struct ChatListView: View {
    let chatService: ChatService

    var body: some View {
        NavigationView {
            List(chatService.fetchChats()) { chat in
                NavigationLink(
                    destination: ChatDetailView(chat: chat, chatService: self.chatService)
                ) {
                    ChatCell(chat: chat)
                }
            }
            .navigationBarTitle("Chats")
        }
    }
}

struct ChatDetailView: View {

    private let chat: Chat
    private let chatService: ChatService
    
    @ObservedObject private var keyboardObserver = KeyboardObserver.shared
    
    @State private var newMessage = ""
    @State private var messages: [Message] = []

    /* ... */

    var body: some View {
        VStack {
            List(messages) { message in
                MessageView(message: message,
                            isMine: self.chatService.currentUser == message.sender)
            }

            Divider()

            HStack {
                TextField("New message",
                          text: $newMessage,
                          onCommit: sendMessage)
                Button(action: sendMessage) {
                    Text("Send")
                }
            }
            .padding([.leading, .top, .trailing])
        }
        .padding(.bottom, keyboardObserver.height)
        .navigationBarTitle(Text(chat.title), displayMode: .inline)
        .animation(.easeInOut)
        .onAppear { self.reloadMessages() }
    }

    private func sendMessage() {
        chatService.addMessage(newMessage, to: chat)
        newMessage = ""
        reloadMessages()
    }

    private func reloadMessages() {
        self.messages = chatService.fetchMessages().filter { $0.chatId == chat.id }
    }

}

import SwiftUI

struct ContentView: View {
    
    @ObservedObject private var counterVM: CounterViewModel
    
    init() {
        counterVM = CounterViewModel()
    }
    
    var body: some View {
        VStack {
            
            Text(counterVM.premium ? "PREMIUM" : "")
                .foregroundColor(Color.green)
                .frame(width: 200, height: 100)
                .font(.largeTitle)
            
            Text("\(counterVM.value)")
                .font(.title)
            Button("Increment") {
                self.counterVM.increment()
            }
        }
    }
}

import Foundation
import SwiftUI

class CounterViewModel: ObservableObject {
    
    @Published private var counter: Counter = Counter()
    
    var value: Int {
        counter.value
    }
    
    var premium: Bool {
        counter.isPremium
    }
    
    func increment() {
        counter.increment()
    }
}

class IntPublisher: Publisher {

    typealias Output = Int
    typealias Failure = Never

    func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {

    }
}

class IntSubscriber: Subscriber {
    typealias Input = Int
    typealias Failure = Never

    func receive(subscription: Subscription) {

    }

    func receive(_ input: Int) -> Subscribers.Demand {
        return .unlimited
    }

    func receive(completion: Subscribers.Completion<Never>) {

    }
}

class TestSubscription: Subscription {
    func request(_ demand: Subscribers.Demand) {

    }

    func cancel() {

    }
}