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https://github.com/belozierov/SwiftCoroutine

Swift coroutines for iOS, macOS and Linux.
https://github.com/belozierov/SwiftCoroutine

async async-await asynchronous atomic await combine coroutine coroutines dispatch futures ios linux lock-free macos multithreading promises scheduler structured-concurrency swift thread

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Swift coroutines for iOS, macOS and Linux.

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README 

        

  

![Swift Coroutine](../master/Sources/logo.png)



##



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Many languages, such as Kotlin, Go, JavaScript, Python, Rust, C#, C++ and others, already have [coroutines](https://en.wikipedia.org/wiki/Coroutine) support that makes the [async/await](https://en.wikipedia.org/wiki/Async/await) pattern implementation possible. This feature is not yet supported in Swift, but this can be improved by a framework without the need to change the language.



#### Main features



- It is the first implementation of [coroutines](https://en.wikipedia.org/wiki/Coroutine) for Swift with iOS, macOS and Linux support. 

- It includes [futures](https://en.wikipedia.org/wiki/Futures_and_promises) and [channels](https://en.wikipedia.org/wiki/Channel_(programming)) that supplement coroutines for more flexibility.

- It is completely [lock-free](https://en.wikipedia.org/wiki/Non-blocking_algorithm) and uses only [atomic](https://en.wikipedia.org/wiki/Compare-and-swap) primitives for synchronizations.



### Motivation



Asynchronous programming is usually associated with callbacks. It is quite convenient until there are too many of them and they start nesting. Then it's called a **pyramid of doom** or even **callback hell**.



Another problem of asynchronous programming is **error handling**, because Swift's natural error handling mechanism cannot be used.



#### What about Rx and other such frameworks?



There are many other frameworks that make it easy to use asynchronous code, such as Combine, RxSwift, PromiseKit and so on. They use other approaches that have some drawbacks:



- Similar to callbacks, you also need to create chained calls, that’s why you can’t normally use loops, exception handling, etc.

- Usually you need to learn a complex new API with hundreds of methods.

- Instead of working with the actual data, you need to operate with some wrappers all the time.

- Chaining of errors can be really complicated to handle.



### Async/await



The [async/await](https://en.wikipedia.org/wiki/Async/await) pattern is an alternative that allows an asynchronous, non-blocking function to be structured in a way similar to an ordinary synchronous function. 



It is already well-established in other programming languages and is an evolution in asynchronous programming. The implementation of this pattern is possible thanks to coroutines.



Let’s have a look at the example with coroutine inside of which `await()` suspends it and resumes when the result is available without blocking the thread.



```swift

//executes coroutine on the main thread

DispatchQueue.main.startCoroutine {

    

    //extension that returns CoFuture<(data: Data, response: URLResponse)>

    let dataFuture = URLSession.shared.dataTaskFuture(for: imageURL)

    

    //await CoFuture result that suspends coroutine and doesn't block the thread

    let data: Data = try dataFuture.await().data



    //create UIImage from the data

    guard let image = UIImage(data: data) else { return }

    

    //execute heavy task on global queue and await the result without blocking the thread

    let thumbnail: UIImage = try DispatchQueue.global().await { image.makeThumbnail() }



    //set image in UIImageView on the main thread

    self.imageView.image = thumbnail

    

}

```



### Documentation



[API documentation](https://belozierov.github.io/SwiftCoroutine)



### Requirements



- Support only 64-bit architectures

- iOS 10+ / macOS 10.12+ / Ubuntu

- Xcode 10.4+

- Swift 5.2+



### Installation



- It's available through the [Swift Package Manager](https://swift.org/package-manager) for iOS, macOS and Linux.

- It's available through [CocoaPods](https://cocoapods.org/pods/swiftcoroutine) for iOS and macOS.



## Working with SwiftCoroutine



### Coroutines



A [coroutine](https://en.wikipedia.org/wiki/Coroutine) is a computation that can be suspended and resumed at a later time without blocking a thread. Coroutines build upon regular functions and can be executed on any scheduler with a possibility to switch among them during execution.



#### Key benefits



- **Suspend instead of block**. The main advantage of coroutines is the ability to suspend their execution at some point without blocking a thread and resuming later on.

- **Fast context switching**. Switching between coroutines is much faster than switching between threads as it does not require the involvement of operating system.

- **Asynchronous code in synchronous manner**. The use of coroutines allows an asynchronous, non-blocking function to be structured in a manner similar to an ordinary synchronous function. And even though coroutines can run in multiple threads, your code will still look consistent and therefore easy to understand.



#### Usage



The coroutines API design is as minimalistic as possible. It consists of the `CoroutineScheduler` protocol that describes how to schedule coroutines (`DispatchQueue` already conforms it), and the `Coroutine` structure with utility methods. This API is enough to do amazing things.



The following example shows the usage of  `await()` inside a coroutine to wrap asynchronous calls.



```swift

//execute coroutine on the main thread

DispatchQueue.main.startCoroutine {

    

    //await URLSessionDataTask response without blocking the thread

    let (data, response, error) = try Coroutine.await { callback in

        URLSession.shared.dataTask(with: url, completionHandler: callback).resume()

    }

    

    . . . use response on the main thread . . . 

}

```



Here's how we can conform `NSManagedObjectContext` to `CoroutineScheduler` for launching coroutines on it.



```swift

extension NSManagedObjectContext: CoroutineScheduler {



    func scheduleTask(_ task: @escaping () -> Void) {

        perform(task)

    }

    

}



//execute coroutine on the main thread

DispatchQueue.main.startCoroutine {

    let context: NSManagedObjectContext //context with privateQueueConcurrencyType

    let request: NSFetchRequest //some complex request



    //execute request on the context without blocking the main thread

    let result: [NSDictionary] = try context.await { try context.fetch(request) }

}

```



### Futures and Promises



A future is a read-only holder for a result that will be provided later and the promise is the provider of this result. They represent the eventual completion or failure of an asynchronous operation.



The [futures and promises](https://en.wikipedia.org/wiki/Futures_and_promises) approach itself has become an industry standart. It is a convenient mechanism to synchronize asynchronous code. But together with coroutines, it takes the usage of asynchronous code to the next level and has become a part of the async/await pattern. If coroutines are a skeleton, then futures and promises are its muscles.



#### Main features



- **Performance**. It is much faster than most of other futures and promises implementations.

- **Awaitable**. You can await the result inside the coroutine.

- **Cancellable**. You can cancel the whole chain as well as handle it and complete the related actions.



#### Usage



Futures and promises are represented by the corresponding `CoFuture` class and its `CoPromise` subclass. 



```swift

//wraps some async func with CoFuture

func makeIntFuture() -> CoFuture {

    let promise = CoPromise()

    someAsyncFunc { int in

        promise.success(int)

    }

    return promise

}

```



It allows to start multiple tasks in parallel and synchronize them later with `await()`.



```swift

//create CoFuture that takes 2 sec. from the example above 

let future1: CoFuture = makeIntFuture()



//execute coroutine on the global queue and returns CoFuture with future result

let future2: CoFuture = DispatchQueue.global().coroutineFuture {

    try Coroutine.delay(.seconds(3)) //some work that takes 3 sec.

    return 6

}



//execute coroutine on the main thread

DispatchQueue.main.startCoroutine {

    let sum: Int = try future1.await() + future2.await() //will await for 3 sec.

    self.label.text = "Sum is \(sum)"

}

```



It's very easy to transform or compose `CoFuture`s into a new one.



```swift

let array: [CoFuture]



//create new CoFuture with sum of future results

let sum = CoFuture { try array.reduce(0) { try $0 + $1.await() } }

```



### Channels



Futures and promises provide a convenient way to transfer a single value between coroutines. [Channels](https://en.wikipedia.org/wiki/Channel_(programming)) provide a way to transfer a stream of values. Conceptually, a channel is similar to a queue that allows to suspend a coroutine on receive if it is empty, or on send if it is full.



This non-blocking primitive is widely used in such languages as Go and Kotlin, and it is another instrument that improves working with coroutines.



#### Usage



To create channels, use the `CoChannel` class.



```swift

//create a channel with a buffer which can store only one element

let channel = CoChannel(capacity: 1)



DispatchQueue.global().startCoroutine {

    for i in 0..<100 {

        //imitate some work

        try Coroutine.delay(.seconds(1))

        //sends a value to the channel and suspends coroutine if its buffer is full

        try channel.awaitSend(i)

    }

    

    //close channel when all values are sent

    channel.close()

}



DispatchQueue.global().startCoroutine {

    //receives values until closed and suspends a coroutine if it's empty

    for i in channel.makeIterator() {

        print("Receive", i)

    }

    

    print("Done")

}

```



### Scope



All launched coroutines, `CoFuture`s and `CoChannel`s, usually do not need to be referenced. They are deinited after their execution. But often there is a need to complete them earlier, when they are no longer needed. For this, `CoFuture` and `CoChannel` have methods for canceling.



`CoScope` makes it easier to manage the life cycle of these objects. It allows you to keep weak references to them and cancel if necessary or on deinit.



#### Usage



You can add coroutines, `CoFuture`s, `CoChannel`s and other `CoCancellable` to `CoScope` to cancel them when they are no longer needed or on deinit.



```swift

class ViewController: UIViewController {



    let scope = CoScope() //will cancel all objects on `cancel()` or deinit

    

    func performSomeWork() {

        //create new `CoChannel` and add to `CoScope`

        let channel = makeSomeChannel().added(to: scope)

        

        //execute coroutine and add to `CoScope`

        DispatchQueue.main.startCoroutine(in: scope) { [weak self] in

            for item in channel.makeIterator() {

                try self?.performSomeWork(with: item)

            }

        }

    }

    

    func performSomeWork(with item: Item) throws {

        //create new `CoFuture` and add to `CoScope`

        let future = makeSomeFuture(item).added(to: scope)

        

        let result = try future.await()

        . . . do some work using result . . .

    }



}

```