天气应用：再试一次

本课属于 Coddy Flutter 学习旅程的“数据与网络”单元。第 26 课，共 27 课。

天气应用可能会在火车上、隧道里和山上打开。没有信号时，应用应该清楚地说明这一点，并提供返回的方法，而不是显示一个永远转不停的加载图标，或一行错误文本。

为错误单独设计一个小屏幕：一个图标、一句容易理解的话、下面的详细信息，以及一个重试按钮，用它在 setState 中创建一个新的 Future。把它放在自己的方法 buildError 中，与 buildWeather 并列，这样 build 就能保持简短。

if (snapshot.connectionState == ConnectionState.waiting) {
  return const CircularProgressIndicator();
}
if (snapshot.hasError) {
  return buildError(snapshot.error!);
}
return buildWeather(snapshot.data!);

先检查 ConnectionState.waiting，这样每次加载时都会显示加载图标，包括第二次尝试。Future 完成后，结果要么是错误，要么是数据，因此最后一行可以使用 snapshot.data!。

要在预览中查看效果，请在 main 的第一行写入 offline = true;，然后按“运行”。按“检查”之前，请先删除这行代码。

挑战

中等

为应用添加错误界面：

一个 buildError(Object error) 方法：显示 Icons.cloud_off（大小为 64）、Could not load the weather（字号为 20）、错误文本，以及一个 Try again 按钮
一个 tryAgain 方法，在 setState 中设置新的 loadWeather()
在 builder 中，先检查 ConnectionState.waiting，然后检查错误，最后显示天气

亲手试试

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:intl/intl.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));
  }

  IconData iconFor(String condition) {
    return switch (condition) {
      'Sunny' => Icons.wb_sunny,
      'Cloudy' => Icons.cloud,
      'Rainy' => Icons.umbrella,
      'Snowy' => Icons.ac_unit,
      _ => Icons.help_outline,
    };
  }

  @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)),
        Icon(iconFor(weather.condition), size: 96, color: Colors.orange),
        Text(
          '${weather.temperature}°',
          style: const TextStyle(fontSize: 72, fontWeight: FontWeight.bold),
        ),
        Text(weather.condition, style: const TextStyle(fontSize: 24)),
        const SizedBox(height: 32),
        for (final day in weather.forecast)
          Padding(
            padding: const EdgeInsets.symmetric(horizontal: 32, vertical: 8),
            child: Row(
              children: [
                Icon(iconFor(day.condition)),
                const SizedBox(width: 16),
                Expanded(
                  child: Text(
                    DateFormat('EEEE').format(day.date),
                    style: const TextStyle(fontSize: 18),
                  ),
                ),
                Text('${day.high}° / ${day.low}°', style: const TextStyle(fontSize: 18)),
              ],
            ),
          ),
      ],
    );
  }
}

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

自己练习： Flutter 在线编译器