Instead of complex network managers, or view controllers containing hundreds of lines of networking code, I always try to reduce my networking into functions that I can pass into my view controllers:

// With Futures/Promises:
typealias Loading<T> = () -> Future<T>

// Without Futures/Promises:
typealias Loading<T> = (@escaping (T) -> Void) -> Void

// If we model our networking code using functions, our view
// controllers no longer need to be aware of any networking,
// and can simply call those functions in order to retrieve
// their data:
class ProductViewController: UIViewController {
    private let loading: Loading<Product>

    init(loading: @escaping Loading<Product>) {
        self.loading = loading
        super.init(nibName: nil, bundle: nil)
    }

    func loadProduct() {
        loading().observe {
            ...
        }
    }
}