并发与 async/await

Swift 并发模型：async/await、Task、Actor 保证线程安全。对照 /roadmaps/swift-ui「并发」节点，网络加载与 UI 更新必须正确分离主线程与后台任务。

学习目标

• 理解 @MainActor 与主线程 UI 更新规则
• 用 Task、TaskGroup 执行异步工作
• 掌握 Actor 隔离可变状态
• 在 SwiftUI 中用 .task 与 async ViewModel 加载数据

核心概念

async/await 与 MainActor

struct Weather: Decodable {
    let city: String
    let temp: Double
    let condition: String
}

enum WeatherError: Error {
    case networkFailed
}

func fetchWeather(city: String) async throws -> Weather {
    try await Task.sleep(nanoseconds: 500_000_000)
    return Weather(city: city, temp: 22.5, condition: "晴")
}

@MainActor
final class WeatherViewModel: ObservableObject {
    @Published var weather: Weather?
    @Published var isLoading = false
    @Published var errorMessage: String?

    func load(city: String) async {
        isLoading = true
        errorMessage = nil
        do {
            weather = try await fetchWeather(city: city)
        } catch {
            errorMessage = "加载失败"
        }
        isLoading = false
    }
}

struct WeatherView: View {
    @StateObject private var viewModel = WeatherViewModel()
    @State private var city = "北京"

    var body: some View {
        VStack(spacing: 16) {
            TextField("城市", text: $city)
                .textFieldStyle(.roundedBorder)

            if viewModel.isLoading {
                ProgressView("加载中…")
            } else if let w = viewModel.weather {
                Text("\(w.city) \(w.temp, specifier: "%.1f")°C")
                Text(w.condition).foregroundStyle(.secondary)
            }

            Button("刷新") {
                Task { await viewModel.load(city: city) }
            }
        }
        .padding()
        .task(id: city) { await viewModel.load(city: city) }
    }
}

@MainActor 保证 @Published 变更在主线程；ViewModel 标注 @MainActor 是 SwiftUI 最佳实践。

Actor 与 TaskGroup

actor ImageCache {
    private var storage: [URL: Data] = [:]

    func data(for url: URL) -> Data? {
        storage[url]
    }

    func store(_ data: Data, for url: URL) {
        storage[url] = data
    }
}

func fetchMultiple(urls: [URL]) async -> [Data] {
    await withTaskGroup(of: Data?.self) { group in
        for url in urls {
            group.addTask {
                try? Data(contentsOf: url)
            }
        }
        var results: [Data] = []
        for await data in group {
            if let data { results.append(data) }
        }
        return results
    }
}