NetworkFuture.swift

private typealias Handler<T> = (GenericResult<T>) -> Void
private enum Sealant<T> {

    case sealed([Handler<T>])
    case resolved(GenericResult<T>)
}

private final class Resolver<T> {

    private let lock = NSLock()
    private var value: Sealant<T> = .sealed([])

    func resolve(_ result: GenericResult<T>) {
        lock.lock()

        var handlers: [Handler<T>]?

        switch value {
        case .resolved:
            return // already resolved
        case .sealed(let _handlers):
            handlers = _handlers
            value = .resolved(result)
        }

        lock.unlock()

        handlers?.forEach { $0(result) }
    }

    func inspect(_ call: @escaping (GenericResult<T>) -> Void) {
        let result: GenericResult<T>

        lock.lock()

        switch value {
        case .sealed(var handlers):
            handlers.append(call)
            value = .sealed(handlers)
            return
        case .resolved(let _result):
            result = _result
        }

        lock.unlock()

        call(result)
    }

    func fulfill(_ newValue: T) {
        resolve(.success(newValue))
    }

    func reject(_ error: Error) {
        resolve(.failure(error))
    }
}

public struct NetworkFuture<T> {

    private let resolver = Resolver<T>()

    public func onResolved(_ call: @escaping (GenericResult<T>) -> Void) {
        resolver.inspect(call)
    }

    func fulfill(_ newValue: T) {
        resolver.fulfill(newValue)
    }

    func reject(_ error: Error) {
        resolver.reject(error)
    }

    func resolve(_ result: GenericResult<T>) {
        resolver.resolve(result)
    }
}