Pass @Published where @Binding is required (SwiftUI, Combine)

Question

A ViewModel class has a sourceProperty that is being edited by the TextField. That property is @Published. I'd like to pass it to the Logic class which has an initializer with Binding<String>. That class will be listening to the sourceProperty changes, react on them and set it's output to the @Published output property.

How can I pass @Published sourceProperty as an initializer parameter to the Logic class?

Relevant code:

final class ViewModel {
    @Published var sourceProperty: String = ""
    private var logic: Logic?

    init() {
        self.logic = Logic(data: $sourceProperty)
        $logic.output.sink({result in 
            print("got result: \(result)")
        })
    }

}

final class Logic: ObservableObject {
    private var bag = Set<AnyCancellable>()
    @Published var output: String = ""
    @Binding var data: String
    init(data: Binding<String>) {
        self._data = data

        $data.sink({ newValue in 
            output = newvalue + "ABCDE"
        }).store(in: &bag)
    }
}

So far I'm getting the following error:

Cannot convert value of type 'Published.Publisher' to expected argument type 'Binding'

The goal is to use a change in the object's own property to trigger a method invocation in another object and then bind that second object's output to some view.

View Layer:

public struct ViewLayer: View {
    @Binding private var sourceProperty: String

    public init(_ placeholder: String,
                sourceProperty: Binding<String>,
    ) {
        self.placeholder = placeholder
        self._sourceProperty = sourceProperty
    }

    public var body: some View {
        TextField(placeholder, text: $sourceProperty)
    }

 }

Zebadiah

@Binding should only be used inside `Views. It would be easier to help if your question included the view layer as well. – Braga

Are you sure you want a Binding? You're trying to use sink on it, which is what comes from a Publisher – Ingamar

No, I'm not sure if I need binding. What I'd like to achieve is to connect two classes together, in a way that when a sourceproperty gets written to (by the view), Logic class will react on that change and update it's own properties. – Zebadiah

I agree with the top comment -- probably good to include the view layer. It definitely sounds like you don't need the binding -- probably just pass the publisher to Logic. But, I'm struggling to see why you need nested ObservableObjects as well. That seems suspicious. I get why your ViewModel would be an ObservableObject, but not why Logic would. And right now, it's the reverse in your code. – Ingamar

Added simplified View Layer as an example – Zebadiah

Where is the ViewModel owned? And why is Logic and ViewModel both an ObservableObject? Neither is really addressed by the view layer that got added. – Ingamar

1. VM is owned by the view. 2. Feel free to remove ObservableObject from one of the classes. – Zebadiah

Answer 1

If I understand your question correctly, you are probably looking for something like that:

final class ViewModel: ObservableObject {
    
    @Published var sourceProperty: String = ""
    private lazy var logic = Logic(data: $sourceProperty)
    private var cancellable: AnyCancellable?

    init() {
        cancellable = logic.$output
            .sink { result in
                print("got result: \(result)")
            }
    }

}

final class Logic: ObservableObject {
    
    @Published private(set) var output: String = ""
    
    init(data: Published<String>.Publisher) {
        data
            .map { $0 + "ABCDE" }
            .assign(to: &$output)
    }
}

Braga

Perfect, this looks like it achieves the goal I had in mind. Thanks! – Zebadiah

you just solved my annoying memory leak :) – Valdavaldas