how to correctly bind a View control to a ViewMode

Question

I need to bind a List to a UniformGrid into a WP Window using WPF MVVM.

I had in mind to do something like this:

Into my VM:

   private List<Rat> rats;
        private UniformGrid uniformGrid;
        public List<Rat> Rats
        {
            get { return rats; }
            set
            {

                if (rats != value)
                    {
                    //update local list value
                        rats = value;

                    //create View UniformGrid
                        if (uniformGrid == null)
                            uniformGrid = new UniformGrid() { Rows=10};
                        else
                            uniformGrid.Children.Clear();
                    foreach(Rat rat in value)
                    {
                        StackPanel stackPanel = new StackPanel();
                        Ellipse ellipse = new Ellipse(){Height=20, Width=20, Stroke= Brushes.Black};
                        if (rat.Sex== SexEnum.Female)
                            ellipse.Fill= Brushes.Pink;
                        else 
                             ellipse.Fill= Brushes.Blue;

                        stackPanel.Children.Add(ellipse  );
                        TextBlock textBlock = new TextBlock();
                        textBlock.Text= rat.Name + " (" + rat.Age +")";
                        stackPanel.Children.Add( textBlock  );
                        uniformGrid.Children.Add(stackPanel);
                    }


                        OnPropertyChanged("Rats");
                    }            
            }
        }

The VM is correctly informed when the list needs to be refreshed into the view via an event. So at this point I would need my View to be correctly bound to the VM. I made it this way:

 <GroupBox x:Name="GB_Rats" Content="{Binding Rats}"  Header="Rats" HorizontalAlignment="Left" Height="194" Margin="29,10,0,0" VerticalAlignment="Top" Width="303">

Is this the correct global approch?

Concretely, when attempting to run the code, this line fails to execute:

 uniformGrid = new UniformGrid() { Rows=10};

->

An unhandled exception of type 'System.InvalidOperationException' occurred in PresentationCore.dll
Additional information: The calling thread must be STA, because many UI components require this.

This lets me think that I should not proceed this way from a MVVM point of view.

Thx for your kind assistance.

Answer 1

The ViewModel isn't supposed to instantiate any UI controls, this should be the View's responsibility.

So in your code you shouldn't try to create StackPanels, Ellipses etc.

Also try to use the types that already have Change notification - for instead of List<T> use ObservableCollection<T> MSDN, i wouldn't recommend replacing a whole list when its value change.

The right way to do this in the MVVM pattern is to create a DataTemplate for the Rat like this:

ViewModel:

public class MainWindowViewModel
{
    public ObservableCollection<Rat> Rats { get; set; } =
        new ObservableCollection<Rat>()
        {
            new Rat()
            {
                Name = "Fred",
                Age = "19",
                Sex = SexEnum.Male
            },
            new Rat()
            {
                Name = "Martha",
                Age = "21",
                Sex = SexEnum.Female
            }
        };

}

Model - Rat and Sex:

public class Rat
{
    public SexEnum Sex { get; set; }
    public string Name { get; set; }
    public string Age { get; set; }
}

public enum SexEnum
{
    Female,
    Male
}

As you want to present the Models value of Sex in one of two colors you should use a IValueConverter for that:

[ValueConversion(typeof(SexEnum), typeof(Brush))]
public class SexToColorConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (!(value is SexEnum))
            throw new ArgumentException("value not of type StateValue");
        SexEnum sv = (SexEnum)value;
        //sanity checks
        if (sv == SexEnum.Female)
            return Brushes.Red;
        return Brushes.Blue;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

This is then used in your window:

Window:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525"
        xmlns:ViewModel="WpfApplication1.VM"
        xmlns:Converters ="clr-namespace:WpfApplication1.Converters"
        >
    <Grid>
        <Grid.Resources>
            <Converters:SexToColorConverter x:Key="SexToBrushConverter"></Converters:SexToColorConverter>
        </Grid.Resources>
        <ComboBox x:Name="comboBox" ItemsSource="{Binding Rats}" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Width="120">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <Ellipse Width="10" Height="10" Fill="{Binding Sex, Converter={StaticResource SexToBrushConverter}}"></Ellipse>
                        <TextBlock Margin="5" Text="{Binding Name}"></TextBlock>
                        <TextBlock Margin="5" Text="{Binding Age}"></TextBlock>
                    </StackPanel>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>

    </Grid>
</Window>

Note the DataTemplate that is assigned to the ComboBox.ItemTemplate property and the declaration of the Converters:SexToColorConverter and its usage to change the color of the ellipse in the Fill binding.

Update 4.4.2016 16:30 Window using a GroupBox with CheckBoxes to display the list

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525"
        xmlns:Converters ="clr-namespace:WpfApplication1.Converters"
        xmlns:vm="clr-namespace:WpfApplication1.VM">
    <Window.DataContext>
        <vm:MainWindowViewModel/>
    </Window.DataContext>
    <Grid>
        <Grid.Resources>
            <Converters:SexToColorConverter x:Key="SexToBrushConverter"></Converters:SexToColorConverter>
        </Grid.Resources>
        <GroupBox>
            <ItemsControl ItemsSource="{Binding Rats}" >
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal">
                            <CheckBox Margin="5"></CheckBox>
                            <Ellipse Width="10" Height="10" Fill="{Binding Sex, Converter={StaticResource SexToBrushConverter}}"></Ellipse>
                            <TextBlock Margin="5" Text="{Binding Name}"></TextBlock>
                            <TextBlock Margin="5" Text="{Binding Age}"></TextBlock>
                        </StackPanel>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </GroupBox>
    </Grid>
</Window>

I guess the target is to also select the Rats, depending on how MVVM purist you want to be you'd add a List of RatViewModels, that have a bool IsChecked property and bind the ItemsSource to a ObservableCollection<RatViewModel> and synchronize this list with your Models List<Rat>