How to bind to nested ObservableObject in SwiftUI

Issue #831

We can normally listen to sub ObservableObject by either listen to its objectWillChange or its Publisher

class SubModel: ObservableObject {
    @Published var count = 0
}

class AppModel: ObservableObject {
    @Published var submodel: SubModel = SubModel()

    private var bag = Set<AnyCancellable>()

    init() {
        submodel.$count
            .sink(receiveValue: { [weak self] _ in
                self?.objectWillChange.send()
            })
    }
}

We can then make a convenient extension to ease this. We use DispatchQueue to trigger objectWillChange in another run loop, to achieve didSet behavior

import SwiftUI
import Combine

extension ObservableObject where ObjectWillChangePublisher == ObservableObjectPublisher {
    func dependsOn<T>(publisher: Published<T>.Publisher) -> AnyCancellable {
        publisher
            .sink(receiveCompletion: { _ in }, receiveValue: { [weak self] _ in
                DispatchQueue.main.async {
                    self?.objectWillChange.send()
                }
            })
    }
}

class AppModel: ObservableObject {
    @Published var submodel: SubModel = SubModel()

    private var bag = Set<AnyCancellable>()

    init() {
        dependsOn(publisher: submodel.$count)
            .store(in: &bag)
    }
}