Promises simplify asynchronous programming, freeing you up to focus on the more important things. They are easy to learn, easy to master and result in clearer, more readable code. Your co-workers will thank you.

UIApplication.shared.isNetworkActivityIndicatorVisible = true

let fetchImage = URLSession.shared.dataTask(.promise, with: url).compactMap{ UIImage(data: $0.data) }
let fetchLocation = CLLocationManager.requestLocation().lastValue

firstly {
    when(fulfilled: fetchImage, fetchLocation)
}.done { image, location in
    self.imageView.image = image
    self.label.text = "\(location)"
}.ensure {
    UIApplication.shared.isNetworkActivityIndicatorVisible = false
}.catch { error in
    self.show(UIAlertController(for: error), sender: self)
}

PromiseKit is a thoughtful and complete implementation of promises for any platform that has a swiftc. It has excellent Objective-C bridging and delightful specializations for iOS, macOS, tvOS and watchOS. It is a top-100 pod used in many of the most popular apps in the world.

PromiseKit 6 Released

PromiseKit 6 has been released; read the release notes and migration guide.

Quick Start

In your Podfile:

use_frameworks!

target "Change Me!" do
  pod "PromiseKit", "~> 6.0"
end

The above gives an Xcode warning? See our Installation Guide.

PromiseKit 6, 5 and 4 support Xcode 8.3, 9.x and 10.0; Swift 3.1, 3.2, 3.3, 3.4, 4.0, 4.1 and 4.2; iOS, macOS, tvOS, watchOS, Linux and Android; CocoaPods, Carthage and SwiftPM; (CI Matrix).

For Carthage, SwiftPM, etc., or for instructions when using older Swifts or Xcodes, see our Installation Guide. We recommend Carthage.

PromiseKit is Thousands of Hours of Work

This project is a labor of love: to the pursuit of making quality, reusable, compartmentalized software that has general use to the wider development community. It is a widely used project where every change, fix and plan can have serious repercussions to its users and thus must be carefully considered. If you use PromiseKit in your apps, please consider supporting its development with a donation to my patreon. Thank you.

Documentation

Handbook
Manual
- Installation Guide
- Objective-C Guide
- Troubleshooting (e.g., solutions to common compile errors)
- Appendix
API Reference

Extensions

Promises are only as useful as the asynchronous tasks they represent. Thus, we have converted (almost) all of Apple’s APIs to promises. The default CocoaPod provides Promises and the extensions for Foundation and UIKit. The other extensions are available by specifying additional subspecs in your Podfile, e.g.:

pod "PromiseKit/MapKit"          # MKDirections().calculate().then { /*…*/ }
pod "PromiseKit/CoreLocation"    # CLLocationManager.requestLocation().then { /*…*/ }

All our extensions are separate repositories at the PromiseKit organization.

I don't want the extensions!

Then don’t have them:

pod "PromiseKit/CorePromise", "~> 6.0"

Note: Carthage installations come with no extensions by default.

Choose Your Networking Library

Promise chains commonly start with a network operation. Thus, we offer extensions for URLSession:

// pod 'PromiseKit/Foundation'  # https://github.com/PromiseKit/Foundation

firstly {
    URLSession.shared.dataTask(.promise, with: try makeUrlRequest()).validate()
    // ^^ we provide `.validate()` so that eg. 404s get converted to errors
}.map {
    try JSONDecoder().decode(Foo.self, with: $0.data)
}.done { foo in
    //…
}.catch { error in
    //…
}

func makeUrlRequest() throws -> URLRequest {
    var rq = URLRequest(url: url)
    rq.httpMethod = "POST"
    rq.addValue("application/json", forHTTPHeaderField: "Content-Type")
    rq.addValue("application/json", forHTTPHeaderField: "Accept")
    rq.httpBody = try JSONEncoder().encode(obj)
    return rq
}

And Alamofire:

// pod 'PromiseKit/Alamofire'  # https://github.com/PromiseKit/Alamofire-

firstly {
    Alamofire
        .request("http://example.com", method: .post, parameters: params)
        .responseDecodable(Foo.self)
}.done { foo in
    //…
}.catch { error in
    //…
}

Nowadays, considering that:

We almost always POST JSON
We now have JSONDecoder
PromiseKit now has map and other functional primitives
PromiseKit (like Alamofire, but not raw-URLSession) also defaults to having callbacks go to the main thread

We recommend vanilla URLSession. It uses fewer black boxes and sticks closer to the metal. Alamofire was essential until the three bulletpoints above became true, but nowadays it isn’t really necessary.

Support

Please check our Troubleshooting Guide, and if after that you still have a question, ask at our Gitter chat channel or on our bug tracker.

stephen-uk/PromiseKit