I'm working on a custom WPF control. The main purpose of this control is to visualize thousands of graphical primitives in a scrollable area. The core part of the control's template looks like this:
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:ItemVisualizer}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<local:ItemAreaElement Grid.Row="0" Grid.Column="0" x:Name="PART_ItemArea" />
<ScrollBar Grid.Row="0" Grid.Column="1" x:Name="PART_ScrollBarVert" Orientation="Vertical" Maximum="100" />
<ScrollBar Grid.Row="1" Grid.Column="0" x:Name="PART_ScrollBarHorz" Orientation="Horizontal" Maximum="100" />
<Rectangle Grid.Row="1" Grid.Column="1" x:Name="PART_SizeGrip" Focusable="False" Fill="#F0F0F0" />
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
The ItemAreaElement is responsible for drawing the items. For simplicity, we can think that its core part looks like this:
class ItemAreaElement : FrameworkElement
{
protected override void OnRender(DrawingContext drawingContext)
{
base.OnRender(drawingContext);
for (int i = 0; i < _data.ItemCount; i++)
{
drawingContext.DrawLine(_penLine, new Point(0, i * 10), new Point(100, i * 10));
}
}
}
I need to repaint the ItemAreaElement every time when a related property in the whole ItemVisualizer control changes. However, I didn't find a way to do that in WPF. The well know trick with the Dispatcher object does not work in my case:
private static void OnItemCountPropertyChanged(DependencyObject source,
DependencyPropertyChangedEventArgs e)
{
ItemVisualizer vis = (ItemVisualizer)source;
vis._itemArea.Dispatcher.Invoke(delegate { }, DispatcherPriority.Render);
}
, where _itemArea is a local reference to the ItemAreaElement got in OnApplyTemplate():
public override void OnApplyTemplate()
{
base.OnApplyTemplate();
if (this.Template != null)
{
_itemArea = this.Template.FindName("PART_ItemArea", this) as ItemAreaElement;
_itemArea.Grid = this;
}
}
Are there other ways to force an update of the UIElement in my construction? Or maybe, I need to redesign the whole control to make it possible?
It should usually be sufficient to specify FrameworkPropertyMetadataOptions.AffectsRender
on registration of the ItemCount
property:
public static readonly DependencyProperty ItemCountProperty =
DependencyProperty.Register(
"ItemCount", typeof(int), typeof(ItemVisualizer),
new FrameworkPropertyMetadata(FrameworkPropertyMetadataOptions.AffectsRender));
If that doesn't help, you could force a redraw by calling InvalidVisual()
on the ItemAreaElement:
var vis = (ItemVisualizer)source;
vis._itemArea.InvalidVisual();