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Promissum is a promises library written in Swift. It features some known functions from Function Programming like, map and flatMap.

It has useful combinators for working with promises like; whenAll for doing something when multiple promises complete, and whenAny for doing something when a single one of a list of promises completes. As well as their binary counterparts: whenBoth and whenEither.

Promissum really shines when used to combine asynchronous operations from different libraries. There are currently some basic extensions to UIKit, Alamofire and CoreDataKit, and contributions for extensions to other libraries are very welcome.

This library has an extensive set of regression tests, documentation, and has been used for months in several high profile production apps at Q42.

This example demonstrates the Alamofire+Promise and CoreDataKit+Promise extensions.

In this example, JSON data is loaded from the Github API. It is then parsed, and stored into CoreData. If both those succeed the result is shown to the user, if either of those fail, a description of the error is shown to the user.

let url = "https://api.github.com/repos/tomlokhorst/Promissum"
Alamofire.request(.GET, url).responseJSONPromise()
  .map(parseJson)
  .flatMap(storeInCoreData)
  .then { project in

    // Show project name and description
    self.nameLabel.text = project.name
    self.descriptionLabel.text = project.descr

    UIView.animateWithDuration(0.5) {
      self.detailsView.alpha = 1
    }
  }
  .trap { e in

    // Either an Alamofire error or a CoreData error occured
    self.errorLabel.text = e.localizedDescription
    self.errorView.alpha = 1
  }

See FadeExample/ViewController.swift for an extended version of this example.

Promissum does not support cancellation, because cancellation does not work well with promises. Promises are future values, values can't be cancelled. If you do need cancellation (quite often useful), take a look at Tasks or Rx instead of promises. I don't have experience with any Swift Task/Rx libraries, so I can't recommend a specific one.

Although, if you're looking at adding cancellation to a PromiseSource, you could use the swift-cancellationtoken library I wrote. This is orthogonal to promises, however.

Listed below are some of the methods and functions provided this library. More documentation is available inline.

• .map(transform: Value -> NewValue)
  Returns a Promise containing the result of mapping a function over the promise value.

• .flatMap(transform: Value -> Promise<NewValue, Error>)
  Returns the flattened result of mapping a function over the promise value.

• .mapError(transform: Error -> NewError)
  Returns a Promise containing the result of mapping a function over the promise error.

• .flatMapError(transform: Error -> Promise<Value, NewError>)
  Returns the flattened result of mapping a function over the promise error.

• .dispatchOn(queue: dispatch_queue_t) Returns a new promise that will execute all callbacks on the specified dispatch_queue. See dispatch queues

• flatten(promise: Promise<Promise<Value, Error>, Error>)
  Flattens a nested Promise of Promise into a single Promise.

• whenBoth(promiseA: Promise<A, Error>, _ promiseB: Promise<B, Error>)
  Creates a Promise that resolves when both arguments to whenBoth resolve.

• whenAll(promises: [Promise<Value, Error>])
  Creates a Promise that resolves when all provided Promises resolve.

• whenEither(promise1: Promise<Value, Error>, _ promise2: Promise<Value, Error>)
  Creates a Promise that resolves when either argument to resolves.

• whenAny(promises: [Promise<Value, Error>])
  Creates a Promise that resolves when any of the argument Promises resolves.

Promises can call handlers on different threads or queues. Handlers are all closures supplied to methods like .then, .trap, .map, and .flatMap.

If nothing else is specified, by default, all handlers will be called on the main queue. This way, you're free to update the UI, without having to worry about manually calling dispatch_async.

However, it's easy to change the dispatch queue used by a promise. In one of two ways:

1. Set the dispatch queue when creating a PromiseSource, e.g.:

let background = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)
let source = PromiseSource<Int, NoError>(dispatch: .OnQueue(background))
source.promise
  .then { x in
    // Handler is called on background queue
  }

2. Or, create a new promise using the .dispatchOn combinator:

let background = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)
somePromise()
  .dispatchOn(background)
  .then { x in
    // Handler is called on background queue
  }

For convenience, there's also .dispatchMain to move back to the main queue, after doing some work on a background queue:

let background = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)
somePromise()
  .dispatchOn(background)
  .map { expensiveComputation($0) }
  .dispatchMain()
  .then { x in
    self.updateUi(x)
  }

CocoaPods is a dependency manager for Cocoa projects. You can install it with the following command:

$ gem install cocoapods

To integrate Promissum into your Xcode project using CocoaPods, specify it in your Podfile:

source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '8.0'

pod 'Promissum'

Then, run the following command:

$ pod install

• 0.5.0 - 2016-01-19 - Add dispatchOn methods for dispatching on different queues
• 0.4.0 - 2015-11-04 - Update Alamofire+Promise to Alamofire 3
• 0.3.0 - 2015-09-11 - Swift 2 support, added custom error types
• 0.2.4 - 2015-05-31 - Fixed examples. Updated CoreDataKit+Promise
• 0.2.3 - 2015-04-13 - Swift 1.2 support
• 0.2.2 - 2015-03-01 - Mac OS X support
• 0.2.1 - 2015-02-16 - Update for new CoreDataKit version
• 0.2.0 - 2015-02-15 - Side-effects happen in a better order. Regression tests added.
• 0.1.1 - 2015-05-31 - whenAnyFinalized combinator added
• 0.1.0 - 2015-01-27 - Initial public release
• 0.0.0 - 2014-10-12 - Initial private version for project at Q42

Promissum is written by Tom Lokhorst and available under the MIT license, so feel free to use it in commercial and non-commercial projects.