将异步值返回到initState()

问题描述

我下面的代码运行正常，可以从url读取csv数据并输出输出:

I've the below code that is working fine, reading the csv data from url and printing the output:

import 'package:flutter/material.dart';
import 'dart:convert';
import 'dart:io';
import 'dart:async';
import 'package:csv/csv.dart';

void fetchUserData() async {
  final request = await HttpClient().getUrl(Uri.parse(
      'https://docs.google.com/spreadsheets/d/e/2PACX-1vQvf9tp4-fETDJbC-HRmRKvVFAXEAGO4lrYPpVeiYkB6nqqXdSs3CjX0eBMvjIoEeX9_qU6K2RWmzVk/pub?gid=0&single=true&output=csv'));
  final response = await request.close();
  List<List<dynamic>> rowsAsListOfValues;
  await for (final csvString in response.transform(const Utf8Decoder())) {
    rowsAsListOfValues =
        const CsvToListConverter().convert(csvString);
  }
  print(rowsAsListOfValues);
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    super.initState();
    fetchUserData();
  }

  @override
  Widget build(BuildContext context) { // ... // }
}

我不需要将输出打印出来，而是需要将其返回到一个变量中，该变量可以显示在y小部件中，我尝试如下进行操作:

Instead of getting the output printed, I need it to be returned into a variable, which I can display in y widget, I tried to do it as below:

Future<List<List<dynamic>>> fetchUserData() async {   /// change
  final request = await HttpClient().getUrl(Uri.parse(
      'https://docs.google.com/spreadsheets/d/e/2PACX-1vQvf9tp4-fETDJbC-HRmRKvVFAXEAGO4lrYPpVeiYkB6nqqXdSs3CjX0eBMvjIoEeX9_qU6K2RWmzVk/pub?gid=0&single=true&output=csv'));
  final response = await request.close();
  List<List<dynamic>> rowsAsListOfValues;
  await for (final csvString in response.transform(const Utf8Decoder())) {
    rowsAsListOfValues =
        const CsvToListConverter().convert(csvString);
  }

  return rowsAsListOfValues;    /// change
}

class _MyHomePageState extends State<MyHomePage> {
  var rowsAsListOfValues;  /// new

  @override
  void initState() {
    super.initState();
    rowsAsListOfValues = fetchUserData();    /// new
    print(rowsAsListOfValues);   /// new
  }

  @override
  Widget build(BuildContext context) { // ... // }
}

但是我得到的输出为 I/flutter(7505):'Future< List< List< dynamic>>>>'

我该如何解决?

推荐答案

在这种情况下，您需要从initState切换到didChangeDependency.因为您需要等待某些进程，所以不能在initState中等待.但是您可以这样等待

You need to switch from initState to didChangeDependency in this case. Because you need to await some process and you cant wait in initState. However you can wait like this

@override
void didChangeDependencies() async {
  super.didChangeDependencies();
  rowsAsListOfValues = await fetchUserData();
  super.setState(() {}); // to update widget data
  /// new
  print(rowsAsListOfValues);
}

这就是结果

I/flutter (24313): [[vranches], [Dammam, 2], [Khobar, 3]]

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