/// A handle that can cancel a live subscription.
///
/// The subscription lives exactly as long as the token: ``cancel()`` ends it
/// early, and releasing the last reference to the token ends it too. Store the
/// token (or hand it to a ``NodeRegistry``) for as long as the callback should
/// keep firing. ``cancel()`` is idempotent.
@MainActor public final class SubscriptionToken {
    private var onCancel: (() -> Void)?

    @_spi(Portico) public init(onCancel: @escaping () -> Void) {
        self.onCancel = onCancel
    }

    /// Removes the underlying subscription. Idempotent.
    public func cancel() {
        onCancel?()
        onCancel = nil
    }

    /// Ends the subscription when the token is released.
    isolated deinit { cancel() }
}