SE-0475: Transactional Observation of Values

[mtsrodrigues (Mateus Rodrigues)]

@gwendal.roue

In the sample code below, are we guaranteed that "Fresh value" is printed before doSomethingElse() starts?

I tried the following using May 14, 2025 toolchain:

@Observable
class Model {
    var count = 0
}

extension Model {
    @MainActor static var shared = Model()
}

@MainActor
func startModelObservation() {
    print(#function)
    let values = Observations {  Model.shared.count }
    if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
        Task.immediate {
            for await value in values {
                print("Fresh value: \(value)")
            }
        }
    }
}

func doSomethingElse() {
    print(#function)
}

startModelObservation()
doSomethingElse()

Prints:

startModelObservation()
Fresh value: 0
doSomethingElse()