多个期货的Future.wait（） [英] Future.wait() for multiple futures

问题描述

我的设备没有互联网连接时，我试图捕获该错误。我建立了2种将来的方法，一种是导入json的方法，另一种是查找数据库的方法。我有一个将来的构建器，它应该等待两个将来的构建完成之后再构建网格视图，但是由于连接错误，似乎似乎正在过早调用offlineFlashCardList。知道如何使它在调用快照错误之前等待两个期货完成吗？

I'm trying to catch the error when my device has no internet connection. I've built out 2 future methods, 1 to import a json and 1 to look into the database. I have a future builder that's suppose to wait for both futures to finish before building out the grid view but it seems like the offlineFlashCardList is being prematurely called due to the connection error. Any idea how to make it wait for both futures to finish before the snapshot error gets called?

import 'package:flutter/material.dart';
import 'package:flutter/foundation.dart';
import 'package:baby_sound/strings.dart';
import 'package:baby_sound/objects/flashCardList.dart';
import 'package:cached_network_image/cached_network_image.dart';
import 'dart:async' show Future;
import 'dart:convert';
import 'package:baby_sound/database/database.dart';
import 'package:baby_sound/objects/network.dart';
import 'package:http/http.dart' as http;

class GridViewWidget extends StatefulWidget{
  @override
  createState() => new GridViewState();

}

class GridViewState extends State<GridViewWidget>{


  List<FlashCardList> flashCardList;
  List<FlashCardList> offlineFlashCardList;



  Future<List<FlashCardList>> fetchFlashCardList() async{
    debugPrint("before response");
    List<FlashCardList> tempFlashCardList;
    final response = await http.get('some json url');
    //checkConnection(url).then((response){
      debugPrint ("after database load, response code: ${response.statusCode}");
      if (response.statusCode == 200) {
        var data = json.decode(response.body);
        var flashCardListData = data["FlashCardList"] as List;
        tempFlashCardList = flashCardListData.map<FlashCardList>((json) => FlashCardList.fromJson(json)).toList();
        for (int i = 0; i < tempFlashCardList.length; i++){
          debugPrint("DBProvider listID: ${await DBProvider.db.getFlashCardList(tempFlashCardList[i].flashCardListID)}, flashCardID: ${tempFlashCardList[i].flashCardListID}");
          if (await DBProvider.db.getFlashCardList(tempFlashCardList[i].flashCardListID) == null){
            DBProvider.db.newFlashCardList(tempFlashCardList[i]);
            debugPrint("Adding ${tempFlashCardList[i].name}}}");
          } else {
            DBProvider.db.updateFlashCardList(tempFlashCardList[i]);
            debugPrint("Updating ${tempFlashCardList[i].name}, getFlashCardList: ${DBProvider.db.getFlashCardList(tempFlashCardList[i].flashCardListID)}");
          }
        }
        flashCardList = tempFlashCardList;
        debugPrint("Standard flashCardList Size: ${flashCardList.length}");
      }

    debugPrint("flashCardList Size Before Return: ${flashCardList.length}");
    return flashCardList;
    }

  Future<List<FlashCardList>> fetchFlashCardListFromDB() async{

    offlineFlashCardList = await DBProvider.db.getAllFlashCardListFromDB();
    debugPrint("fetchFromDB size: ${offlineFlashCardList.length}");
    return offlineFlashCardList;
  }



  @override
  void initState(){
    debugPrint ('debug main.dart');
    super.initState();

  }


  @override
  Widget build(BuildContext context){
    return new Scaffold(
      appBar: new AppBar(
        centerTitle: true,
        title: new Text(Strings.pageTitle),
      ),
      body: FutureBuilder<List<FlashCardList>>(
        future: new Future(() async{
          await fetchFlashCardList();
          await fetchFlashCardListFromDB();
        }),
        builder: (BuildContext context, AsyncSnapshot<List<FlashCardList>> snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasError) {
              debugPrint("Snapshot has error: ${snapshot.error}");
              return new GridView.builder(
                  gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
                      maxCrossAxisExtent: 200.0,
                      childAspectRatio: 0.5),
                  itemCount: offlineFlashCardList.length,
                  itemBuilder: (BuildContext context, int index) {
                    return _getGridItemUI(context, offlineFlashCardList[index]);
                  });
//              return new Center(child: new CircularProgressIndicator());
            } else {
              debugPrint("Grid ViewBuilder");
              return new GridView.builder(
                  gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
                      maxCrossAxisExtent: 200.0,
                      childAspectRatio:0.5),
                  itemCount: flashCardList.length,
                  itemBuilder: (BuildContext context, int index) {
                    return _getGridItemUI(context, flashCardList[index]);
                  });
            }
          }else {
            debugPrint("CircularProgress");
            return new Center(child: new CircularProgressIndicator());
          }
        })
    );
  }

  _getGridItemUI(BuildContext context, FlashCardList item){
    return new InkWell(
      onTap: () {
        _showSnackBar(context, item);
      },
      child: new Card(
        child: new Column(
          crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[

              new Image(image: new CachedNetworkImageProvider("https://babymozart.org/babymozartq92C9TLa9UMkulL2m81xHdn9u2R92e1e/image/" + item.image)),
              /*new Expanded(
                child:new Center(
                  child: new Column(
                    children: <Widget>[
                      new SizedBox(height: 8.0),
                      new Expanded(
                        child: AutoSizeText(
                          item.name, maxLines: 1,
                        )
                      )
                    ],
                  ),
                )
              )*/
            ],
        ),
        elevation: 2.0,
        margin: EdgeInsets.all(5.0),
      )
    );
  }

  _showSnackBar(BuildContext context, FlashCardList item){

  }



}

推荐答案

您可以使用 Future.wait 等待几个Future完成。

You can use Future.wait to wait for several Future to be completed.

body: FutureBuilder<List<FlashCardList>>(
    future: Future.wait([
        fetchFlashCardList(),
        fetchFlashCardListFromDB(),
    ]),

这篇关于多个期货的Future.wait（）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！