Promises

The Promises are used to write functions that returns the Futures. The Promise contains the Future instance and can complete it.

let promise = Promise<String>()
promise.future // future

promise.complete(.value("value")) // can be completed with .value(value) or .error(error)

All ways to complete the Future in the Promise:

let value = "value"
let error = NSError(domain: "", code: -1, userInfo: nil)

promise.complete(.value(value))

promise.complete(.error(error))

promise.success(value)

promise.error(error)

promise.complete(Future<String>(value: value)) // with the other future

Examples

#1

func loadData() -> Future<String> {
    
    // create the promise with String type
    let promise = Promise<String>()

    // make some async operations
    DispatchQueue.global().async {

        if let data = data() {
            promise.success(data)
        } else {
            promise.error(NSError(domain: "", code: -1, userInfo: nil)) // some error
        }
    }
    // return the future
    return promise.future
}

func data() -> String? {
    return "a lot of data"
}

loadData().onSuccess { data in
    print(data)
}.onError { error in
    print(error)
}

#2

func makeSomething() -> Future<String> {
    
    // create the promise with String type
    let promise = Promise<String>()
    
    // make some async operations
    DispatchQueue.global().async {
        
        if let future = optionalFuture() {
            promise.complete(future)
        } else {
            promise.error(NSError(domain: "", code: -1, userInfo: nil)) // some error
        }
    }
    // return the future
    return promise.future
}

func optionalFuture() -> Future<String>? {
    
    return Future<String>(value: "optional future value")
}

makeSomething().onComplete { result in
    
    switch result {
    case .value(let value):
        print(value) // "optional future value"
    case .error(let error):
        print(error)
    }
}

#3

class ViewController: UIViewController {
    
    let dataManager = DataManager()
    let label = UILabel(frame: UIScreen.main.bounds)
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        dataManager.loadData().onSuccess { data in
            DispatchQueue.main.async {
                self.label.text = data
            }
        }.onError { error in
            print(error)
            // handle error
        }
    }
}

class DataManager {
    
    func loadData() -> Future<String> {
        
        // create the promise with String type
        let promise = Promise<String>()
        
        // check is url valid
        guard let url = URL(string: "https://api.github.com/emojis") else {
            // handle error
            promise.error(ExampleError.invalidUrl)
            return promise.future
        }
        
        // loading data from url
        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
            if let data = data, let string = String(data: data, encoding: .utf8) {
                // return result
                promise.success(string)
            } else {
                // handle error
                promise.error(ExampleError.cantLoadData)
            }
        }
        task.resume()
        
        // return the future
        return promise.future
   }
}

enum ExampleError: Error {
    case invalidUrl
    case cantLoadData
}