How to use ReactiveList so UI is updated when new items are added

Question

I'm creating a Xamarin.Forms application with a list. The itemSource is a reactiveList. Adding new items to the list however does not update the UI. What is the correct way of doing this?

List Definition

_listView = new ListView();
var cell = new DataTemplate(typeof(TextCell));
cell.SetBinding(TextCell.TextProperty, "name");
cell.SetBinding(TextCell.DetailProperty, "location");
_listView.ItemTemplate = cell;

Binding

this.OneWayBind(ViewModel, x => x.monkeys, x => x._listView.ItemsSource);
this.OneWayBind(ViewModel, x => x.save, x => x._button.Command); //save adds new items

View Model

public class MyPageModel : ReactiveObject
{
    public MyPageModel()
    {
        var canSave = this.WhenAny(x => x.username, x => !String.IsNullOrWhiteSpace(x.Value) && x.Value.Length>5);
        save = ReactiveCommand.CreateAsyncTask(canSave, async _ =>
        {
            var monkey = new Monkey { name = username, location = "@ " + DateTime.Now.Ticks.ToString("X"), details = "More here" };
            monkeys.Add(monkey);
            username = "";
        });
        monkeys = new ReactiveList<Monkey>{
            new Monkey { name="Baboon", location="Africa & Asia", details = "Baboons are Africian and Arabian Old World..." }
        };
        _monkeys.ChangeTrackingEnabled = true;
    }
    private string _username = "";
    public string username
    {
        get { return _username; }
        set { this.RaiseAndSetIfChanged(ref _username, value); }
    }
    private double _value = 0;
    public double value
    {
        get { return _value; }
        set { this.RaiseAndSetIfChanged(ref _value, value); }
    }
    public ReactiveCommand<Unit> save { get; set; }
    public ReactiveList<Monkey> _monkeys;
    public ReactiveList<Monkey> monkeys
    {
        get { return _monkeys; }
        set { this.RaiseAndSetIfChanged(ref _monkeys, value); }
    }
}
public class Monkey
{
    public string name { get; set; }
    public string location { get; set; }
    public string details { get; set; }
}

Tried it with the ReactiveList property being a normal auto-property as well as the one in the code above where I used the RaiseAndSetIfChanged method.

Answer 1

Your problem is that you are changing monkeys on a non-UI thread. In other frameworks, this would throw an Exception, but in AppKit / UIKit this just Does Weird Stuff (usually nothing).

    save = ReactiveCommand.Create(canSave);
    save.Subscribe(_ =>
    {
        // Do the part that modifies UI elements (or things bound to them)
        // in the RxCmd's Subscribe. This is guaranteed to run on the UI thread
        var monkey = new Monkey { name = username, location = "@ " + DateTime.Now.Ticks.ToString("X"), details = "More here" };
        monkeys.Add(monkey);
        username = "";
    });

Answer 2

This technique seems awfully complicated. It makes me think that you want to do something more complicated that I think it to be.

Here's a clean solutionfor adding an object to a list box by binding to a ReactiveList<T> in pure MVVM fashion.

First, the xaml

<Window x:Class="ReactiveUIListBox.View.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:viewModel="clr-namespace:ReactiveUIListBox.ViewModel"
        Title="MainWindow" Height="350" Width="525">

    <Window.Resources>
        <viewModel:MainWindowViewModel x:Key="MainWindowViewModel"/>    
    </Window.Resources>

    <Window.DataContext>
        <StaticResource ResourceKey="MainWindowViewModel"/>
    </Window.DataContext>

    <Grid DataContext="{StaticResource MainWindowViewModel}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>

        <Grid Grid.Column="0" Background="Azure">
            <ListBox ItemsSource="{Binding Model.TestReactiveList}"></ListBox>
        </Grid>

        <Grid Grid.Column="1">
            <Button x:Name="button" Command="{Binding TestCommand}" Content="Button" HorizontalAlignment="Left" Margin="78,89,0,0" VerticalAlignment="Top" Width="75"/>
        </Grid>

    </Grid>
</Window>

Here's the ViewModel

using System.Windows.Input;
using ReactiveUIListBox.Model;
using SecretSauce.Mvvm.ViewModelBase;

namespace ReactiveUIListBox.ViewModel
{
    public class MainWindowViewModel : ViewModelBase
    {
        public MainWindowViewModel()
        {
            Model = new ReactiveModel<string>();
        }
        public ReactiveModel<string> Model
        {
            get;
            set;
        }

        public ICommand TestCommand
        {
            get { return new RelayCommand(ExecuteTestCommand); }
        }

        private void ExecuteTestCommand(object obj)
        {
            Model.TestReactiveList.Add("test string");
        }
    }
}

and finally, here's the Model.

using ReactiveUI;

namespace ReactiveUIListBox.Model
{
    public class ReactiveModel<T> : ReactiveObject
    {
        public ReactiveModel()
        {
            TestReactiveList= new ReactiveList<T>();
        }

        public ReactiveList<T> TestReactiveList
        {
            get;
            set;
        }
    }
}

Pressing the button will populate the ListBox. Hopefully I have not completely over simplified what you are trying to do, but your code does seem to be all over the place.

I can't differentiate between your code that fits in the View, Model or ViewModel.

Cheers.