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Q&A

How to dynamically change panel of ItemsControl?

+4
−0

Let's say I have some collection of data. My goal is to provide different kind of view of ItemsControl depending on user`s preference.

For simplicity, we can assume that user can select only between two states - horizontal & vertical.

MCVE

public enum ViewType { H, V }

public class MainWindowViewModel : ViewModelBase
{
    private ViewType _viewType;

    public MainWindowViewModel()
    {
        Values = Enumerable.Range(0, 20).Select(v => v.ToString());
        ViewTypes = new[] { ViewType.H, ViewType.V };
        ViewType = ViewTypes[0];
    }

    public ViewType ViewType
    {
        get => _viewType;
        set => this.RaiseAndSetIfChanged(ref _viewType, value);
    }
    public IEnumerable<string> Values { get; }
    public ViewType[] ViewTypes { get; }
}
  <Grid RowDefinitions="*,Auto">
    <ItemsControl Items="{Binding Values}"
                  Grid.Row="0" />
    <ComboBox Items="{Binding ViewTypes}"
              Grid.Row="1"
              SelectedItem="{Binding ViewType}"/>
  </Grid>

But I wasn't able to find a way to define selector for ItemsPanelTemplate.


When I tried to use converter:

ItemsPanel="{Binding ViewType, Converter={StaticResource ResourceKey=converter}}"
public class ViewTypePanelConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var template = new ItemsPanelTemplate()
        {
            Content = new Func<IServiceProvider, ControlTemplateResult>( _ => {
                var content =
                    value switch
                    {
                        ViewType.V => new StackPanel { Orientation = Orientation.Vertical },
                        ViewType.H => new StackPanel { Orientation = Orientation.Horizontal },
                        _ => throw new NotImplementedException(),
                    };
                return new ControlTemplateResult(content, new NameScope());
            })
        };
        return template;
    }

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

it has zero effect.


This Q is inspired by Use DataTemplate for ItemsControl.ItemsPanel #7268 discussion from Avalonia project repo.

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as (2 comments)

1 answer

+2
−0

This isn't the cleanest approach but at least it works.

One can define two different templates (each of them contains ItemsControl with single difference in ItemsPanelTemplate) and switch between them as needed with a simple "selector":

C#:

public class ViewTypeTemplateSelector : IDataTemplate
{
    [Content]
    public Dictionary<ViewType, IDataTemplate> Templates { get; }
        = new Dictionary<ViewType, IDataTemplate>();
    public IControl Build(object data) => Templates[(ViewType)data].Build(data);
    public bool Match(object data) => data is ViewType;
}

xaml:

    <ContentControl Grid.Row="0" Content="{Binding ViewType}">
      <ContentControl.Resources>
          <DataTemplate x:Key="itemTemplate">
            <TextBlock Text="{Binding}" Margin="2" Padding="2"/>
          </DataTemplate>
        </ContentControl.Resources>
      <ContentControl.DataTemplates>
        <views:ViewTypeTemplateSelector>
          <DataTemplate x:Key="H">
            <ItemsControl ItemTemplate="{StaticResource ResourceKey=itemTemplate}" Items="{Binding Values}" DataContext="{Binding $parent[Window].DataContext}">
              <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                  <StackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
              </ItemsControl.ItemsPanel>
            </ItemsControl>
          </DataTemplate>
          <DataTemplate x:Key="V">
            <ItemsControl ItemTemplate="{StaticResource ResourceKey=itemTemplate}" Items="{Binding Values}" DataContext="{Binding $parent[Window].DataContext}">
              <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                  <StackPanel Orientation="Vertical"/>
                </ItemsPanelTemplate>
              </ItemsControl.ItemsPanel>
            </ItemsControl>
          </DataTemplate>
        </views:ViewTypeTemplateSelector>
      </ContentControl.DataTemplates>
    </ContentControl>
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