PropertyWrapper: @input provides a write-only interface externally, but a subject available internally for modelling reactive streams in Combine

Input.swift

import Combine

@propertyWrapper
public struct Input<T> {
    public struct SendProxy {
        private let send: (T) -> Void

        fileprivate init(_ send: @escaping (T) -> Void) {
            self.send = send
        }

        public func callAsFunction(_ value: T) {
            send(value)
        }
    }

    public let subject = PassthroughSubject<T, Never>()

    public var wrappedValue: T {
        get { fatalError("Inputs are write only") }
        set { fatalError("Inputs should only be written to via the $projectedValue") }
    }
    public var projectedValue: SendProxy { .init(subject.send) }

    public init() { }
}

Usage.swift

class MyViewModel: ObservableObject {
    // MARK: - Inputs
    @Input var add: Int
  
    // MARK: - Outputs
    @Published var count: Int = 0
  
    // MARK: - Lifecycle
    init() {
        _add.subject
            .combineLatest($count, +)
            .assign(to: $count)
    }
}

struct MyView: View {
    @StateObject var viewModel = MyViewModel()
  
    var body: some View {
        VStack {
            Text("\(viewModel.count)")
            Button("Add", action: { viewModel.$add(1) })
        }
    }
}