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json - Flutter：如果 Firebase Storage 中的 JSON 文件已更新，如何使用 Firebase Storage 获取新数据

问题描述

我目前通过从 Firebase 存储调用 JSON 文件来显示数据，但我希望不是每次都下载 JSON 文件来显示数据 => 我将检查来自 Firebase 存储的 JSON 文件是否已更改：

关于 JSON 文件

这是我将 JSON 上传到 Firebase 存储后的 JSON 链接：

https://firebasestorage.googleapis.com/v0/b/tft-test-48c87.appspot.com/o/loadData.json?alt=media&token=92e3d416-62dc-4137-93a3-59ade95ac38f

据我所知，这个链接由两部分组成：

第一部分：https://firebasestorage.googleapis.com/v0/b/tft-test-48c87.appspot.com/o/loadData.json

最后一部分：?alt=media&token=+ 2e3d416-62dc-4137-93a3-59ade95ac38f（它是字符串的值：第一部分中的“downloadTokens” ）

在链接的第一部分，有关于 JSON 文件的所有信息，特别是我认为 String "updated"的值可以用作下载文件或不下载文件的条件。

前任。"updated": "2020-08-04T14:30:10.920Z",

每次我上传与旧 JSON 文件同名的新 JSON 文件时，更新的这个String的值都会改变，但链接下载不会改变。

脚步

所以我想做以下事情：

  1. 创建文件以将字符串“updated”存储在本地目录中（例如“updated”：null）以及下载到本地目录后存储JSON 文件的位置
  2. 打开应用
  3. 检查链接第一部分中的字符串“更新”：

我知道对于一篇文章来说这是很多问题，我是一个有代码的新手，如果我把它分成几篇文章，那么对我来说很难将它们组合起来。所以我希望有完整代码的答案，那会很棒。谢谢。这是主文件：

import 'package:ask/model/load_data_model.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

class LoadDataPage extends StatefulWidget {
  @override
  _LoadDataPageState createState() => _LoadDataPageState();
}

class DataServices {
  static const String url = 'https://firebasestorage.googleapis.com/v0/b/tft-test-48c87.appspot.com/o/loadData.json?alt=media&token=92e3d416-62dc-4137-93a3-59ade95ac38f';

  static Future<List<Data>> getData() async {
    try {
      final response = await http.get(url);
      if (200 == response.statusCode) {
        final List<Data> data = dataFromJson(response.body);
        return data;
      } else {
        return List<Data>();
      }
    } catch (e) {
      return List<Data>();
    }
  }
}

class _LoadDataPageState extends State<LoadDataPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text('Load Data')),
        body: FutureBuilder(
            future: DataServices.getData(),
            builder: (BuildContext context, AsyncSnapshot snapshot) {
              List<Widget> children;
              List<Data> _data = snapshot.data;
              if (snapshot.hasData) {
                return ListView.builder(
                  itemCount: _data.length,
                  itemBuilder: (context, index) {
                    return Column(
                      children: [Text(_data[index].data)],
                    );
                  },
                );
              } else {
                children = <Widget>[SizedBox(child: CircularProgressIndicator(), width: 60, height: 60), const Padding(padding: EdgeInsets.only(top: 16), child: Text('Loading...'))];
              }
              return Center(child: Column(mainAxisAlignment: MainAxisAlignment.center, crossAxisAlignment: CrossAxisAlignment.center, children: children));
            }));
  }
}

另一个步骤

EdwynZN 的回答对我很有用，但是，我编辑帖子以添加一个我认为会尽快加载页面的案例，所以请再次帮助我：

打开页面后 => readFile> compareLastUpdate> _lastUpdateDB&_createFile

P/S：这样的流程，他们第二次打开页面就会显示新的数据，对吧？但是我想知道如果StatefulWidget在新的JSON文件被覆盖到旧的JSON文件之后使用=> => 之后手机屏幕会显示新数据吗？

标签： jsonfirebaseflutterdartfirebase-storage

解决方案

我建议使用shared_preferences将最后更新的日期保存为字符串

import 'package:shared_preferences/shared_preferences.dart';
import 'package:path_provider/path_provider.dart';
import 'dart:convert';


/// Move them outside of the class as Top Level functions
List<Data> readFile(File file) {
  try{
    String data = file.readAsStringSync();
    return dataFromJson(data);
  } catch(e){
    print(e.toString());
    return List<Data>(); // or return an empty list, up to you
  }
}

// No need of encoder now because response body is already a String
void writeFile(Map<String, dynamic> arg) =>
  arg['file']?.writeAsStringSync(arg['data'], flush: true);

class DataServices {

  DateTime dateApi;

  static const String url = 'https://firebasestorage.googleapis.com/v0/b/tft-test-48c87.appspot.com/o/loadData.json?alt=media&token=92e3d416-62dc-4137-93a3-59ade95ac38f';
  static const String urlUpdate = 'https://firebasestorage.googleapis.com/v0/b/tft-test-48c87.appspot.com/o/loadData.json';

  Future<List<Data>> getData() async {
    bool update = await compareLastUpdate;
    if(update) { // that means the update times are the same, so retrieving form json file is better than doing http request
       final file  = await _createFile();
       if(await file.exists()) return await compute(readFile, file);
       else return null; //or an empty List
       // If it doesn't exists (probably first time running the app)
       // then retrieve an empty list, null or check how to fill the list from somewhere else
    }
    try {
      final response = await http.get(url);
      final SharedPreferences preferences = await SharedPreferences.getInstance();
      if (200 == response.statusCode) {
        final String utfData = utf8.decode(response.bodyBytes); //just decode it yourself instead of using response.body which uses [latin1] by default
        final List<Data> data = await compute(dataFromJson, utfData);
        final file  = await _createFile();
        Map<String, dynamic> args = {
          'file': file,
          'data': utfData
          //'data': response.body // pass the return body instead of the data
        };
        await compute(writeFile, args);
        await preferences.setString('updateDate', dateApi.toString()); //Save the new date
        return data;
      } else {
        return List<Data>();
      }
    } catch (e) {
      return List<Data>();
    }
  }

 File _createFile() async{
   Directory tempDir = await getTemporaryDirectory(); // or check for a cache dir also
   return File('${tempDir.path}/Data.json');
 }


Future<bool> get compareLastUpdate async{
  final dateCache = await _lastUpdateDB;
  dateApi = await _lastUpdateApi;

  if(dateCache == null) return false;    
  return dateApi?.isAtSameMomentAs(dateCache) ?? false; // or just isAfter()
  // If dateApi is null (an error conection or some throw) just return false or throw an error and 
  // catch it somewhere else (and give info to the user why it couldn't update)
}

Future<DateTime> get _lastUpdateApi async{
  try {
     final response = await http.get(urlUpdate);
     DateTime dateTime;
     if (200 == response.statusCode) {
       final data = jsonDecode(response.body));
       dateTime = DateTime.tryParse(data['updated'] ?? '');
     } 
     return dateTime;
   } catch (e) {
     return null;
   }
}

  Future<DateTime> get _lastUpdateDB async{
    final SharedPreferences preferences = await SharedPreferences.getInstance();
    return DateTime.tryParse(preferences.getString('updateDate') ?? ''); // Or if it's null use an old date
    // The first time the app opens there is no updateDate value, so it returns null, if that
    // happens replace it by an old date, one you know your api will be always newer,
    // Ex: 1999-08-06 02:07:53.973 Your Api/App didn't even exist back then
    // Or just use an empty String so the tryParser returns null
  }
}

然后在小部件中，您只需将其称为相同的

class _LoadDataPageState extends State<LoadDataPage> {
  final DataServices services = DataServices();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text('Load Data')),
        body: FutureBuilder(
            future: services.getData(),
            builder: (BuildContext context, AsyncSnapshot snapshot) {
              List<Widget> children;
              List<Data> _data = snapshot.data;
              if (snapshot.hasData) {
                return ListView.builder(
                  itemCount: _data.length,
                  itemBuilder: (context, index) {
                    return Column(
                      children: [Text(_data[index].data)],
                    );
                  },
                );
              } else {
                children = <Widget>[SizedBox(child: CircularProgressIndicator(), width: 60, height: 60), const Padding(padding: EdgeInsets.only(top: 16), child: Text('Loading...'))];
              }
              return Center(child: Column(mainAxisAlignment: MainAxisAlignment.center, crossAxisAlignment: CrossAxisAlignment.center, children: children));
            }));
  }
}

您还可以检查Dio 包，该包具有一些通过 http 的功能，可让您将参数添加到 url

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