结构体、类与协议

Swift 是协议导向语言：struct 值类型用于模型，class 引用类型用于共享状态，protocol 定义能力契约。对照 /roadmaps/swift-ui「OOP」节点，这是 SwiftUI 数据建模与 ViewModel 的基础。

学习目标

• 区分 struct 与 class 的语义与拷贝行为
• 使用 protocol 与 extension 组织代码
• 理解 ARC 与 weak/unowned 打破循环引用
• 为 SwiftUI 模型实现 Identifiable、Codable

核心概念

struct vs class

struct Lesson: Identifiable, Codable, Equatable {
    let id: Int
    var title: String
    var completed: Bool
}

// 值类型：拷贝独立
var a = Lesson(id: 1, title: "语法", completed: false)
var b = a
b.completed = true
print(a.completed)  // false，a 不受影响

// 引用类型：共享实例
class LessonStore {
    var lessons: [Lesson] = []
    func markComplete(id: Int) {
        if let i = lessons.firstIndex(where: { $0.id == id }) {
            lessons[i].completed = true
        }
    }
}

let store1 = LessonStore()
let store2 = store1  // 同一实例
store2.lessons.append(Lesson(id: 1, title: "OOP", completed: false))
print(store1.lessons.count)  // 1

SwiftUI 模型优先 struct（线程安全、可预测）；ViewModel 用 class + ObservableObject。

Protocol 与 Extension

protocol TrackDisplayable {
    var displayTitle: String { get }
    var lessonCount: Int { get }
}

struct Track: TrackDisplayable, Identifiable {
    let id: Int
    var title: String
    var lessons: [Lesson]

    var displayTitle: String { "📚 \(title)" }
    var lessonCount: Int { lessons.count }
}

extension Track {
    var completionRate: Double {
        guard !lessons.isEmpty else { return 0 }
        let done = lessons.filter(\.completed).count
        return Double(done) / Double(lessons.count)
    }
}

Protocol 支持组合能力；Extension 为已有类型添加计算属性与方法，无需继承。

ARC 与 weak

class NetworkManager {
    var onComplete: (() -> Void)?

    func fetch(completion: @escaping () -> Void) {
        onComplete = completion
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            completion()
        }
    }
}

// 闭包捕获 self 时用 weak 避免循环引用
class LessonViewModel: ObservableObject {
    private let manager = NetworkManager()

    func load() {
        manager.fetch { [weak self] in
            self?.refreshUI()
        }
    }

    private func refreshUI() { /* 更新 @Published */ }
}