Alternative to @published that works with subclasses

SuperPublished.swift

@propertyWrapper
public struct SuperPublished<Value> {

    public init(wrappedValue: Value) {
        self.wrappedValue = wrappedValue
    }
    
    public var wrappedValue: Value
    
    public static subscript<EnclosingSelf: ObservableObject>(
        _enclosingInstance observed: EnclosingSelf,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
    ) -> Value {
        get {
            observed[keyPath: storageKeyPath].wrappedValue
        }
        set {
            if let publisher = observed.objectWillChange as? Combine.ObservableObjectPublisher {
                publisher.send()
            }
            observed[keyPath: storageKeyPath].wrappedValue = newValue
        }
    }
}

TestPlayground.swift

import PlaygroundSupport
import SwiftUI
import Combine

class Root: ObservableObject {
    @Published var rootPublished: Int = 0
    @SuperPublished var rootSuperPublished: Int = 0
}

class Child: Root {
    @Published var childPublished: Int = 0
    @SuperPublished var childSuperPublished: Int = 0
}

struct MyView: View {

    @ObservedObject var child = Child()
    
    var body: some View {
        VStack(alignment: .leading) {
            button("rootPublished", for: \.rootPublished)
            button("rootSuperPublished", for: \.rootSuperPublished)
            button("childPublished", for: \.childPublished)
            button("childSuperPublished", for: \.childSuperPublished)
        }
    }
    
    func button(
        _ title: String,
        for keyPath: ReferenceWritableKeyPath<Child, Int>
    ) -> some View {
        HStack {
            Text("\(title): \(child[keyPath: keyPath])")
            Button(action: { self.child[keyPath: keyPath] += 1 }) {
                Text("+= 1")
            }
        }
    }
}

PlaygroundPage.current.liveView = NSHostingView(rootView: MyView())

This is very cool. I found it very useful. Pity that Apple doesn't provide this functionality by default.

I just got one question, have you tried to unit tests a @SuperPublished property? I'm not able to do it e.g.

$childPublished // this one works
.sink {
XCTAssertEqual($0, 0)
expectation.fulfill()
}.store(in: &cancelBag)

$rootSuperPublished // $ not found, only rootSuperPublished
.sink {
XCTAssertEqual($0, 0)
expectation.fulfill()
}.store(in: &cancelBag)

Thank you

@atb1983 oh you're right, this property wrapper misses a projectedValue. You can add it like this:

var projectedValue: SuperPublished<Value> {
    return self
}

Oh actually not... This will return a SuperPublished<Value> when using $, while Published<Value> returns a publisher.
But you still should be able to do it by maintaining a CurrentValueSubject<Value>.