How to add a trigger to a WPF custom control without overriding the existing style?

Question

I am creating a simple custom control extending from toggle button that allows the user to specify checked and unchecked content directly in XAML. It works well but it is based on a trigger, and I don't know how to define the trigger except in a style. If I define the style, then I lose anything set outside of the custom control.

What I would like to be able to do is just append this trigger to any existing style set elsewhere on the control.

Here's the XAML for the style/trigger.

<ToggleButton.Style>
    <Style TargetType="{x:Type ToggleButton}" BasedOn="{StaticResource {x:Type ToggleButton}}">
        <Setter Property="Content" Value="{Binding RelativeSource={RelativeSource Self}, Path=UncheckedContent}" />
        <Style.Triggers>
            <Trigger Property="IsChecked" Value="True">
                <Setter Property="Content"
                        Value="{Binding RelativeSource={RelativeSource Self}, Path=CheckedContent}" />
            </Trigger>
        </Style.Triggers>
    </Style>
</ToggleButton.Style>

I tried inheriting the style via the BasedOn with a default type but it won't work if the custom control has an explicit style set by its parent. I also considered EventTriggers but I do not believe there would be an event to initialize the control.

Thanks for any help anyone can offer. :)

Answer 1

Just to clear things up on the terminology here: A user control is a control that derives from the UserControl class. If I understood you right you derived from ToggleButton to add the UncheckedContent and CheckedContent properties. In that case you have created a custom control. Its always easier to follow if we agree on common terminology :)

As far as I know you can not do such a generic style inheritance in xaml. You always have to specifiy explictly what style a anothet style is based upon. Your style can either be based on the default style for ToggleButton or on a specific other style. If you can't build a style inheritance chain that respects that, this approach won't work.

But since you have a custom control, couldn't you write a default style for it that is based on the default toggle button style like this?

<Style TargetType="{x:Type CustomToggleButton}" BasedOn="{StaticResource {x:Type ToggleButton}}">

Then whenever you apply an explicit style to a toggle button you would specify that it is based on the default toggle button style.

Also you could write a (default) control template for your new toggle button in Themes\Generic.xaml that contains the above triggers. In blend you can get a copy of the default template for toggle button ("Edit Template"->"Edit a Copy") so you can make sure that your toggle button looks exactly like the normal toggle button. Then incorporate the triggers above into that template.

BTW: you do not have to create a new control just to add new properties. You can add new properties to an exisiting control using attached properties. They can be used from XAML just like normal properties.