩溃。

一旦它从sharedPreferences获取值,我就调用setState,然后用正确的值再次调用build方法。但是,不应该发生itemno=null的初始生成调用。

我希望有一个共享引用的同步方法,但它似乎不存在。如何运行应用程序,以便在启动时在颤振中正确初始化变量?

我试图通过使用同步方法来解决这个问题,通过写入一个JSON文件,然后使用下面的短flutter测试应用程序从中读取变量来初始化我的变量-对我来说,保存一个变量来初始化似乎是太过杀伤力了,但我还是尝试了一下:

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");
  }
}

在变量完全初始化之前调用build方法的结果仍然存在,这会导致应用程序崩溃。

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

如何在应用程序启动时初始化颤振中的变量?