gistfile1.cs

internal IList<string> OpenMessageSubscriptions = new List<string>();

protected bool ShowSubViewModel<TSubViewModel, TResult>(SubViewModelBase<TResult>.ParametersBase parameters, Action<TResult> onResult)
    where TSubViewModel : SubViewModelBase<TResult>
{
    MvxSubscriptionToken token = null;
    token = Messenger.Subscribe<SubNavigationResultMessage<TResult>>(
        msg =>
        {
            if (token != null)
                Messenger.Unsubscribe<SubNavigationResultMessage<TResult>>(token);

            if (!OpenMessageSubscriptions.Contains(msg.MessageId))
                return;

            OpenMessageSubscriptions.Remove(msg.MessageId);

            onResult(msg.Result);
        });

    parameters.MessageId = Guid.NewGuid().ToString();
    OpenMessageSubscriptions.Add(parameters.MessageId);

    return ShowViewModel<TSubViewModel>(parameters);
}