SwiftUI passing data between multiple classes

Question

After scouring Apple's SwiftUI docs, the web and stackoverflow, I can't seem to figure out how to use multiple classes and passing EnviromentObject data between them. All the articles on SwiftUI's EnvironmentObject, ObservableObjects, Bindings show how to pass data from a model to a view, but how about from one model to another. Or am I thinking about this the wrong way altogether.

How can I pass a @Published var from Class A to Class B?

In this simple SwiftUI example app I am trying to pass @Published data from a Settings class to a NetworkManager class. This app has a single ContentView with two form fields for username and password with a button that when pressed will contact an API and display the results.

The below code will crash because the NetworkManager mockService function does not know what "settings" is. I can read the "Settings" observed EnvironmentObject in a View, but how do I get that data in another class? I am guessing there is something to do with Bindings here, but not sure what to do.

SceneDelegate:

...
var settings = Settings()
var networkManager = NetworkManager()

...
let contentView = ContentView()
    .environmentObject(settings)
    .environmentObject(networkManager)
...

ContentView.swift

class Settings: ObservableObject {
    @Published var username: String = ""
    @Published var password: String = ""    
}

// This function in reality will be rewritten and expanded with multiple networking calls
class NetworkManager: ObservableObject {
    @Published var token: String = ""

    func mockService() {
        token = settings.username + settings.password
    }
}

struct ContentView: View {
    @EnvironmentObject var settings: Settings
    @EnvironmentObject var networkManager: NetworkManager
    var body: some View {
        VStack {
            TextField("Username", text: $settings.username)
            TextField("Password", text: $settings.password)
            Button("Update"){
                self.networkManager.mockService()
            }
            Divider()
            Text("\(networkManager.token)")

        }
    }
}

Answer 1

You can use .sink to observe changes from @Published properties

Code Example:

import Foundation
import Combine

class ItemList:ObservableObject {
    @Published var items: [Item] = []
}

struct Item {
    let name:String
    let image:String
}

class viewModel {
    var itemsCancellable:AnyCancellable
    init(il:ItemList){
        self.itemsCancellable = il.$items.sink { items in
        print("items array changed")
        }
    }
}

If you store your data inside a singleton class, then you can easily read from viewModel and sink changes:

import Foundation
import Combine

class ItemList:ObservableObject {

    static var shared:ItemList = ItemList()
    @Published var items: [Item] = []
}

struct Item {
    let name:String
    let image:String
}

class viewModel {
    var itemsCancellable = ItemList.shared.$items.sink { items in
        print("items array changed")
    }
}

Answer 2

This's one way of making the network object aware of settings object, but i don't think that it's the best way to do it, you can try it:

class Settings: ObservableObject {
    @Published var username: String = ""
    @Published var password: String = ""
    var currentSettingsPublisher: PassthroughSubject<Settings,Never> = .init()

    var cancellablesBag: Set<AnyCancellable> = []


    init() {
        observeChanges()
    }

    // Object will change is called to notify subscribers about changes( That's how swiftUI react to changes)
    private func observeChanges() {

        self.objectWillChange.sink { [weak self] (_) in
            guard let self = self else { return }
            self.currentSettingsPublisher.send(self)
        }.store(in: &cancellablesBag)
    }
}

// This function in reality will be rewritten and expanded with multiple networking calls
class NetworkManager: ObservableObject {
    
    @Published var token: String = ""
    var cancellablesBag: Set<AnyCancellable> = []
    var currentSettingsPublisher: AnyPublisher<Settings,Never>
    private var settings: Settings?

    init(_ settings: AnyPublisher<Settings,Never>) {
        self.currentSettingsPublisher = settings
        observeSettings()
    }

    func observeSettings() {
        currentSettingsPublisher
            // .debounce(for: .seconds(0.5), scheduler: RunLoop.main) maybe you can use debounce to debounce updating of your settings object
            .sink { [weak self] (newSettings) in
            guard let self = self else { return }
                print("i have the new settings")
            self.settings = newSettings
        }.store(in: &cancellablesBag)
    }
    

    
    func mockService() {
        guard let settings = settings else {
            return assertionFailure("Settings is nil, what to do ?")
        }

        token = settings.username + settings.password
        print("new token: \(token)")
    }
}

struct ContentView: View {
    @EnvironmentObject var settings: Settings
    @EnvironmentObject var networkManager: NetworkManager
    var body: some View {
        VStack {
            TextField("Username", text: $settings.username)
            TextField("Password", text: $settings.password)
            Button("Update"){
                self.networkManager.mockService()
            }
            Divider()
            Text("\(networkManager.token)")

        }
    }
}

and for SceneDelegate

var settings = Settings()
var networkManager: NetworkManager!

        func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {

networkManager = 
    NetworkManager(settings.currentSettingsPublisher.eraseToAnyPublisher())
            
    
            let contentView = ContentView()
                .environmentObject(settings)
                .environmentObject(networkManager)

}