我目前正在通过从Firebase Storage调用JSON文件来显示数据，但是我希望这样做，而不是每次都下载JSON文件来显示数据=>我将检查Firebase Store中的JSON文件是否已更改:
关于JSON文件
将JSON上传到Firebase存储后，这是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，此链接由两部分组成:
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(这是中字符串:“downloadTokens”的值，第一部分)在该链接的第一部分中，包含有关JSON文件的所有信息，尤其是我认为String “已更新” 的值可以用作是否下载文件的条件。
例如
"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，此字符串已更新的值都会更改，但链接下载不会更改。脚步
因此，我想执行以下操作:
!=值本地目录 =>First part + ?alt=media&token= + downloadTokens)下载到本地目录(如果旧的json文件已经存在，它将被替换)我知道对于一个帖子，这是一个很多问题，我是一个有代码的新手，如果我将其分成几篇文章，那么对我来说很难将它们组合在一起。因此，我希望完整代码的答案会很棒。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的答案对我来说非常有效，但是，我编辑该帖子以添加更多案例，我认为这将使加载页面尽快，所以请再次帮助我:
打开页面后=>
==> readFile> compareLastUpdate和_lastUpdateDB_createFile:否> readFile和_lastUpdateDB>再次是_createFile readFile:compareLastUpdate和_lastUpdateDB P / S:通过此流程，他们第二次打开页面，然后将显示新数据，对吗？但是我想知道如果在新的JSON文件被覆盖到旧的JSON文件=>之后使用
_createFile =>，那么电话屏幕会在那之后显示新数据吗？
最佳答案
我建议使用shared_preferences将最后更新的日期另存为字符串
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
}
}
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));
}));
}
}
关于json - Flutter:如果Firebase Storage中的JSON文件已更新，则如何使用Firebase Storage获取新数据，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/63251355/