WPF - ItemTemplate not acting as expected

问题:

I have a UserControl which I'm using to display a list of UIElements. The control consists of a single ItemsControl with it's ItemPanelTemplate switched for a horizontal StackPanel, its ItemsSource bound to a DependencyProperty exposed by the UserControl and its ItemTemplate set in the UserControl.Resources.

Everything works fine except the ItemTemplate never get's applied and I can't see why. The full source is below.

UserControl.xaml -

<UserControl x:Name="UC" x:FieldModifier="private" x:Class="ContentSliderControl.ContentSlider"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<UserControl.Resources>

    <DataTemplate x:Key="pageTemplate">
        <Border CornerRadius="10" Padding="5" Height="200" Width="200" Background="#333">
            <ContentControl Content="{Binding}"/>
        </Border>
    </DataTemplate>

    <ItemsPanelTemplate x:Key="template">
        <StackPanel IsItemsHost="True"
            Orientation="Horizontal"
            ScrollViewer.HorizontalScrollBarVisibility="Disabled"
            ScrollViewer.VerticalScrollBarVisibility="Disabled"/>
    </ItemsPanelTemplate>
</UserControl.Resources>

<ItemsControl ItemsPanel="{StaticResource template}" 
              ItemTemplate="{StaticResource pageTemplate}" 
              ItemsSource="{Binding ElementName=UC,Path=Pages}"/>

UserControl.xaml.cs -

[ContentProperty("Pages")]
public partial class ContentSlider : UserControl
{


    public List<UIElement> Pages
    {
        get { return (List<UIElement>)GetValue(PagesProperty); }
        //set { SetValue(PagesProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Pages.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty PagesProperty =
        DependencyProperty.Register("Pages", typeof(List<UIElement>), typeof(ContentSlider), new UIPropertyMetadata(null));



    public ContentSlider()
    {
        InitializeComponent();
    }
}

}

I consume the control in my main window like this -

    <slider:ContentSlider >
    <slider:ContentSlider.Pages>
        <Button>1</Button>
        <Button>2</Button>
        <Button>3</Button>
        <Button>4</Button>
    </slider:ContentSlider.Pages>
</slider:ContentSlider>

The buttons appear fine but not inside the 200px square border.

Any help would be greatlly appriciated. Thanks.

回答1:

It's because it's a list of UIElement, the item template is only applied if the items can't be displayed directly.

回答2:

Nir is correct, ItemsControl will add item directly to its Panel if they are UIElements. I couldn't find any mention of this behavior in MSDN, but Dr. WPF mentions it in his article on item containers:

If a UIElement is added to the Items collection of an explicit ItemsControl instance (as opposed to an instance of a derived class like ListBox), it will become a direct child of the items panel. If a non-UIElement is added, it will be wrapped within a ContentPresenter.

Your solution is probably to use a ListBox instead, and set ItemContainerStyle to a new Style for ListBoxItem, and in that style, use a ControlTemplate with your Border in it.

回答3:

Nir is right, this custom ItemsControl implementation will solve the issue and let use your own ItemTemplate:

public class ItemsControlForUIElement : ItemsControl
{
   protected override DependencyObject GetContainerForItemOverride()
   {
       return new ContentPresenter();
   }
   protected override bool IsItemItsOwnContainerOverride(object item)
   {
       return false;
   }
}

回答4:

Robert Macnee nailed the reason on the head. His solution involves using the control template which might be overkill for a given scenario. Alternatively, use a ListBox - set the ItemContainerStyle to a new Style for ListBoxItem, and in that style, set the ContentTemplate to the DataTemplate that you wanted to use in the ListBox ItemTemplate.

回答5:

If you set the DataType property on the DataTemplate it would start working.