天气应用：未来三天

本课属于 Coddy Flutter 学习旅程的“数据与网络”单元。第 24 课，共 27 课。

城市对应的答案还包含天气预报：对象中的一个 JSON 列表，每天一项。

"forecast": [
  {"date": "2026-10-20", "condition": "Cloudy", "high": 22, "low": 15},
  ...
]

读取对象中的列表和单独读取列表的方式相同：先将它取出为 as List<dynamic>，然后用各自的 fromJson 处理每一项。看看文件底部的 Weather.fromJson：它已经完成了这些操作，所以 weather.forecast 是一个 List<DayForecast>。

final days = json['forecast'] as List<dynamic>;

forecast: days
    .map((day) => DayForecast.fromJson(day as Map<String, dynamic>))
    .toList(),

每天都有一个 DateTime。DateFormat('EEEE') 会将它转换为星期几的全称，例如“星期二”。在 Column 的 children 中使用 for，可以为每天创建一行。

挑战

中等

在当前天气下方，显示接下来的 3 天：

导入 package:intl/intl.dart
在 SizedBox(height: 32) 后，为每天添加一个 Row：在 Expanded 中显示星期几，使用 DateFormat('EEEE')，然后显示最高温度和最低温度，例如 22° / 15°
为每一行的左右两侧各添加 32 像素的内边距

亲手试试

import 'dart:convert';

import 'package:flutter/material.dart';

import 'api.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: WeatherScreen(),
    );
  }
}

class WeatherScreen extends StatefulWidget {
  const WeatherScreen({super.key});

  @override
  State<WeatherScreen> createState() => _WeatherScreenState();
}

class _WeatherScreenState extends State<WeatherScreen> {
  late Future<Weather> _weather;

  @override
  void initState() {
    super.initState();
    _weather = loadWeather();
  }

  Future<Weather> loadWeather() async {
    final body = await fetch('/weather/lisbon');
    return Weather.fromJson(jsonDecode(body));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Weather')),
      body: Center(
        child: FutureBuilder<Weather>(
          future: _weather,
          builder: (context, snapshot) {
            if (snapshot.hasError) {
              return Text('Could not load: ${snapshot.error}');
            }
            if (snapshot.hasData) {
              return buildWeather(snapshot.data!);
            }
            return const CircularProgressIndicator();
          },
        ),
      ),
    );
  }

  Widget buildWeather(Weather weather) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      children: [
        Text(weather.city, style: const TextStyle(fontSize: 32)),
        Text(
          '${weather.temperature}°',
          style: const TextStyle(fontSize: 72, fontWeight: FontWeight.bold),
        ),
        Text(weather.condition, style: const TextStyle(fontSize: 24)),
      ],
    );
  }
}

class Weather {
  Weather({
    required this.city,
    required this.temperature,
    required this.condition,
    required this.forecast,
  });

  final String city;
  final int temperature;
  final String condition;
  final List<DayForecast> forecast;

  factory Weather.fromJson(Map<String, dynamic> json) {
    final days = json['forecast'] as List<dynamic>;
    return Weather(
      city: json['city'] as String,
      temperature: json['temperature'] as int,
      condition: json['condition'] as String,
      forecast: days
          .map((day) => DayForecast.fromJson(day as Map<String, dynamic>))
          .toList(),
    );
  }
}

class DayForecast {
  DayForecast({
    required this.date,
    required this.condition,
    required this.high,
    required this.low,
  });

  final DateTime date;
  final String condition;
  final int high;
  final int low;

  factory DayForecast.fromJson(Map<String, dynamic> json) {
    return DayForecast(
      date: DateTime.parse(json['date'] as String),
      condition: json['condition'] as String,
      high: json['high'] as int,
      low: json['low'] as int,
    );
  }
}

自我测试

这节课包含一个小测验。开始学习即可作答，并记录你的进度。

数据与网络中的所有课

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