Description
When is a lightweight implementation of Promises in Swift. It doesn't include any helper functions for iOS and OSX and it's intentional, to remove redundant complexity and give you more freedom and flexibility in your choices. It is type safe, thanks Swift generics, so you could create promises with whatever type you want.
When can easily be integrated into your projects and libraries to move your asynchronous code up to the next level.
Table of Contents
Why?
To make asynchronous code more readable and standardized:
fetchJSON().then({ data: NSData -> [[String: AnyObject]] in
// Convert to JSON
return json
}).then({ json: [[String: AnyObject]] -> [Entity] in
// Map JSON
// Save to database
return items
}).done({ items: [Entity] in
self.items = items
self.tableView.reloadData()
}).error({ error in
print(error)
})Usage
Promise
A promise represents the future value of a task. Promises start in a pending state and then could be resolved with a value or rejected with an error.
// Creates a new promise that could be resolved with a String value
let promise = Promise<String>()
// Resolves the promise
promise.resolve("String")
// Or rejects the promise
promise.reject(Error.notFound)// Creates a new promise that is resolved with a String value
let promise = Promise({
return "String"
})// Creates a new promise that is rejected with an Error
let promise = Promise({
//...
throw Error.notFound
})Callbacks of the current promise and all the chained promises
(created with then) are executed on the main queue by default, but
you can always specify the needed queue in init:
let promise = Promise<String>(queue: dispatch_get_main_queue())Done
Adds a handler to be called when the promise object is resolved with a value:
// Create a new promise in a pending state
let promise = Promise<String>()
// Add done callback
promise.done({ value in
print(value)
})
// Resolve the promise
promise.resolve("String")Fail
Adds a handler to be called when the promise object is rejected with
an Error:
// Create a new promise in a pending state
let promise = Promise<String>()
// Add fail callback
promise.fail({ error in
print(error)
})
// Reject the promise
promise.reject(Error.notFound)It's also possible to cancel a promise, which means it will be rejected with
PromiseError.cancelled error. FailurePolicy can be used if you want to
ignore this error in your fail handler:
// Create a new promise in a pending state
let promise = Promise<String>()
// This callback will not be called when a promise is cancelled
promise.fail({ error in
print(error)
})
// This callback will be called when a promise is cancelled
promise.fail(policy: .allErrors, { error in
print(error)
})
// Cancel the promise
promise.cancel()Always
Adds a handler to be called when the promise object is either resolved or rejected. This callback will be called after done or fail handlers.
// Create a new promise in a pending state
let promise = Promise<String>()
// Add always callback
promise.always({ result in
switch result {
case let .success(value):
print(value)
case let .failure(error):
print(error)
}
})
// Resolve or reject the promise
promise.resolve("String") // promise.reject(Error.notFound)Then
Returns a new promise that can use the result value of the current promise. It means that you could easily create chains of promises to simplify complex asynchronous operations into clear and simple to understand logic.
A new promise is resolved with the value returned from the provided closure:
let promise = Promise<NSData>()
promise
.then({ data -> Int in
return data.length
}).then({ length -> Bool in
return length > 5
}).done({ value in
print(value)
})
promise.resolve("String".dataUsingEncoding(NSUTF8StringEncoding)!)A new promise is resolved when the promise returned from the provided closure resolves:
struct Networking {
static func GET(url: NSURL) -> Promise<NSData> {
let promise = Promise<NSData>()
//...
return promise
}
}
Networking.GET(url1)
.then({ data -> Promise<NSData> in
//...
return Networking.GET(url2)
}).then({ data -> Int in
return data.length
}).done({ value in
print(value)
})then closure is executed on the main queue by default, but you can pass a needed queue as a parameter:
promise.then(on: dispatch_get_global_queue(QOS_CLASS_UTILITY, 0))({ data -> Int in
//...
})If you want to use background queue there are the helper methods for this case:
promise1.thenInBackground({ data -> Int in
//...
})
promise2.thenInBackground({ data -> Promise<NSData> in
//...
})Recover
Returns a new promise that can be used to continue the chain when an error was thrown.
let promise = Promise<String>()
// Recover the chain
promise
.recover({ error -> Promise<String> in
return Promise({
return "Recovered"
})
})
.done({ string in
print(string) // Recovered
})
// Reject the promise
promise.reject(Error.notFound)When
Provides a way to execute callback functions based on one or more promises. The when method returns a new "master" promise that tracks the aggregate state of all the passed promises. The method will resolve its "master" promise as soon as all the promises resolve, or reject the "master" promise as soon as one of the promises is rejected. If the "master" promise is resolved, the done callback is executed with resolved values for each of the promises:
let promise1 = Promise<Int>()
let promise2 = Promise<String>()
let promise3 = Promise<Int>()
when(promise1, promise2, promise3)
.done({ value1, value2, value3 in
print(value1)
print(value2)
print(value3)
})
promise1.resolve(1)
promise2.resolve("String")
promise3.resolve(3)Reactive extensions
Use the following extension in order to integrate When with RxSwift:
import RxSwift
extension Promise: ObservableConvertibleType {
public func asObservable() -> Observable<T> {
return Observable.create({ observer in
self
.done({ value in
observer.onNext(value)
})
.fail({ error in
observer.onError(error)
})
.always({ _ in
observer.onCompleted()
})
return Disposables.create()
})
}
}Installation
When is available through CocoaPods. To install it, simply add the following line to your Podfile:
pod 'When'For RxSwift extensions you can use CocoaPods subspecs:
pod 'When/RxSwift'When is also available through Carthage. To install just write into your Cartfile:
github "vadymmarkov/When"Author
Vadym Markov, [email protected]
Credits
Credits for inspiration go to PromiseKit and Then.
Contributing
Check the CONTRIBUTING file for more info.
License
When is available under the MIT license. See the LICENSE file for more info.