【2025年版】Swift Concurrencyとは？初心者でもわかる非同期処理の基礎から実践まで

func fetchUserData(completion: @escaping (Result<User, Error>) -> Void) {
    URLSession.shared.dataTask(with: url) { data, response, error in
        if let error = error {
            completion(.failure(error))
            return
        }
        // データ処理...
        completion(.success(user))
    }.resume()
}

// 使用時
fetchUserData { result in
    switch result {
    case .success(let user):
        print(user.name)
    case .failure(let error):
        print(error)
    }
}

func fetchUserData() async throws -> User {
    let (data, _) = try await URLSession.shared.data(from: url)
    let user = try JSONDecoder().decode(User.self, from: data)
    return user
}

// 使用時
Task {
    do {
        let user = try await fetchUserData()
        print(user.name)
    } catch {
        print(error)
    }
}

actor BankAccount {
    private var balance: Double = 0
    
    func deposit(_ amount: Double) {
        balance += amount
    }
    
    func withdraw(_ amount: Double) -> Bool {
        guard balance >= amount else { return false }
        balance -= amount
        return true
    }
    
    func getBalance() -> Double {
        return balance
    }
}

// 使用例
let account = BankAccount()

Task {
    await account.deposit(1000)
    let balance = await account.getBalance()
    print("残高: \(balance)円")
}

// 基本的なTask
Task {
    let data = try await fetchData()
    print(data)
}

// キャンセル可能なTask
let task = Task {
    for i in 1...10 {
        try Task.checkCancellation() // キャンセルチェック
        print(i)
        try await Task.sleep(nanoseconds: 1_000_000_000) // 1秒待機
    }
}

// 3秒後にキャンセル
Task {
    try await Task.sleep(nanoseconds: 3_000_000_000)
    task.cancel()
}

func downloadImages(urls: [URL]) async throws -> [UIImage] {
    try await withThrowingTaskGroup(of: UIImage.self) { group in
        for url in urls {
            group.addTask {
                let (data, _) = try await URLSession.shared.data(from: url)
                guard let image = UIImage(data: data) else {
                    throw ImageError.invalidData
                }
                return image
            }
        }
        
        var images: [UIImage] = []
        for try await image in group {
            images.append(image)
        }
        return images
    }
}

@MainActor
class ViewModel: ObservableObject {
    @Published var userName: String = ""
    
    func loadUser() async {
        let user = try? await fetchUserData()
        // MainActorなので、自動的にメインスレッドで実行される
        self.userName = user?.name ?? "Unknown"
    }
}

// または部分的に指定
func updateUI() async {
    let data = await fetchData() // バックグラウンドで実行
    
    await MainActor.run {
        // この中だけメインスレッドで実行
        label.text = data
    }
}

struct WeatherResponse: Codable {
    let temperature: Double
    let description: String
}

actor WeatherService {
    private let baseURL = "https://api.weather.com"
    private var cache: [String: WeatherResponse] = [:]
    
    func getWeather(for city: String) async throws -> WeatherResponse {
        // キャッシュチェック
        if let cached = cache[city] {
            return cached
        }
        
        // API呼び出し
        let url = URL(string: "\(baseURL)/weather?city=\(city)")!
        let (data, _) = try await URLSession.shared.data(from: url)
        let weather = try JSONDecoder().decode(WeatherResponse.self, from: data)
        
        // キャッシュに保存
        cache[city] = weather
        return weather
    }
}

@MainActor
class WeatherViewModel: ObservableObject {
    @Published var temperature: String = ""
    @Published var description: String = ""
    @Published var isLoading: Bool = false
    
    private let service = WeatherService()
    
    func loadWeather(for city: String) async {
        isLoading = true
        defer { isLoading = false }
        
        do {
            let weather = try await service.getWeather(for: city)
            self.temperature = "\(weather.temperature)°C"
            self.description = weather.description
        } catch {
            self.description = "読み込みエラー"
        }
    }
}

// ❌ エラー：通常の関数からasync関数は呼べない
func normalFunction() {
    let data = await fetchData() // コンパイルエラー
}

// ✅ 正しい：Taskで囲む
func normalFunction() {
    Task {
        let data = await fetchData()
    }
}

actor Counter {
    var count = 0
}

let counter = Counter()

// ❌ エラー
print(counter.count)

// ✅ 正しい
Task {
    let value = await counter.count
    print(value)
}