sjordine

DispatchQueue.global().async {
   let result = doSomething()
   let transformedResult = transform(result)
   completionHandler(transformedResult)
}

sjordine

DispatchQueue.global().async {
   let result = doSomething()
   completionHandler(result)
}

sjordine

DispatchQueue.global().async {
    let result = doSomethingAsynchronously()
    let transformedToAsyncFunction = transformToAsynchronous(result)
    DispatchQueue.global().async {
        let transformedValue = transformedToAsyncFunction()
         completionHandler(transformedValue)
    }
}

sjordine

class Promise<T> {
    
    var task:(() -> T)?

    init(_ value: T) {
        task = { return value }
    }
    
    init(_ task:@escaping() -> T) {
        self.task = task

sjordine

func map<Destination>(_ transform:@escaping(T)->Destination) -> Promise<Destination> {
  return PromiseMap(container: self, transform: transform)
}

sjordine

class PromiseMap<Source, Destination>: Promise<Destination> {
    
    let sourcePromise: Promise<Source>
    let transform: (Source)->(Destination)
    
    init(container:Promise<Source>, transform: @escaping(Source)->Destination) {
        self.sourcePromise = container
        self.transform = transform
        super.init()
    }

sjordine

originalPromise            // that is the original Promise A
  .map(transformFunction)  // this function returns a new Promise B
  .onCompletion {...       // this onCompletion refers to Promise B
                           // so the Promise A onCompletion is not "exposed"

sjordine

func flatMap<Destination>(_ transform:@escaping(T) -> Promise<Destination>) -> Promise<Destination> {
  let mappedTransform = self.map(transform)
  return NestedPromise(mappedTransform)
}

sjordine

class NestedPromise<T>: Promise<T> {
    
    var outerPromise:Promise<Promise<T>>
    
    init(_ nestedPromises:Promise<Promise<T>>) {
        outerPromise = nestedPromises
        super.init()
    }
    
    override func onCompletion(_ completionHandler: @escaping (T) -> Void) {

sjordine

let nasaURL = Promise("https://api.nasa.gov/planetary/apod?api_key=DEMO_KEY")

func imageURL(from urlString:String) -> URL {
    let url = URL(string:urlString)!
    let data = try! Data(contentsOf: url)
    let jsonDictionary = try! JSONDecoder().decode([String:String].self, from: data)
    let imageURL = jsonDictionary["hdurl"] as! String
    return URL(string:imageURL)!
}