显示ItemsControl.ItemsSource是否为null

Question

Greetings,

I have a ItemsControl which template I changed to show a RadioButton for every object in the binded ItemsSource.

However the ItemsSource can be empty and when it is empty I'd like to show a default value. Something like "The binded list contains no items for you to select"...

One way I thought of is to set the ItemsControl.Visibility to Collapsed and have a TextBlock.Vsibility to Visible which shows the text.. But this would include a lot more data.

Is it possible to show a default value if the ItemsControl.ItemsSource is null?

Answer 1

After creating this simple converter:

public class AnyItemsToVisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var collection = value as IEnumerable;
        if (collection == null)
            return Visibility.Collapsed;

        return collection.OfType<object>().Any() ? Visibility.Collapsed : Visibility.Visible;
    }

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

You can override the ItemsControl Template to suppor this using RelativeSource Binding.

<UserControl x:Class="SilverlightApplication1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:SilverlightApplication1">
    <UserControl.Resources>
        <local:AnyItemsToVisibilityConverter x:Key="AnyItemsToVisibilityConverter" />
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" Background="White">
        <ItemsControl>
            <ItemsControl.Template>
                <ControlTemplate TargetType="ItemsControl">
                    <Grid>
                        <TextBlock Text="No Items to Display" 
Visibility="{Binding Items, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource AnyItemsToVisibilityConverter}}" />
                        <ItemsPresenter />
                    </Grid>
                </ControlTemplate>     
            </ItemsControl.Template>
        </ItemsControl>
    </Grid>
</UserControl>

Answer 2

如果我理解正确，我认为您可以通过创建IValueConverter来解决您的问题。

Answer 3

You should not create a Converter which shows wether your List is empty or not. It is better when your XAML,Converter and data source are totally independent items. Isn't MVVM about loose coupling?

OK, code behind is evil. Thanks for pointing that out. I corrected the source code, it is totally declarative style now:

       <ControlTemplate x:Key="ListBoxTemplate" TargetType="ListBox">
            <StackPanel>
            <ItemsPresenter  
                 Visibility="{Binding Path=NotEmpty,
                Converter={StaticResource BoolToVisibilityConverter}}">
            </ItemsPresenter>
                <TextBlock Text="No items to select from" 
                 Visibility="{Binding Path=Empty,
                 Converter={StaticResource BoolToVisibilityConverter}}"/>
            </StackPanel>
        </ControlTemplate>

        <Style x:Key="ListBoxStyle2" TargetType="ListBox"  >
            <Setter Property="Template" Value="{StaticResource ListBoxTemplate}">
            </Setter>
            <Setter Property="ItemsPanel">
                <Setter.Value>
                    <ItemsPanelTemplate>
                        <StackPanel />
                    </ItemsPanelTemplate>
                </Setter.Value>
            </Setter>
        </Style>

Answer 4

One thing you could do is that after checking ItemsControl.ItemsSource is null, you could add a single item "The binded list contains no items for you to select" . I hope this would server your purpose.