AsyncTask 0.1.3

AsyncTask 0.1.3

LangLanguage SwiftSwift
License MIT
ReleasedLast Release Jun 2016
SPMSupports SPM

Maintained by Zhixuan Lai.

AsyncTask 0.1.3

  • By
  • Zhixuan Lai


An asynchronous programming library for Swift


AsyncTask is much more than Future and Promise.

  • It is composable, allowing you to build complex workflow.
  • It supports native error handling with do-catch and try.
  • It is protocol oriented; so you can turn any object into a Task.

Without AsyncTask:

// get a global concurrent queue
let queue = dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)
// submit a task to the queue for background execution
dispatch_async(queue) {
    let enhancedImage = self.applyImageFilter(image) // expensive operation taking a few seconds
    // update UI on the main queue
    dispatch_async(dispatch_get_main_queue()) {
        self.imageView.image = enhancedImage
        UIView.animateWithDuration(0.3, animations: {
            self.imageView.alpha = 1
        }) { completed in
            // add code to happen next here

With AsyncTask:

Task {
    let enhancedImage = self.applyImageFilter(image)
    Task {self.imageView.image = enhancedImage}.async(.Main)
    let completed = UIView.animateWithDurationAsync(0.3) { self.label.alpha = 1 }.await(.Main)
    // add code to happen next here

It even allows you to extend existing types:

let (data, response) = try! NSURL(string: "")!.await()




In AsyncTask, a Task represents the eventual result of an asynchronous operation, as do Future and Promise in other libraries. It can wrap both synchronous and asynchronous APIs. To create a Task, initialize it with a closure. To make it reusable, write functions that return a task.

// synchronous API
func encrypt(message: String) -> Task<String> {
    return Task {
// asynchronous API
func get(URL: NSURL) -> Task<(NSData?, NSURLResponse?, NSError?)> {
    return Task {completionHandler in
        NSURLSession().dataTaskWithURL(URL, completionHandler: completionHandler).resume()

To get the result of a Task, use async or await. async is just like dispatch_async, and you can supply a completion handler. await, on the contrary, blocks the current thread and waits for the task to finish.

// async
encrypt(message).async { ciphertext in /* do somthing */ }
get(URL).async {(data, response, error) in /* do somthing */ }

// await
let ciphertext = encrypt(message).await()
let (data, response, error) = get(URL).await()

Composing Tasks

You can use multiple await expressions to ensure that each statement completes before executing the next statement:

Task {
    print(“downloading image”)
    var image = UIImage(data: downloadImage.await())!
    print(“processing image”)
    image = processImage(image).await()

You can also call awaitFirst and awaitAll on a collection of tasks to execute them in parallel:

let replicatedURLs = ["", ""]
let first =

let messages = ["1", "2", "3"]
let all =

Handling Errors

Swift provide first-class support for error handling. In AsyncTask, a ThrowableTask takes a throwing closure and propagates the error.

func load(path: String) -> ThrowableTask<NSData> {
    return ThrowableTask {
        switch path {
        case "profile.png":
            return NSData()
        case "index.html":
            return NSData()
            throw Error.NotFound

expect{try load("profile.png").await()}.notTo(throwError())
expect{try load("index.html").await()}.notTo(throwError())
expect{try load("random.txt").await()}.to(throwError())

Extending Tasks

AsyncTask is protocol oriented; it defines TaskType and ThrowableTaskType and provides the default implementation of async and await using protocol extension. In other words, these protocols are easy to implement, and you can await on any object that confronts to them. Being able to extend tasks powerful because it allows tasks to encapsulate states and behaviors.

In the following example, by extending NSURL to be TaskType, we make data fetching a part of the NSURL class. To confront to the TaskType protocol, just specify an action and the return type.

extension NSURL : ThrowableTaskType {

    public typealias ReturnType = (NSData, NSURLResponse)

    public func action(completion: Result<ReturnType> -> ()) {
        ThrowableTask<ReturnType> {
            let session = NSURLSession(configuration: .ephemeralSessionConfiguration())
            let (data, response, error) = Task { callback in session.dataTaskWithURL(self, completionHandler: callback).resume()}.await()
            guard error == nil else { throw error! }
            return (data!, response!)
        }.asyncResult(completion: completion)


This extension allows us to write the following code:

let (data, response) = try! NSURL(string: "")!.await()

A Task can represent more complicated activities, even those involving UI. In the following example, we use an ImagePickerTask to launch a UIImagePickerViewController and wait for the user to choose an image. Once the user selects an image or press the cancel button, the view controller dismisses, and the task returns with the selected image.

class ImagePickerDemoViewController: UIViewController {

    let imageView = UIImageView()

    func launchImagePicker() {
        Task {
            do {
                let data = try ImagePickerTask(viewController: self).await()
            } catch Error.PhotoLibraryNotAvailable {
                alert("Photo Library is Not Available")
            guard let image = data?[UIImagePickerControllerOriginalImage] as? UIImage else {
                self.imageView.image = nil
            self.imageView.image = image


The ImagePickerTask knows when the user has picked an image or canceled because it is the UIImagePickerViewController’s delegate. For more details, see its implementation and the example folder.


To run the example project, clone the repo, and run pod install from the Example directory first.

You may also want to take a look at the test cases.


Zhixuan Lai, [email protected]


AsyncTask is available under the MIT license. See the LICENSE file for more info.