Swift concurrency

Introduction:

In Swift, asynchronous programming is becoming increasingly important as applications need to be able to handle long-running tasks without blocking the user interface. The latest version of Swift, Swift 5.5, introduced a number of new features to make asynchronous programming easier and more expressive. In this article, we will explore the new async/await syntax, Tasks, async let, Task groups, actors and global actors, and how each of these features can help us write more efficient, readable and maintainable code.

1. Async/Await:

Before Swift 5.5, asynchronous programming in Swift relied on the completion handler pattern, where a function would take a closure that would be called when the operation was completed. This pattern can lead to code that is difficult to read and understand, especially when multiple asynchronous operations are chained together. With the new async/await syntax, we can write asynchronous code that looks and behaves like synchronous code.

Async/await allows us to write asynchronous functions that return a value, without having to use completion handlers. The async keyword is used to mark a function as asynchronous, and the await keyword is used to wait for the completion of an asynchronous operation. Here is an example:

func fetchImage() async throws -> UIImage {
 let url = URL(string: "https://example.com/image.jpg")!
 let (data, response) = try await URLSession.shared.data(from: url)
 guard (response as? HTTPURLResponse)?.statusCode == 200 else {
 throw NetworkError.invalidResponse
 }
 return UIImage(data: data)!
}

In this example, we use the new async/await syntax to fetch an image from a URL using the URLSession API. We use the await keyword to wait for the data to be downloaded, and then we use the guard statement to ensure that the response has a status code of 200 before returning the image.

2 . Tasks:

In Swift 5.5, Tasks are a new concurrency primitive that allow us to represent a unit of work that can be run concurrently with other tasks. Tasks can be used to perform both synchronous and asynchronous work, and can be cancelled if they are no longer needed.

Tasks are created using the Task.init function, and can be run using the Task.run function. Here is an example:

let task = Task {
    print("Hello, world!")
}
task.run()

In this example, we create a task that simply prints “Hello, world!” to the console, and then run the task using the Task.run function.

3. Async Let:

Async let is a new feature in Swift 5.5 that allows us to declare and initialize variables asynchronously. This is useful when we need to wait for the result of an asynchronous operation before we can use a variable.

Async let is used by prefixing the let keyword with the async keyword. Here is an example:

func fetchImage() async throws -> UIImage {
    let url = URL(string: "https://example.com/image.jpg")!
    let (data, _) = try await URLSession.shared.data(from: url)
    let image async = try UIImage(data: data)
    return image
}

In this example, we use async let to wait for the result of the UIImage initialization before returning the image.

4. Task Group:

Task groups are a new feature in Swift 5.5 that allow us to run multiple tasks concurrently and wait for all of them to complete. Task groups are useful when we have a number of independent tasks that can be run concurrently, and we need to wait for all of them to complete before continuing.

Task groups are created using the TaskGroup.init function, and tasks can be added to the group using the TaskGroup.add function. Here

let group = TaskGroup<Int, Error>()
for i in 1...10 {
    group.addTask {
        await Task.sleep(1_000_000_000)
        return i * 2
    }
}

do {
    let results = try await group.waitForAll()
    print(results)
} catch {
    print("An error occurred: \(error)")
}

In this example, we create a task group and add 10 tasks to the group. Each task waits for one second using the Task.sleep function, and then returns its index multiplied by 2. We then wait for all of the tasks to complete using the TaskGroup.waitForAll function, and print out the results.

5. Actors:

Actors are a new feature in Swift 5.5 that allow us to write thread-safe code in a more declarative way. Actors are objects that can only be accessed by one task at a time, and they use message passing to communicate with other actors. Actors can be used to avoid common concurrency issues such as data races and deadlocks.

Here is an example:

actor BankAccount {
    private var balance: Int

    init(balance: Int) {
        self.balance = balance
    }

    func deposit(amount: Int) {
        balance += amount
    }

    func withdraw(amount: Int) -> Bool {
        if amount <= balance {
            balance -= amount
            return true
        } else {
            return false
        }
    }
}

let account = BankAccount(balance: 100)
Task.detached {
    account.deposit(amount: 50)
}
Task.detached {
    if account.withdraw(amount: 75) {
        print("Withdrawal succeeded")
    } else {
        print("Withdrawal failed")
    }
}

In this example, we create an actor that represents a bank account. We use the deposit and withdraw functions to modify the balance of the account, and we ensure that only one task can access the account at a time. We then create two detached tasks that deposit 50 dollars and withdraw 75 dollars, respectively.

6 . Global Actors:

Global actors are a new feature in Swift 5.5 that allow us to write concurrent code in a more declarative way. A global actor is a special type of actor that can be accessed by any task in the system. Global actors can be used to ensure that certain types or functions are always accessed in a thread-safe manner, without the need to define a custom actor for each one.

Here is an example:

@globalActor
struct Logger {
    static let shared = Logger()

    func log(_ message: String) {
        print(message)
    }
}

@Logger func logMessage() {
    print("Hello, world!")
}

Task.detached {
    logMessage()
}

In this example, we define a global actor called Logger that can be used to log messages. We then define a function called logMessage that is marked as being part of the Logger actor. This means that any calls to this function will be serialized and executed one at a time, ensuring that the logging is thread-safe. We then create a detached task that calls the logMessage function.

Conclusion:

Swift 5.5 introduces several new features that make it easier to write concurrent and asynchronous code. Async/await simplifies the syntax for working with asynchronous functions, while Task and TaskGroup provide a more flexible way to manage multiple concurrent operations. Finally, global actors make it easier to ensure that certain types or functions are always accessed in a thread-safe manner. These features should make it easier for developers to write more efficient and reliable code, especially when working with complex concurrent systems.