// ❌ Compile-time error in Swift 6
var globalCounter = 0

func increment() {
    globalCounter += 1  // Error: concurrent access
}

// ✅ Safe with actor isolation
actor Counter {
    private var value = 0
    func increment() { value += 1 }
}

struct User: Sendable {
    let id: Int
    let name: String
}

@Sendable func process() async {
    // Captures must be Sendable
}

@MainActor
class ViewModel: ObservableObject {
    @Published var items = [Item]()
}

@globalActor
actor DatabaseActor {
    static let shared = DatabaseActor()
    private init() {}
}

func fetchData() async throws -> Data {
    let (data, _) = try await URLSession.shared.data(from: url)
    return data
}

// Fire-and-forget
Task {
    let result = await asyncOperation()
}

// With priority
Task(priority: .background) {
    await heavyComputation()
}

// With cancellation
let task = Task {
    try await longRunningOperation()
}
task.cancel()

// Async let (parallel await)
async let task1 = fetchUser()
async let task2 = fetchSettings()
let (user, settings) = try await (task1, task2)

// TaskGroup
try await withThrowingTaskGroup(of: Item.self) { group in
    for id in ids {
        group.addTask { try await fetchItem(id: id) }
    }
    return try await group.reduce(into: []) { $0.append($1) }
}

actor BankAccount {
    private var balance: Double = 0
    
    func deposit(_ amount: Double) {
        balance += amount
    }
    
    func getBalance() -> Double {
        return balance
    }
}

let account = BankAccount()
await account.deposit(100)

// Crossing isolation boundaries
@MainActor
func updateUI() async {
    // On main thread
    let data = await fetchData()  // Switch to non-isolated
    label.text = data  // Back to main thread
}

// Region transfer
func process() async {
    let data = Data()  // Disconnected
    await save(data)   // Transfer to actor
    // ❌ Can't use data here anymore
}

// Before (Swift 5)
func fetchUser(completion: @escaping (Result<User, Error>) -> Void) {
    URLSession.shared.dataTask(with: url) { data, response, error in
        // Handle result
        completion(result)
    }.resume()
}

// After (Swift 6)
func fetchUser() async throws -> User {
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}