WPF Trigger and Binding problem

I am creating a button with a simple Polyline which I want to change color when the button is Disabled (btnUp.IsEnabled = false).

I tried this:

<RepeatButton x:Name="btnUp" Foreground="Green">
  <RepeatButton.Style>
    <开发者_如何学Go;Style TargetType="{x:Type RepeatButton}">
      <Setter Property="Content">
        <Setter.Value>
          <Polyline x:Name="arrowUp" Points="0,2 3,0 6,2" Stroke="{Binding RelativeSource={RelativeSource AncestorType=RepeatButton}, Path=Foreground, Mode=TwoWay}" StrokeThickness="2"/>
        </Setter.Value>
      </Setter>
      <Style.Triggers>
        <Trigger Property="IsEnabled" Value="False">
          <Setter Property="Foreground" Value="#FFFFFFFF"/>
        </Trigger>
      </Style.Triggers>
    </Style>
  </RepeatButton.Style>
</RepeatButton>

But the Polyline still has the same (Green) color when the button is disabled. Even though I did expect it to be white because of the databinding between the button.foreground and polyline.stroke.

However if I change the trigger to this it works (the button is collapsed):

<Trigger Property="IsEnabled" Value="False">
  <Setter Property="Visibility" Value="Collapsed"/>
</Trigger>

I found the answer on my own. I moved the Foreground="Green" to a separate setter, <Setter Property="Foreground" Value="#FF383838"/> inside the <Style>, then it worked. I'm not sure why though.

Here's the complete solution:

<RepeatButton x:Name="btnUp">
  <RepeatButton.Style>
    <Style TargetType="{x:Type RepeatButton}">
      <Setter Property="Foreground" Value="Green"/>
      <Setter Property="Content">
        <Setter.Value>
          <Polyline x:Name="arrowUp" Points="0,2 3,0 6,2" Stroke="{Binding RelativeSource={RelativeSource AncestorType=RepeatButton}, Path=Foreground, Mode=TwoWay}" StrokeThickness="2"/>
        </Setter.Value>
      </Setter>
      <Style.Triggers>
        <Trigger Property="IsEnabled" Value="False">
          <Setter Property="Foreground" Value="#FFFFFFFF"/>
        </Trigger>
      </Style.Triggers>
    </Style>
  </RepeatButton.Style>
</RepeatButton>