The didSet observer does not fire when the value is changed due to the assignment. Is this a bug or a feature?

struct ContentView: View {
    @StateObject var v = vm()
    var body: some View {
        Text(String(v.b))
            .padding()
    }
}

class vm: ObservableObject {
    @Published var b = false {
        didSet {
            print("hello didSet \(b)")
        }
    }

    let pub = CurrentValueSubject<Bool, Never>(false)

    init() {
        self.b = true
        pub.assign(to: &$b)
        self.pub.send(true)
        self.pub.send(false)
    }
}

Expected output:

hello didSet true

hello didSet false

hello didSet true

hello didSet false

Actual output:

hello didSet true

Assigning the value with .sink() is a workaround but why doesn't assign(to:) trigger the didSet observer?

didSet is not trigger for assignments made in init.

Although I am not entirely sure where are you getting the single true value

didSet does fire for assignments made in init, it just doesn't fire after the intial assignment which was at the declaration. That's where the single true value comes from. You can also delay the sending to the subject until after the init and it still doesn't fire.