在 API 调用中不返回响应，带有 http 包

Question

我正在使用flutter的Http包连接API，但我看到它没有返回任何内容，也没有捕获错误。

这是代码。

只打印第一个print()

   Future getCountryByName(String name) async {
    try {
      print(name);

      final url = 'https://restcountries.eu/rest/v2/name/colombia';
      final response = await http.get(url);
      print(response);
      return [];
    } catch (e) {
      print(e);
    }
  }

Answer 1

网络服务：

class NetService {
  static Future<T> getJson<T>(String url) {
    return http.get(Uri.parse(url))
      .then((response) {
        if (response.statusCode == 200) {
          return jsonDecode(response.body) as T;
        }
        print('Status Code : ${response.statusCode}...');
        return null;
      })
      .catchError((err) => print(err));
  }
}

主要：

import 'package:_samples2/networking.dart';

class Country {
  static const url = 'https://restcountries.eu/rest/v2/name/colombia';

  static Future<List> getColombiaInfo() async {
    print('Start fetching...');
    return await NetService.getJson<List>(url).whenComplete(() => print('Fetching done!'));
  }
}

void main(List<String> args) async {
  var info = await Country.getColombiaInfo();

  print(info);
}

结果：

Start fetching...
Fetching done!
[{name: Colombia, topLevelDomain: [.co], alpha2Code: CO, alpha3Code: COL, callingCodes: [57], capital: Bogotá, altSpellings: [CO, Republic of Colombia, República de Colombia], region: Americas, subregion: South America, population: 48759958, latlng: [4.0, -72.0], demonym: Colombian, area: 1141748.0, gini: 55.9, timezones: [UTC-05:00], borders: [BRA, ECU, PAN, PER, VEN], nativeName: Colombia, numericCode: 170, currencies: [{code: COP, name: Colombian peso, symbol: $}], languages: [{iso639_1: es, iso639_2: spa, name: Spanish, nativeName: Español}], translations: {de: Kolumbien, es: Colombia, fr: Colombie, ja: コロンビア, it: Colombia, br: Colômbia, pt: Colômbia, nl: Colombia, hr: Kolumbija, fa: کلمبیا}, flag: https://restcountries.eu/data/col.svg, regionalBlocs: [{acronym: PA, name: Pacific Alliance, otherAcronyms: [], otherNames: [Alianza del Pacífico]}, {acronym: USAN, name: Union of South American Nations, otherAcronyms: [UNASUR, UNASUL, UZAN], otherNames: [Unión de Naciones Suramericanas, União de Nações Sul-Americanas, Unie van Zuid-Amerikaanse Naties, South American Union]}], cioc: COL}]

Answer 2

1/ 您必须为您的 Future 定义类型（在 之间），一个表示延迟计算的对象，才能正常工作

Future 用于表示将来某个时间可用的潜在值或错误。 Future 的接收者可以注册回调来处理可用的值或错误。例如：

2/还请仔细阅读documentation for http：

http.get() 方法返回一个包含响应的 Future

3/ 也请阅读documentation for FutureBuilder，因为您可能会需要它。

完整的工作示例

您可以按照文档的示例正确获取您的数据：

class Country {
  final String name;
  final String capital;
  final String region;

  Country({this.name, this.capital, this.region});

  factory Country.fromJson(Map<String, dynamic> json) {
    return Country(
      name: json['name'],
      capital: json['capital'],
      region: json['region'],
    );
  }

  @override
  String toString() { // nice override to quickly get info about your json
    return 'Country{name: $name, capital: $capital, region: $region)';
  }
}

Future<Country> getCountryByName  (String name) async {
  Country country;
  try {
    final url = 'https://restcountries.eu/rest/v2/name/$name';
    final response = await http.get(url);
    if (response.statusCode == 200) {
      print(response.body);

      country = Country.fromJson(json.decode(response.body)[0]); // the response that you get is an array, you just want to first item.
    }
    else
      print("error: ${response.statusCode}");
  }
  catch (e) {
    print(e);
  }
    print(country.toString());
    return country;
}

您的有状态小部件应如下所示：

class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);

final String title;

@override
_MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child:  FutureBuilder<Country>(
        future: getCountryByName("colombia"),
        builder: (context, AsyncSnapshot<Country> snapshot) {
          if (snapshot.connectionState == ConnectionState.done &&
              snapshot.hasData) {
            return Text(snapshot.data.toString());
          }
          else
            return CircularProgressIndicator();
    })

      ),
    );
  }
}

当然不要忘记导入：

import 'package:flutter/material.dart';
import 'dart:convert'; // for json.decode
import 'package:http/http.dart' as http; // for get request

我在Github上传了这个项目，如果你想试试的话。

Answer 3

更正的代码：

import 'dart:convert' as convert; // for jsonDecode
import 'package:http/http.dart' as http;

 Future getCountryByName(String name) async {
  try {
    print(name);

    final url = 'https://restcountries.eu/rest/v2/name/colombia';
    // http.get returns future of type Future<Response>
    final response = await http.get(url);
    // parse the response and get the resulting json reponse
    var jsonResponse = convert.jsonDecode(response.body);
    print(jsonResponse);
    return [];
  } catch (e) {
    print(e);
  }
}

参考：http package usage guide