PromiseKit – Promises for Swift & ObjC

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.

Quick Start

In your Podfile:

use_frameworks!

target "Change Me!" do
  pod "PromiseKit", "~> 6.8"
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, 4.2, 4.3 and 5.0 (development snapshots); iOS, macOS, tvOS, watchOS, Linux and Android; CocoaPods, Carthage and SwiftPM; (CI Matrix).

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

PromiseKit and Swift 5.5+ Async/Await

As of Swift 5.5, the Swift language now offers support for built-in concurrency with async / await. See Async+ for a port of PromiseKit’s most useful patterns to this new paradigm.

Documentation

• Handbook
  • Getting Started
  • Promises: Common Patterns
  • Frequently Asked Questions
• 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", "~> 6.8"

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 bullet points 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.

PromiseKit on GitHub: https://github.com/mxcl/PromiseKit
Platform: iOS
⭐️: 14K