Simplest way to use promises

Promise.swift

import Foundation

class Promise<Value> {

    enum State<T> {

        case completed(T)
        case pending
    }

    private var state: State<Value> = .pending
    private var callbacks: [(Value) -> Void] = []

    init(resolver: (_ resolve: @escaping (Value) -> Void) -> Void) {

        resolver(resolve)
    }

    func then(_ onResolved: @escaping (Value) -> Void) {

        callbacks.append(onResolved)
        triggerCallbacksIfResolved()
    }

    func then<T>(_ body: @escaping (Value) -> Promise<T>) -> Promise<T> {

        return Promise<T> { resolve in

            then { value in

                body(value).then(resolve)
            }
        }
    }


    func then<T>(_ body: @escaping (Value) -> T) -> Promise<T> {

        return then { value in

            return Promise<T> { resolve in

                resolve(body(value))
            }
        }
    }

    private func resolve(value: Value) {

        updateState(to: .completed(value))
    }

    private func updateState(to newState: State<Value>) {

        guard case .pending = state else { return }

        state = newState
        triggerCallbacksIfResolved()
    }

    private func triggerCallbacksIfResolved() {

        guard case let .completed(value) = state else { return }

        callbacks.forEach { callback in

            callback(value)
        }
        callbacks.removeAll()
    }
}