Flutter: How to use Firebase Storage to get new data if JSON file in Firebase Storage updated

Question

I am currently displaying the data by calling the JSON file from Firebase Storage, but I want that instead of download JSON file every single time to show data => I will check if the JSON file from the Firebase Store has changed:

• If it changed => download the new JSON file to Local directory and display it.
• Otherwise => display the old JSON file in Local directory (This old JSON file will be downloaded when first time App open)

---

About JSON File

This is JSON link after I upload JSON to Firebase Storage:

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

As far as I know, this link is made up of 2 parts:

First part: https://firebasestorage.googleapis.com/v0/b/tft-test-48c87.appspot.com/o/loadData.json

Last part: ?alt=media&token= + 2e3d416-62dc-4137-93a3-59ade95ac38f (it is value of String: "downloadTokens" in First part)

In the First part of the link, there is all information about JSON file, and especially I think that value of String "updated" can be used as a condition for the purpose of downloading files or not.

Ex. "updated": "2020-08-04T14:30:10.920Z",

The value of this String updated will change every time I upload a new JSON file with the same name as the old JSON file but the link download will not change.

---

Steps

So I want to do the following:

1. Create file to store String "updated" in Local directory (Ex. "updated": null) and where to store the JSON file after download to Local directory
2. Open App
3. Check String "updated" in link First Part:

• Case A: if value of String "updated" in First Part != value of String "updated" in Local directory =>

  • Step 1: download JSON file (by link: First part + ?alt=media&token= + downloadTokens) to Local directory (If the old json file already exists, it will be replaced)
  • Step 2: overwrite value of String "updated" in Local directory by value of String "updated" in Firebase Storage
  • Step 3: access JSON file in Local directory to display data

• Case B: if value of String "updated" in First Part == value of String "updated" in Local directory => do nothing, just access JSON file in Local directory to display data

---

I know this is a lot of questions for one post, I'm a newbie with code and if I split it up into a few posts then it is very difficult to combine them for me. So I hope the answer with full code, that would be great. Thanks. This is the main file:

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));
            }));
  }
}

---

Another Steps

EdwynZN's answer worked great for me, however, I edit the post to add one more case which I think will make load page ASAP, So please help me again:

After open Page => readFile > compareLastUpdate > _lastUpdateDB & _createFile

• Case A: The first time the app opens => readFile: false > _lastUpdateDB & _createFile > readFile again
• Case B: Not the first time the app opens:
  • the data is still loaded immediately from the old JSON, at the same time, run in background: compareLastUpdate:
    • If update times are the same => do nothing
    • If update times are different => _lastUpdateDB & _createFile

P/S: With this flow, the second time they open the page then new data will be displayed, right? But I wonder that if using StatefulWidget => after the new JSON file is overwritten to the old JSON file => will the phone screen display new data after that?

Answer 1

I would recommend using shared_preferences to save the last updated date as a String

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
  }
}

Then in the widget you just call it the same

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));
            }));
  }
}

Also yu could check Dio package which have some functions over http that let you add parameters to the url