gualtierofrigerio / PromiseThen

Swift package with the implementation of a Promise

PromiseThen

An implementation of Future and Promises with the ability to chain multiple Promises. See my blog post Callbacks vs Promises for examples and details

How to use it

Use a Promise to avoid completion handlers

func getData(atURL url: URL) -> Promise<Data> {
    let promise = Promise<Data>()
    let session = URLSession.shared
    let task = session.dataTask(with: url) { (data, response, error) in
        if let err = error {
            promise.reject(error: err)
        }
        else {
            if let data = data {
                promise.resolve(value: data)
            }
            else {
                let unknowError = NSError(domain: "", code : 0, userInfo: nil)
                promise.reject(error: unknowError)
            }
        }
    }
    task.resume()
    return promise
}


let p = getData()
p.observe { promiseReturn in
    switch promiseReturn {
    case .success(let data):
        // here is our data!
    case .error(let error):
        // we have an error :(
    }

it is possible to chain multiple operations by calling then

func getUsersWithMergedData() -> Promise<[User]> {
    return getPictures().then({self.addPicturesToAlbums($0)})
                        .then({self.addAlbumsToUsers($0)})
}

you can optionally specify an object conforming to CancellableTask to cancel a task from the Promise. This example cancels a URLSessionDataTask

struct URLCancellableTask: CancellableTask {
    var dataTask: URLSessionDataTask
    
    func cancel() {
        dataTask.cancel()
    }
}

func getData(atURL url: URL) -> Promise<Data> {
    let promise = Promise<Data>()
    let session = URLSession.shared
    let task = session.dataTask(with: url) { (data, response, error) in
        if let error = error {
            promise.reject(error: NetworkClientError.networkError(error))
            return
        }
        if let data = data {
            promise.resolve(value: data)
        }
        else {
            promise.reject(error: NetworkClientError.noData)
        }
    }
    promise.setCancellableTask(URLCancellableTask(dataTask: task))
    task.resume()
    return promise
}

let promise = getData(atURL: url)
promise.cancel()

How to install

Use SPM by importing the package at this link https://github.com/gualtierofrigerio/PromiseThen.git