WPF Toggle Combo Box Item Text Colour MVVM ReactiveUI

Question

OK so this is definitely a newbie question that unfortunately could not figure/find the answer to.

Essentially binding a list of objects to a Combobox, when the Disabled property on the object is set to true I want the text colour of the Combobox item to be set to gray.

This is what I have so far:

Combobox item datatype

public class ListItem
{
    public ListItem(string text)
    {
        Text = text;
    }

    public string Text { get; set; }
    public bool Disabled { get; set; }
}

Viewmodel setup


public class MainPageViewModel : ReactiveObject
{
    // In ReactiveUI, this is the syntax to declare a read-write property
    // that will notify Observers, as well as WPF, that a property has 
    // changed. If we declared this as a normal property, we couldn't tell 
    // when it has changed!
    private ListItem _selectedItem;
    public ListItem SelectedItem
    {
        get => _selectedItem;
        set => this.RaiseAndSetIfChanged(ref _selectedItem, value);
    }

    public List Items { get; set; }

    public MainPageViewModel()
    {
        Items = new List
        {
            new ListItem ("A Cat"),
            new ListItem ("A Dog"),
            new ListItem ("A Mouse"),
            new ListItem ("A Frog") { Disabled = true }
        };
    }
}

ReactiveUI Binding

public MainPage()
{
  InitializeComponent();

  ViewModel = new MainPageViewModel();
  
  this.WhenActivated(d =>
  {
    this.OneWayBind(ViewModel, vm => vm.Items, v => v.MyComboBox.ItemsSource)
                    .DisposeWith(d);

    this.Bind(ViewModel, vm => vm.SelectedItem, v => v.MyComboBox.SelectedItem)
                    .DisposeWith(d);

  });
}

Xaml markup

Any help is appreciated let me know if you need more information.

Solution: It looks like in future I need to test the example code before puttin it up - our actual code had the Disabled property set as a readonly which must mess with WPF binding. Changing it to public set and get solved the first issue of not seeing it greyed out! It would seem staring at a problem for so long blinds you and it really is that simple. As for graying out the selected item I will try it out and see.

user_4838058 · Answer

The last item in the dropdown already has its text grayed out, so I assume you're asking about the selected item. The ComboBox uses separate data templates for the selected item and the items in the dropdown. You can use a DataTemplateSelector to set both.

public class ComboBoxTemplateSelector : DataTemplateSelector
{
    public DataTemplate SelectedItemTemplate { get; set; }
    public DataTemplate DropdownItemsTemplate { get; set; }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        var itemToCheck = container;

        // Search up the visual tree, stopping at either a ComboBox or a ComboBoxItem (or null).
        // This will determine which template to use.
        while (itemToCheck is not null and not ComboBox and not ComboBoxItem)
            itemToCheck = VisualTreeHelper.GetParent(itemToCheck);

        // If you stopped at a ComboBoxItem, you're in the dropdown.
        return itemToCheck is ComboBoxItem ? DropdownItemsTemplate : SelectedItemTemplate;
    }
}

Xaml markup

We have some repetition in the DataTemplate definitions, but these tend to grow apart in production code.

Resources

user_2411960 · Answer

I'm assuming your problem is that ComboBoxItems do not get grayed once the app is running.

I'm not familiar with ReactiveUI, but since I found a problem in your code, I tried it in a CommunityToolkit.Mvvm version of your code and verified my theory.

Bottom of line, you need to implement the ReactiveUI version of INotifyPropertyChanged to the Disabled property.

If you are interested in, I can post the CommunityToolkit.Mvvm version of this code.

user_6070324 · Answer

Here is an approach that worked in my tests:

Combobox item datatype:

//-- Unchanged
public class ListItem
{
    public ListItem( string text )
    {
        Text = text;
    }

    public string Text { get; set; }

    public bool Disabled { get; set; }
}

Viewmodel setup:

public class MainPageViewModel : INotifyPropertyChanged
{
    private ListItem? _selectedItem;

    public event PropertyChangedEventHandler? PropertyChanged;

    public ListItem? SelectedItem
    {
        get => _selectedItem;
        set
        {
            //-- I didn't had the "RaiseAndSetIfChanged" method, so I just implemented the functionality manually
            if( value != _selectedItem )
            {
                //-- Update the value ...
                _selectedItem = value;

                //-- ... AND inform everyone (who is interested) about the change
                this.PropertyChanged?.Invoke( this, new PropertyChangedEventArgs( nameof( this.SelectedItem ) ) );
            }
        }
    }

    //-- Use always an ObservableCollection when you want to achieve reactivity
    public ObservableCollection Items 
    { get; } = new ObservableCollection();

    public MainPageViewModel()
    {
        //-- Add some test data
        this.Items.Add( new ListItem( "A Cat" ) );
        this.Items.Add( new ListItem( "A Dog" ) );
        this.Items.Add( new ListItem( "A Mouse" ) );
        this.Items.Add( new ListItem( "A Frog" ) { Disabled = true } );

        //-- Just select the first item
        this.SelectedItem = this.Items[0];
    }
}

Main page:

public MainPage()
    {
        //-- Define the DataContext BEFORE the UI will be initialized ;)
        this.DataContext = new MainPageViewModel();

        InitializeComponent();

        //-- Never saw such code before -> just don't do that ;)

        //this.WhenActivated( d =>
        //{
        //    this.OneWayBind( ViewModel, vm => vm.Items, v => v.MyComboBox.ItemsSource )
        //                    .DisposeWith( d );

        //    this.Bind( ViewModel, vm => vm.SelectedItem, v => v.MyComboBox.SelectedItem )
        //                    .DisposeWith( d );

        //} );
    }

Xaml markup:

I hope this will satisfy your requirements. Have fun :)

WPF Toggle Combo Box Item Text Colour MVVM ReactiveUI

3 Answers

Xaml markup

Resources