MvvmCross - Passing a string with IMvxNavigationService

Question

I'm currently working on a Xamarin.iOS project that uses a web-api to gather data. However, I'm running into some problems trying to pass the user input from a textfield to the Tableview that gets the result from the api.

To do this I've followed the example on the MvvmCross documentation.

The problem is that the input from the Textfield never reaches the 'Filter' property in my TableviewController's viewmodel. I think I'm not passing the string object correctly to my IMvxNavigationService when called.

To clarify, in my UserinputViewController I'm binding the textfield's text like so:

    [MvxFromStoryboard(StoryboardName = "Main")]
    public partial class SearchEventView : MvxViewController
    {
        public SearchEventView (IntPtr handle) : base (handle)
        {
        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();

            MvxFluentBindingDescriptionSet<SearchEventView, SearchEventViewModel> set = new MvxFluentBindingDescriptionSet<SearchEventView, SearchEventViewModel>(this);

            set.Bind(btnSearch).To(vm => vm.SearchEventCommand);
            set.Bind(txtSearchFilter).For(s => s.Text).To(vm => vm.SearchFilter);
            set.Apply();
        }
      }

The Viewmodel linked to this ViewController looks like this:

public class SearchEventViewModel : MvxViewModel
{
    private readonly IMvxNavigationService _navigationService;  

    private string _searchFilter;

    public string SearchFilter
    {
        get { return _searchFilter; }
        set { _searchFilter = value; RaisePropertyChanged(() => SearchFilter); }
    }

    public SearchEventViewModel(IMvxNavigationService mvxNavigationService)
    {
        this._navigationService = mvxNavigationService;
    }

    public IMvxCommand SearchEventCommand {
        get {
            return new MvxCommand<string>(SearchEvent);
        }
    }

    private async void SearchEvent(string filter)
    {
        await _navigationService.Navigate<EventListViewModel, string>(filter);
    }
}

And finally, TableviewController's viewmodel looks like this:

public class EventListViewModel : MvxViewModel<string>
{
    private readonly ITicketMasterService _ticketMasterService;
    private readonly IMvxNavigationService _navigationService;
    private List<Event> _events;

    public List<Event> Events
    {
        get { return _events; }
        set { _events = value; RaisePropertyChanged(() => Events); }
    }

    private string _filter;

    public string Filter
    {
        get { return _filter; }
        set { _filter = value; RaisePropertyChanged(() => Filter); }
    }


    public EventListViewModel(ITicketMasterService ticketMasterService, IMvxNavigationService mvxNavigationService)
    {
        this._ticketMasterService = ticketMasterService;
        this._navigationService = mvxNavigationService;
    }

    public IMvxCommand EventDetailCommand {
        get {
            return new MvxCommand<Event>(EventDetail);
        }
    }

    private void EventDetail(Event detailEvent)
    {   
        _navigationService.Navigate<EventDetailViewModel, Event>(detailEvent);
    }

    public override void Prepare(string parameter)
    {
        this.Filter = parameter;
    }

    public override async Task Initialize()
    {
        await base.Initialize();
        //Do heavy work and data loading here
        this.Events = await _ticketMasterService.GetEvents(Filter);
    }
}

Whenever trying to run, the string object 'parameter' in my TableviewController's Prepare function remains 'null' and I have no idea how to fix it. Any help is greatly appreciated!

Answer 1

I believe the issue is with your command setup

new MvxCommand<string>(SearchEvent);

As this command is being bound to a standard UIButton. It will not pass through a parameter value of your filter but null instead. So the string parameter generic can be removed. Additionally, as you want to execute an asynchronous method I would suggest rather using MvxAsyncCommand

new MvxAsyncCommand(SearchEvent);

Then in terms of SearchEvent method you can remove the parameter. The value of filter is bound to your SearchFilter property. It is this property's value that you want to send as the navigation parameter.

private async Task SearchEvent()
{
    await _navigationService.Navigate<EventListViewModel, string>(SearchFilter);
}