Change common color in ControlTemplate

Question

I have a ControlTemplate which uses the same color in multiple elements. On certain triggers (e.g. OnMouseOver) I'd like to change that color. As far as I can see I have to define a setter for every element to change its color. Is there a way to reference a shared resource in the template that all contained elements can access, and which can be changed by a trigger, so I don't have to address each and every element?

Here's an (made up) example:

<ControlTemplate x:Key="myTemplate" TargetType="{x:Type Button}">
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition/>
      <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <Ellipse Fill="red" Grid.Column="0"/>
    <Ellipse Fill="red" Grid.Column="1"/>
    <ContentPresenter Grid.ColumnSpan="2" VerticalAlignment="Center"/>
 </Grid>
</ControlTemplate>

When the control is disabled, I want the ellipses to be grey, without setting both of them explicitly, e.g. I don't want to write

<Trigger Property="IsEnabled" Value="False">
  <Setter TargetName="_ellipse1" Property="Fill" Value="Grey"/>
  <Setter TargetName="_ellipse2" Property="Fill" Value="Grey"/>
</Trigger>

but set the color of both ellipses with just one setter.

Answer 1

I think the best way to do this is with a value converter. Then you can avoid a messy trigger altogether. Here is your example, but with a converter added.

<Window.Resources>
  <local:EnabledToColorConverter x:Key="enabledToColorConverter"/>
  <ControlTemplate x:Key="myTemplate" TargetType="{x:Type Button}">
    <Grid>
      <Grid.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition/>
      </Grid.ColumnDefinitions>
      <Ellipse Name="_ellipse1" Fill="{TemplateBinding IsEnabled, Converter={StaticResource enabledToColorConverter}}" Grid.Column="0"/>
      <Ellipse Name="_ellipse2" Fill="{TemplateBinding IsEnabled, Converter={StaticResource enabledToColorConverter}}" Grid.Column="1"/>
      <ContentPresenter Grid.ColumnSpan="2" VerticalAlignment="Center"/>
    </Grid>
  </ControlTemplate>
</Window.Resources>

<StackPanel>
  <Button Template="{StaticResource myTemplate}">Enabled Button</Button>
  <Button Template="{StaticResource myTemplate}" IsEnabled="False">Disabled Button</Button>
</StackPanel>

And here is the converter:

public class EnabledToColorConverter : IValueConverter
{
 public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
 {
  bool isEnabled = (bool)value;
  return isEnabled ?
   Brushes.Red :
   Brushes.Gray;
 }

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

Answer 2

Place the trigger on a style for the ellipses (ellipsi?) instead of the button. IsEnabled will propagate down if you set IsEnabled = false on the Button.

<ControlTemplate x:Key="myTemplate" TargetType="{x:Type Button}">
    <ControlTemplate.Resources>
        <Style TargetType="{x:Type Ellipse}">
            <Setter Property="Fill" Value="Red" />
            <Style.Triggers>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter Property="Fill" Value="Gray" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </ControlTemplate.Resources>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Ellipse Grid.Column="0"/>
        <Ellipse Grid.Column="1"/>
        <ContentPresenter Grid.ColumnSpan="2" VerticalAlignment="Center"/>
    </Grid>
</ControlTemplate>