Binding custom dependency property to custom WPF style

Question

I have an issue when designing a inherited Expander. My intention is to have a progress bar behind the toggle button and text in the default Expander header.

I have

Answer 1

In the style, we can use a standard Binding with a RelativeSource to set up the property.

<ProgressBar Name="ProgressBar"
             Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Progress}"
             Minimum="0"
             Maximum="100" />

Then, in the window we just add Progress="50" or a binding to somewhere else.

You will also need to set the Button's background to transparent, or change some manner of the layout, in order to see it.