Swift: Swift 属性深入：存储属性、计算属性、属性观察器与懒加载

class UserProfile {
    var avatarUrl: String
    func displayAvatar() {
        // 每次都从网络下载头像
        downloadImage(from: avatarUrl) { image in
            // 显示图片
        }
    }
}

class UserProfile {
    var avatarUrl: String
    lazy var cachedAvatar: UIImage? = {
        // 只执行一次——首次访问时下载
        return downloadImage(from: avatarUrl)
    }()
}

// ============================================
// 存储属性的基本用法
// ============================================
struct User {
    let id: Int           // 常量存储属性
    var name: String       // 变量存储属性
    var email: String
}
var user = User(id: 1, name: "Alice", email: "alice@example.com")
user.name = "Alice Smith"  // 变量属性可以修改
// user.id = 2             // 编译错误！常量属性不可修改
print("\(user.name) (\(user.email))")

// ============================================
// 计算属性——摄氏/华氏温度互转
// ============================================
struct Temperature {
    var celsius: Double
    // 计算属性——从 celsius 计算 fahrenheit
    var fahrenheit: Double {
        get {
            return celsius * 9 / 5 + 32
        }
        set(newFahrenheit) {
            celsius = (newFahrenheit - 32) * 5 / 9
        }
    }
}
var temp = Temperature(celsius: 25)
print("Celsius: \(temp.celsius)°C")
print("Fahrenheit: \(temp.fahrenheit)°F")
// 通过设置 fahrenheit 间接修改 celsius
temp.fahrenheit = 100
print("After setting to 100°F:")
print("Celsius: \(temp.celsius)°C")

// ============================================
// 属性观察器监控用户分数变化
// ============================================
class Player {
    var name: String
    var score: Int = 0 {
        willSet {
            print("分数将从 \(score) 变为 \(newValue)")
        }
        didSet {
            print("分数已从 \(oldValue) 变为 \(score)")
            if score > 100 {
                print("🎉 恭喜 \(name) 获得高分！")
            }
        }
    }
    var level: Int {
        // 根据分数计算的只读计算属性
        switch score {
        case 0..<50: return 1
        case 50..<100: return 2
        default: return 3
        }
    }
    init(name: String) {
        self.name = name
    }
}
let player = Player(name: "Alice")
player.score = 60
print("Level: \(player.level)")
player.score = 120
print("Level: \(player.level)")

// ============================================
// lazy 懒加载：数据库配置
// ============================================
class DatabaseManager {
    let configFile: String
    // 懒加载——只有执行查询时才初始化数据库连接
    lazy var connection: String = {
        print("正在建立数据库连接（只执行一次）...")
        // 模拟建立连接
        return "Connected to \(configFile)"
    }()
    init(configFile: String) {
        self.configFile = configFile
    }
    func query(_ sql: String) {
        print("使用连接：\(connection) 执行查询：\(sql)")
    }
}
let db = DatabaseManager(configFile: "app.db")
print("DatabaseManager 已创建，连接未初始化")
db.query("SELECT * FROM users")
db.query("SELECT * FROM orders")
// 第二次访问 connection 时不会重新初始化

// ============================================
// static 类型属性
// ============================================
struct AppConfig {
    static let appName = "MySwiftApp"
    static var version = "1.0"
    static var launchCount = 0
    static func incrementLaunch() {
        launchCount += 1
    }
}
// 通过类型名直接访问，无需实例
print(AppConfig.appName)
AppConfig.version = "1.1"
AppConfig.incrementLaunch()
AppConfig.incrementLaunch()
print("Version: \(AppConfig.version), Launches: \(AppConfig.launchCount)")

// ============================================
// 完整示例：个人资料管理系统
// 功能：存储/计算属性 + 属性观察器 + lazy + static
// ============================================
import Foundation
// 1. 全局配置（类型属性）
struct Config {
    static let maxAvatarSizeMB = 5.0
    static var apiBaseURL = "https://api.example.com"
    static var userCount = 0
}
// 2. 用户资料
class UserProfile {
    // 存储属性
    let id: Int
    var name: String {
        didSet {
            print("用户名已更新：\(oldValue) → \(name)")
        }
    }
    var avatarUrl: String
    // 计算属性——根据姓名生成缩写
    var initials: String {
        name.split(separator: " ").compactMap { $0.first }.map { String($0) }.joined()
    }
    // 懒加载——首次访问时计算年龄
    lazy var ageDescription: String = {
        print("首次计算年龄描述...")
        return "\(name)'s profile"
    }()
    // 属性观察器
    var email: String {
        willSet {
            print("正在更新邮箱...")
        }
        didSet {
            print("邮箱已更新为 \(email)")
        }
    }
    init(id: Int, name: String, email: String, avatarUrl: String) {
        self.id = id
        self.name = name
        self.email = email
        self.avatarUrl = avatarUrl
        Config.userCount += 1
    }
    deinit {
        Config.userCount -= 1
    }
}
// 3. 使用
let profile = UserProfile(
    id: 1,
    name: "Alice Johnson",
    email: "alice@example.com",
    avatarUrl: "https://example.com/avatar.jpg"
)
print("缩写：\(profile.initials)")
print("年龄：\(profile.ageDescription)")
print("活跃用户数：\(Config.userCount)")
// 修改触发观察器
profile.email = "alice@newdomain.com"
profile.name = "Alice Smith"