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Promis

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The easiest Future and Promises framework in Swift. No magic. No boilerplate.

Overview

While starting from the Objective-C implementation of JustPromises and keeping the code minimalistic, this library adds the following:

You can read about the theory behind Future and Promises on Wikipedia, here are the main things you should know to get started.

Promis brags about being/having:

Alternatives

Other open-source solutions exist such as:

Promis takes inspiration from the Objective-C version of JustPromises developed by the iOS Team of Just Eat which is really concise and minimalistic, while other libraries are more weighty.

Usage

The following example should outline the main benefits of using futures via chaining.

let request = URLRequest(url: URL(string: "http://example.com")!)

// starts by hitting an API to download data
getData(request: request).thenWithResult { data in
    // continue by parsing the retrieved data
    parse(data: data)
}.thenWithResult { parsedData in
    // continue by mapping the parsed data
    map(data: parsedData)
}.onError { error in
    // executed only in case an error occurred in the chain
    print("error: " + String(describing: error))
}.finally(queue: .main) { future in
    // always executed, no matter the state of the previous future or how the chain did perform
    switch future.state {
        case .result(let value):
            print(String(describing: value))
        case .error(let err):
            print(String(describing: err))
        case .cancelled:
            print("future is in a cancelled state")
        case .unresolved:
            print("this really cannot be if any chaining block is executed")
        }
}

The functions used in the example have the following signatures:

func getData(request: URLRequest) -> Future<Data>
func parse(data: Data) -> Future<[Dictionary<String,AnyObject>]>
func map(data: [Dictionary<String,AnyObject>]) -> Future<[FooBar]>

Promises and Futures are parametrized leveraging the power of the generics, meaning that Swift can infer the type of the result compile type. This was a considerable limitation in the Objective-C world and we can now prevent lots of issues at build time thanks to the static typing nature of the language. The state of the future is an enum defined as follows:

enum FutureState<ResultType> {
    case unresolved
    case result(ResultType)
    case error(Error)
    case cancelled
}

Promises are created and resolved like so:

let promise = Promise<ResultType>()
promise.setResult(value)
// or
promise.setError(error)
// or
promise.cancel()

Continuation methods used for chaining are the following:

func then<NextResultType>(queue: DispatchQueue? = nil, task: @escaping (Future) -> Future<NextResultType>) -> Future<NextResultType>
func thenWithResult<NextResultType>(queue: DispatchQueue? = nil, continuation: @escaping (ResultType) -> Future<NextResultType>) -> Future<NextResultType> {
func onError(queue: DispatchQueue? = nil, continuation: @escaping (Error) -> Void) -> Future {
func finally(queue: DispatchQueue? = nil, block: @escaping (Future<ResultType>) -> Void)

All the functions can accept an optional DispatchQueue used to perform the continuation blocks.

Best practices

Functions wrapping async tasks should follow the below pattern:

func wrappedAsyncTask() -> Future<ResultType> {

    let promise = Promise<Data>()
    someAsyncOperation() { data, error in
        // resolve the promise according to how the async operations did go
        switch (data, error) {
        case (let data?, _):
            promise.setResult(data)
        case (nil, let error?):
            promise.setError(error)
        // etc.
        }
    }
    return promise.future
}

You could chain an onError continuation before returning the future to allow in-line error handling, which I find to be a very handy pattern.

// ...
return promise.future.onError {error in
    // handle/log error
}

Pitfalls

When using then or thenWithResult, the following should be taken in consideration.

...}.thenWithResult { data -> Future<NextResultType> in
    /**
    If a block is not trivial, Swift cannot infer the type of the closure and gives the error
    'Unable to infer complex closure return type; add explicit type to disambiguate'
    so you'll have to add `-> Future<NextResultType> to the block signature
    
    You can make the closure complex just by adding any extra statement (like a print).
    
    All the more reason to structure your code as done in the first given example :)
    */
    print("complex closure")
    return parse(data: data)
}

Please check the GettingStarted playground in the demo app to see the complete implementation of the above examples.

Installation

CocoaPods

Add Promis to your Podfile

use_frameworks!
target 'MyTarget' do
    pod 'Promis', '~> x.y.z'
end

$ pod install

Carthage

github "albertodebortoli/Promis" ~> "x.y.z"

Then on your application target Build Phases settings tab, add a "New Run Script Phase". Create a Run Script with the following content:

/usr/local/bin/carthage copy-frameworks

and add the following paths under "Input Files":

$(SRCROOT)/Carthage/Build/iOS/Promis.framework

Author

Alberto De Bortoli albertodebortoli.website@gmail.com Twitter: @albertodebo GitHub: albertodebortoli website: albertodebortoli.com

License

Promis is available under the Apache 2 license in respect of JustPromises which this library takes inspiration from. See the LICENSE file for more info.