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Modern development is highly asynchronous: isn’t it about time we had tools that made programming asynchronously powerful, easy and delightful?

UIApplication.shared.isNetworkActivityIndicatorVisible = true

firstly {
    when(URLSession.dataTask(with: url).asImage(), CLLocationManager.promise())
}.then { image, location -> Void in
    self.imageView.image = image
    self.label.text = "\(location)"
}.always {
    UIApplication.shared.isNetworkActivityIndicatorVisible = false
}.catch { error in
    UIAlertView(/*…*/).show()
}

PromiseKit is a thoughtful and complete implementation of promises for any platform with a swiftc (indeed, this includes Linux), it has excellent Objective-C bridging and delightful specializations for iOS, macOS, tvOS and watchOS.

Quick Start

We recommend CocoaPods or Carthage, however you can just drop PromiseKit.xcodeproj into your project and add PromiseKit.framework to your app’s embedded frameworks.

Xcode 9 / Swift 4

# CocoaPods
swift_version = "4.0"
pod "PromiseKit", branch: "swift4-beta1"

# Carthage
github "mxcl/PromiseKit" "swift4-beta1"

# SwiftPM
package.dependencies.append(
    .package(url: "https://github.com/mxcl/PromiseKit", .branch("swift4-beta1"))
)

Please Note

We have not ported the extensions yet. This is your opportunity to contribute.

1. Clone:

   cd your-project/..
   git clone https://github.com/mxcl/PromiseKit PMK-swift4-beta1 --recursive -b swift4-beta1
   

2. Amend your Podfile:

   pod "PromiseKit", path: "../PMK-swift4-beta1"
   

3. Make your fixes for the PromiseKit extensions you use.

4. Fork, push & pull-request.

Xcode 8 / Swift 3 and Xcode 9 / Swift 3.2

Indeed, PromiseKit 4 already supports Swift 3.2 and thus Xcode 9.

# CocoaPods
swift_version = "3.0"
pod "PromiseKit", "~> 4.0"

# Carthage
github "mxcl/PromiseKit" ~> 4.0

# SwiftPM
let package = Package(
    dependencies: [
        .Package(url: "https://github.com/mxcl/PromiseKit", majorVersion: 4)
    ]
)

Xcode 8 / Swift 2.3 or Xcode 7

# CocoaPods
swift_version = "2.3"
pod "PromiseKit", "~> 3.5"

# Carthage
github "mxcl/PromiseKit" ~> 3.5

Documentation

We have thorough and complete documentation at promisekit.org.

Overview

Promises are defined by the function then:

login().then { json in
    //…
}

They are chainable:

login().then { json -> Promise<UIImage> in
    return fetchAvatar(json["username"])
}.then { avatarImage in
    self.imageView.image = avatarImage
}

Errors cascade through chains:

login().then {
    return fetchAvatar()
}.then { avatarImage in
    //…
}.catch { error in
    UIAlertView(/*…*/).show()
}

They are composable:

let username = login().then{ $0["username"] }

when(username, CLLocationManager.promise()).then { user, location in
    return fetchAvatar(user, location: location)
}.then { image in
    //…
}

They are trivial to refactor:

func avatar() -> Promise<UIImage> {
    let username = login().then{ $0["username"] }

    return when(username, CLLocationManager.promise()).then { user, location in
        return fetchAvatar(user, location: location)
    }
}

You can easily create a new, pending promise.

func fetchAvatar(user: String) -> Promise<UIImage> {
    return Promise { fulfill, reject in
        MyWebHelper.GET("\(user)/avatar") { data, err in
            guard let data = data else { return reject(err) }
            guard let img = UIImage(data: data) else { return reject(MyError.InvalidImage) }
            guard let img.size.width > 0 else { return reject(MyError.ImageTooSmall) }
            fulfill(img)
        }
    }
}

Continue Learning…

Complete and progressive learning guide at promisekit.org.

PromiseKit vs. Xcode

PromiseKit contains Swift, so we engage in an unending battle with Xcode:

Swift	Xcode	PromiseKit	CI Status	Release Notes
3	8	4		2016/09
2	7/8	3		2015/10
1	7	3	–	2015/10
N/A	*	1†		–

† PromiseKit 1 is pure Objective-C and thus can be used with any Xcode, it is also your only choice if you need to support iOS 7 or below.

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We also maintain some branches to aid migrating between Swift versions:

Xcode	Swift	PromiseKit	Branch	CI Status
8.0	2.3	2	swift-2.3-minimal-changes
7.3	2.2	2	swift-2.2-minimal-changes
7.2	2.2	2	swift-2.2-minimal-changes
7.1	2.1	2	swift-2.0-minimal-changes
7.0	2.0	2	swift-2.0-minimal-changes

We do not usually backport fixes to these branches, but pull-requests are welcome.

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 comes with promises UIKit and Foundation, the rest are accessed by specifying additional subspecs in your Podfile, eg:

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

All our extensions are separate repositories at the PromiseKit org .

For Carthage specify the additional repositories in your Cartfile:

github "PromiseKit/MapKit" ~> 1.0

Choose Your Networking Library

NSURLSession is typically inadequate; choose from Alamofire or OMGHTTPURLRQ:

// pod 'PromiseKit/Alamofire'  
Alamofire.request("http://example.com", withMethod: .GET).responseJSON().then { json in
    //…
}.catch { error in
    //…
}

// pod 'PromiseKit/OMGHTTPURLRQ'
URLSession.GET("http://example.com").asDictionary().then { json in
    
}.catch { error in
    //…
}

For AFNetworking we recommend csotiriou/AFNetworking.

Need to convert your codebase to Promises?

From experience it really improves the robustness of your app, feel free to ask us how to go about it.

Support

Ask your question at our Gitter chat channel or on our bug tracker.