Flutter系列文章-实战项目-编程知识

在本篇文章中，我们将通过一个实际的 Flutter 应用来综合运用最近学到的知识，包括保存到数据库、进行 HTTP 请求等。我们将开发一个简单的天气应用，可以根据用户输入的城市名获取该城市的天气信息，并将用户查询的城市列表保存到本地数据库中。

第一步：需求分析和设计

1. 确定项目目标

我们的目标是开发一个天气应用，用户可以在应用中输入城市名，然后获取该城市的天气信息。

2. 设计界面

我们的应用包含两个页面：主页面用于输入城市名，显示天气信息，以及查询历史记录页面用于显示用户查询的城市列表。

第二步：开发

1. 创建 Flutter 项目

首先，在命令行中创建一个新的 Flutter 项目：

flutter create my_first_flutter_app

然后，进入项目目录：

cd my_first_flutter_app

2. 编码实现

a. 创建页面和路由

在 lib 文件夹中创建两个文件：home_page.dart、history_page.dart。

home_page.dart：

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';class HomePage extends StatefulWidget {@override_HomePageState createState() => _HomePageState();
}class _HomePageState extends State<HomePage> {String city = '';String weather = '';void getWeather() async {final response = await http.get(Uri.parse('https://api.openweathermap.org/data/2.5/weather?q=$city&appid=YOUY_API_KEY'));if (response.statusCode == 200) {final data = jsonDecode(response.body);setState(() {weather ='Temperature: ${data['main']['temp']}°C, Weather: ${data['weather'][0]['main']}';});} else {setState(() {weather = 'Failed to get weather data';});}}@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text('Weather App'),),body: Padding(padding: EdgeInsets.all(16),child: Column(crossAxisAlignment: CrossAxisAlignment.stretch,children: [TextField(onChanged: (value) {setState(() {city = value;});},decoration: InputDecoration(labelText: 'City Name'),),ElevatedButton(onPressed: () {getWeather();},child: Text('Get Weather'),),SizedBox(height: 20),Text(weather,style: TextStyle(fontSize: 18),),],),),);}
}

history_page.dart：

import 'package:flutter/material.dart';
import 'package:my_first_flutter_app/database_helper.dart';
import 'package:my_first_flutter_app/city.dart';class HistoryPage extends StatefulWidget {@override_HistoryPageState createState() => _HistoryPageState();
}class _HistoryPageState extends State<HistoryPage> {List<City> cities = [];@overridevoid initState() {super.initState();fetchCities();}void fetchCities() async {final dbHelper = DatabaseHelper.instance;final allRows = await dbHelper.queryAllRows();setState(() {cities = allRows.map((row) => City.fromMap(row)).toList();});}@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text('Search History'),),body: ListView.builder(itemCount: cities.length,itemBuilder: (context, index) {final city = cities[index];return ListTile(title: Text(city.name),subtitle: Text('Weather: ${city.weather}'),);},),);}
}

在 main.dart 文件中，设置路由：

import 'package:flutter/material.dart';
import 'package:my_first_flutter_app/home_page.dart';
import 'package:my_first_flutter_app/history_page.dart';void main() {runApp(MyApp());
}class MyApp extends StatelessWidget {@overrideWidget build(BuildContext context) {return MaterialApp(initialRoute: '/',routes: {'/': (context) => HomePage(),'/history': (context) => HistoryPage(),},);}
}

b. 实现数据库功能

我们将使用 SQLite 来保存用户查询的城市列表。在 lib 文件夹中创建新的文件 city.dart、database_helper.dart，用于创建和管理数据库。

city.dart

class City {int id;String name;String weather;City({this.id = 0, required this.name, required this.weather});Map<String, dynamic> toMap() {return {'id': id, 'name': name, 'weather': weather};}City.fromMap(Map<String, dynamic> map): id = map['id'],name = map['name'],weather = map['weather'];
}

database_helper.dart

import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';
import 'package:my_first_flutter_app/city.dart';class DatabaseHelper {static final _databaseName = 'weather_database.db';static final _databaseVersion = 1;static final table = 'cities';static final columnId = '_id';static final columnName = 'name';static final columnWeather = 'weather';DatabaseHelper._privateConstructor();static final DatabaseHelper instance = DatabaseHelper._privateConstructor();static Database? _database;Future<Database?> get database async {if (_database != null) return _database;_database = await _initDatabase();return _database;}_initDatabase() async {String path = join(await getDatabasesPath(), _databaseName);return await openDatabase(path,version: _databaseVersion, onCreate: _onCreate);}Future<void> _onCreate(Database db, int version) async {await db.execute('''CREATE TABLE $table ($columnId INTEGER PRIMARY KEY,$columnName TEXT NOT NULL,$columnWeather TEXT NOT NULL)''');}Future<int> insert(City city) async {Database? db = await instance.database;return await db?.insert(table, city.toMap()) ?? 0;}Future<List<Map<String, dynamic>>> queryAllRows() async {Database? db = await instance.database;return await db?.query(table) ?? [];}
}