How to grow/shrink a TextBlock (Font Size) to the available space in WPF?

I've seen this question asked a lot, however, to the opposite of what I'm looking for. While other people want a control to size itself based on the开发者_开发百科 size of text, I'm trying to figure out is if there is a way to grow the size of text to the amount of space available.

Take the idea of a digital clock window and you want the numbers stating the time to grow (or shrink) based on the size of the window the clock is in. If there isn't a way to automatically do this any pointers to a programmatic way I can get this accomplished?

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The WPF Viewbox control will grow / shrink its contents to the available space:

http://www.wpftutorial.net/ViewBox.html

Just place your TextBlock within a ViewBox:

<Viewbox Stretch="Uniform" Width="50" Height="50">
    <TextBlock Text="Test" />
</Viewbox>

Of course, your Viewbox is typically scaled by its container, but hopefully you get the idea!