AsyncPublished.swift

@available(iOS 16.0, macOS 13.0, watchOS 9.0, *)
@propertyWrapper
public struct AsyncPublished<Value> {
    @available(*, unavailable, message: "@AsyncPublished is only available on properties of classes")
    public var wrappedValue: Value {
        get { fatalError() }
        set { fatalError() } // swiftlint:disable:this unused_setter_value
    }
    public let projectedValue: AsyncStream<Value>
    public init(initialValue: Value) {
        self.init(wrappedValue: initialValue)
    }
    public init(wrappedValue: Value) {
        storage = wrappedValue
        var con: AsyncStream<Value>.Continuation!
        let str = AsyncStream<Value> {
            con = $0
        }
        self.projectedValue = str
        self.continuation = con
    }
    public static subscript<EnclosingSelf: AnyObject>(
        _enclosingInstance object: EnclosingSelf,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, AsyncPublished<Value>>
    ) -> Value {
        get {
            return object[keyPath: storageKeyPath].storage
        }
        set {
            object[keyPath: storageKeyPath].storage = newValue
            object[keyPath: storageKeyPath].continuation.yield(newValue)
        }
    }
    private let continuation: AsyncStream<Value>.Continuation
    private var storage: Value
}

actor Thingy {
    @AsyncPublished var value: String = ""
    func update(_ value: String) {
        self.value = value
    }
}

let thingy = Thingy()

Task {
    for await value in await thingy.$value {
        print(value)
    }
}

Task {
    await thingy.update("1")
    await thingy.update("2")
    await thingy.update("3")
}