SwiftUI - how to observe(.sink/ .onReceive) PassthroughSubject inside a View

Question

If you encountered any of the next limitations while writing your app:

1. The type of your ViewModel or Model is Struct not Class. This way you will not be able to use ObservableObject + ObservedObject.
2. Also a second limitation is to avoid the use of Property Wrappers as: @Binding and @Published, in your Struct as they are harder to define as optional properties.
3. You don't want bidirectional biding like the one resulting from using @State + @Binding. You want to be open - allow others to observ changes, but be closed - don't allow for other to make changes (Delegate pattern in UIKit, published pattern in RXSwift, @StateObject in SwiftUI but only for Class).

You could consider the next solution.

Answer 1

If you have a Struct as Model or ViewModel and you need to update your SwiftUI View here it is how to do just that with SwiftUI + Combine.

import Combine

struct ViewModel {

    var textChangedPublisher: AnyPublisher<String, Never> {
        textChangedSubject.eraseToAnyPublisher()
    }
    private let textChangedSubject = PassthroughSubject<String, Never>()

    func triggerUpdate() {
        let newStr = DateFormatter().string(from: Date())
        textChangedSubject.send(newStr)
    }

}

And a screen:

import SwiftUI

struct ContentView: View {
    
    private let vieModel = ViewModel()
    @State private var title = "Let's start"
    
    var body: some View {
        VStack {
            Text(title)
            Button.init("Show cuurent date") {
                vieModel.triggerUpdate()
            }
            .onReceive(vieModel.textChangedPublisher) { newValue in
                title = newValue
            }
        }
        .padding()
    }
}