Question

I’m trying to extend an Expander’s header (the ToggleButton part) to allow for some more custom features. Being rather new to extending WPF controls using ControlTemplates, I have already accepted by now, that I will have to completely replace the Expander’s template by my own (without being able to reuse originally defined components, or even styles, which makes this quite a bit annoying in my opinion – but that’s not what the question is about).

However, I wanted to introduce some properties which change the way the component looks and works. So I created a new subtype of Expander which simply defines the new dependency properties I want to use. But I noticed that, given that I want to use those properties within the template for the ToggleButton, which is nested inside of the one for the Expander, I cannot use those properties like that. So I had to subtype ToggleButton too, define the dependency properties for that type as well, and make the Expander’s template pass the values through to the ToggleButton template.

So I ended up with two new types in the namespace (nesting the new ToggleButton type inside of the Expander subtype didn’t work?), a complete copy of the Expander’s template with only a single line changed (where the ToggleButton was inserted, passing through the new properties), and then the actual desired changes for the ToggleButton template.

And here comes the question: Is there any way to make this less complicated? To reduce the amount of work that needs to be done just for extending a subcomponent of another’s template? Or just a way to use the parent’s (Expander) properties inside of the ToggleButton template without having to subtype that one as well?

Answer 1

I ended up using what jberger suggested in the comments:

Using the following binding expression I did not need to duplicate the DependencyProperties into another component for the ToggleButton, but could just access the Expander’s properties inside the nested template:

{Binding CustomProperty, RelativeSource={RelativeSource
    FindAncestor, AncestorType=local:CustomExpander}}

Sadly, but as expected, one apparently cannot reuse template parts from the default. However in this case I could leave out parts from the original template that I didn’t need in my copy. For example the different styles for non-default expanding (i.e. other than Top), as my expander is specialized to that layout.

This however rendered the ExpandDirection property useless, but I personally can accept that. It might lead to a follow-up question though; if one should rather build the component from scratch (based on Expander’s base, instead of Expander itself).