Promissum is a promises library written in Swift. It features some known functions from Functional 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 and Alamofire, contributions for extensions to other libraries are very welcome.
This library has an extensive set of regression tests, documentation, and has been used in several high profile production apps at Q42.
This example demonstrates the Alamofire+Promise extension.
In this example, JSON data is loaded from the Github API. It is then parsed, and stored into a local cache. 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(url).responseJSONPromise()
.map(parseJson)
.flatMap(storeInLocalCache)
.then { project in
// Show project name and description
self.nameLabel.text = project.name
self.descriptionLabel.text = project.descr
UIView.animate(withDuration: 0.5) {
self.detailsView.alpha = 1
}
}
.trap { e in
// Either an Alamofire error or a LocalCache error occured
self.errorLabel.text = e.localizedDescription
self.errorView.alpha = 1
}
Interop with Swift's async await is available via the .asyncValue
and .asyncResult
properties, as well as a custom Promise
initializer.
// Go from Promise to async/await
func downloadProject(url: URL) async throws -> Project {
let data = try await downloadPromise(url: url).asyncValue
let project = try parse(data)
return project
}
// Go from async/await to Promise
func submitProject(project: Project) -> Promise<Void, Error> {
Promise {
let data = try encode(project)
try await upload(data)
}
}
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.
.dispatch(on queue: DispatchQueue)
Returns a new promise that will execute all callbacks on the specified dispatch_queue. See dispatch queues
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:
let background = DispatchQueue.global(qos: .background)
let source = PromiseSource<Int, Never>(dispatch: .queue(background))
source.promise
.then { x in
// Handler is called on background queue
}
.dispatchOn
combinator:let background = DispatchQueue.global(qos: .background)
somePromise()
.dispatch(on: 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 = DispatchQueue.global(qos: .background)
somePromise()
.dispatch(on: background)
.map { expensiveComputation($0) }
.dispatchMain()
.then { x in
self.updateUi(x)
}
SPM is a dependency manager for Swift projects.
Once you have SPM setup, add a dependency using Xcode or by editing Package.swift:
dependencies: [
.package(url: "https://github.com/tomlokhorst/Promissum.git", from: "7.1.0"),
]
Note: Previous versions of Promissum supported CocoaPods, this is no longer supported. If you still need pods support, you can use the 5.x.x versions of this package, while those still work.
delay
and related functionsdispatchOn
methods for dispatching on different queueswhenAnyFinalized
combinator addedPromissum is written by Tom Lokhorst and available under the MIT license, so feel free to use it in commercial and non-commercial projects.