Made PART_Track optional for Slider

[TomEdwardsEnscape]

This part was made required in #14180, and this causes a build error in our product when using 11.1.0-beta1. This PR reverts the changes and also ensures that the _track field is not accessed when no track exists in the control template.

Our use case is that we have specialised slider controls, the templates of which do not contain tracks. One contains a nested slider (plus various other controls), and it is this nested slider which has a track and handles input. The other does not contain a track at all, and we handle all input ourselves.

There is also a workaround for this issue. Add the following XAML to the offending template:

<Track Name="PART_Track" IsVisible="False"/>

What is the current behavior?

Avalonia error AVLN2205: Required template part with x:Name 'PART_Track' must be defined on 'SemiCircularSlider' ControlTemplate.

Breaking changes

None

Obsoletions / Deprecations

None

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0046184-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[timunie]

Hi @TomEdwardsEnscape I can't wrap my head around the usecase here. Could you file a scetch of what you want to achieve?

Thank you

[TomEdwardsEnscape]

Greatly simplified XAML:

<ControlTemplate TargetType="SettingSlider">
	<Grid>
		<Label Content="{TemplateBinding Header}"/>
		<Button Command="Reset" />
		<NumericUpDown x:Name="PART_TextInput" />
		<Slider Name="PART_Slider" />
	</Grid>
</ControlTemplate>

We then bind the properties of PART_Slider when the template is applied. This includes value converters for things like logarithmic value ranges.

In the second use case, the template contains nothing but text and a canvas on which we draw various paths and ellipses. Everything else is done in C# in response to mouse events.

Both cases are part of our control library. We want the controls to be sliders and inherit all of that type's public properties.