Can't use button property in trigger in WPF - wpf

Question

I have button with custom content (a red circle) defined inside a user control and I want the circle to change to gray if the button is disabled.
I tried the following and it does not compile
Error: 'IsEnabled' member is not valid because it does not have a qualifying type name.
Can any explain what I am doing wrong here?
<Button>
<Button.Content>
<Grid>
<Ellipse x:Name="circle" Width="20" Height="20" Fill="Red"/>
</Grid>
</Button.Content>
<Button.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="circle" Property="Fill" Value="Gray"/>
</Trigger>
</Button.Triggers>
</Button>

Answer 1

IsEnabled is a DependecyProperty that's defined on a class that Button inherits from; XAML requires that you explicitly specify the owning type. Try Trigger Property="UIElement.IsEnabled" instead.

Answer 2

Here's a way to do it:
<Button Name="btn" >
<Ellipse Height="20" Width="20">
<Ellipse.Style>
<Style TargetType="Ellipse">
<Setter Property="Fill" Value="Red" />
<!-- Here is the 'normal' content -->
<Style.Triggers>
<DataTrigger Binding="{Binding IsEnabled, ElementName=btn}" Value="False">
<Setter Property="Fill" Value="Blue" />
<!-- Here is the 'override' content -->
</DataTrigger>
</Style.Triggers>
</Style>
</Ellipse.Style>
</Ellipse>
</Button>
DataTriggers only work in styles, so we needed to target the actual element that was receiving the style but bind to another element in the control.