Quick promise implemention

SimplePromise.swift

import Foundation

public class Promise<T> {
	
    public private(set) lazy var future: Future<T> = Future()
    
    public init() { /**/ }

    public func fulfill(_ value: T) {
        future.result = .success(value)
    }

    public func reject(_ error: Error) {
        future.result = .failure(error)
    }
}

public class Future<T> {

    fileprivate var result: Result<T, Error>? { didSet { notifyHandlersIfNeeded() } }
    private var handlers: [ResultHandler] = []
    private let lock = NSRecursiveLock()

    public typealias ResultHandler = (Result<T, Error>) -> Void

    public func value(_ handler: @escaping ResultHandler) {
        locked { self.handlers.append(handler) }
        notifyHandlersIfNeeded()
    }

    private func notifyHandlersIfNeeded() {
        locked {
            guard let result = result else { return }
            handlers.forEach { $0(result) }
            handlers.removeAll()
        }
    }

    private func locked(_ closure: () -> Void) {
        lock.lock()
        closure()
        lock.unlock()
    }
}