Custom Lazy Loading

RemoteValue.swift

import Foundation
import SwiftUI
import Combine
import TinyNetworking

final class RemoteValue<A>: BindableObject {
    let didChange = MyPublisher()
    let endpoint: Endpoint<A>
    var value: A? {
        didSet {
            DispatchQueue.main.async {
                self.didChange.send()
            }
        }
    }
    init(_ endpoint: Endpoint<A>, initial: A? = nil) {
        self.endpoint = endpoint
        self.value = initial
        didChange.onSubscribe = { [weak self] in
            self?.reload()
        }
    }
    
    func reload() {
        print("Loading \(endpoint)")
        URLSession.shared.load(endpoint) { result in
            switch result {
            case .failure:
                ()
                // self.value = nil
            case .success(let x): self.value = x
            }
        }
    }
}

Sub.swift

import Foundation
import Combine
import Register // from https://github.com/chriseidhof/register

class MySubscription: Subscription {
    let cleanup: () -> ()
    init(_ cleanup: @escaping () -> ()) {
        self.cleanup = cleanup
    }
    func request(_ demand: Subscribers.Demand) {
        assert(demand == .unlimited) // todo we don't support other demands yet
    }

    func cancel() {
        cleanup()
    }

    deinit {
        cleanup()
    }
}

class MyPublisher: Publisher {
    typealias Output = ()
    typealias Failure = Never

    var onSubscribe: () -> () = { }

    var subscribers: Register<AnySubscriber<(), Never>> = Register() {
        didSet {
            if oldValue.isEmpty && !subscribers.isEmpty {
                onSubscribe()
            }
        }
    }
    func receive<S>(subscriber: S) where S : Subscriber, S.Failure == Never, S.Input == () {
        let token = subscribers.add(AnySubscriber(subscriber))
        let subscription = MySubscription() { [weak self] in
            self?.subscribers.remove(token)
        }
        subscriber.receive(subscription: subscription)
    }

    func send() {
        for s in subscribers.values {
            let demand = s.receive(())
            // todo we don't do anything with demand yet
        }
    }
}