How can you share a resource between ListBox item instances? - wpf

Question

We have a custom-rendered ListBox which maintains an instance of a StreamGeometry object that is based on its width. The control needs to then share that StreamGeometry instance with all of its items for rendering purposes.
Only way we can think is putting that StreamGeometry instance in the ViewModel for the ListBox, then binding to it in the individual DataTemplates, which just feels dirty to me considering that is a view-only thing and therefore shouldn't be in the ViewModel at all.
Note: We could also just store it via an attached property on the ListBox (or subclass the ListBox), but we're still left with binding of a view-only thing which seems wrong to me for something like this.
Any thoughts?

Answer 1

You can make the StreamGeometry a dependency property on your custom listview, then refer to it through Binding MyGeometry, RelativeSource={RelativeSource AncestorType=ListView}.
This way, there is no ViewModel involved.
Xaml:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
xmlns:s="clr-namespace:System;assembly=mscorlib"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<!-- default lsitviewitem style except for added path -->
<Style TargetType="{x:Type ListViewItem}">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="Padding" Value="2,0,0,0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ListViewItem}">
<Border x:Name="Bd" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
<StackPanel Orientation="Horizontal">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
<!-- added path-->
<Path Stretch="Uniform" Stroke="DarkBlue" Fill="DarkOrchid" Data="{Binding MyGeometry, RelativeSource={RelativeSource AncestorType=ListView}}" />
</StackPanel>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected" Value="true">
<Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsSelected" Value="true"/>
<Condition Property="Selector.IsSelectionActive" Value="false"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightBrushKey}}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightTextBrushKey}}"/>
</MultiTrigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid >
<local:CustomListView Margin="20" >
<local:CustomListView.Items>
<ListViewItem Content="ListViewItem1" />
<ListViewItem Content="ListViewItem2" />
<ListViewItem Content="ListViewItem3" />
</local:CustomListView.Items>
</local:CustomListView>
</Grid>
</Window>
CustomListView:
using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
namespace WpfApplication1
{
public class CustomListView : ListView
{
public StreamGeometry MyGeometry { get { return (StreamGeometry)GetValue(MyGeometryProperty); } set { SetValue(MyGeometryProperty, value); } }
public static readonly DependencyProperty MyGeometryProperty = DependencyProperty.Register("MyGeometry", typeof(StreamGeometry), typeof(CustomListView), new PropertyMetadata(null));
protected override void OnRender(DrawingContext drawingContext)
{
StreamGeometry geometry = new StreamGeometry(); // directly opening MyGeometry results in "must have isfrozen set to false to modify" error
using (StreamGeometryContext context = geometry.Open())
{
Point p1 = new Point(this.ActualWidth * (2d / 5d), 0);
Point p2 = new Point(this.ActualWidth / 2d, -10);
Point p3 = new Point(this.ActualWidth * (3d / 5d), 0);
context.BeginFigure(p1, true, true);
List<Point> points = new List<Point>() { p2, p3 };
context.PolyLineTo(points, true, true);
}
drawingContext.DrawGeometry(Brushes.DarkOrchid, new Pen(Brushes.DarkBlue, 1), geometry);
this.MyGeometry = geometry;
base.OnRender(drawingContext);
}
}
}