Derived dimensions

A Thickness or a CornerRadius declared in a ResourceDictionary is baked at parse time. Its Left/Top/Right/Bottom (or TopLeft/TopRight/…) are plain CLR properties, not dependency properties, so they can only be fed with StaticResource and never follow a later change of the scalar they were built from:

<!-- Baked at load: editing Dimensions.Thickness at runtime does nothing here -->
<Thickness x:Key="MenuBorderThickness"
           Right="{StaticResource {x:Static joufflu:Dimensions.Thickness}}" />

Derive.BorderThickness, Derive.CornerRadius and Derive.Margin build the value on the element instead, from a real DynamicResource. A scalar edited at runtime — by the theme customizer, for instance — flows straight through, and no derived resource key has to be declared or re-pushed by hand.

Factors

Each of the three properties has a matching factor, a Thickness (or a CornerRadius) whose components multiply the derived value side by side:

Factor component	Result
0	The side or corner is dropped
1	It is kept as is
anything else	It is scaled — 2 for a doubled margin, 0.5 for a halved radius

The default is 1,1,1,1, so the derived value is applied whole.

Derive.BorderThickness

Point Derive.BorderThickness at a resource key and pick the sides with Derive.BorderThicknessFactor, in the usual Left,Top,Right,Bottom order.

<!-- Right edge only, following Dimensions.Thickness live -->
<Border toolkit:Derive.BorderThickness="{x:Static joufflu:Dimensions.Thickness}"
        toolkit:Derive.BorderThicknessFactor="0,0,1,0" />

<!-- Open at the bottom, with a doubled top edge -->
<Border toolkit:Derive.BorderThickness="{x:Static joufflu:Dimensions.Thickness}"
        toolkit:Derive.BorderThicknessFactor="1,2,1,0" />

Both properties work in a Style setter, so a control can derive its own border without a keyed Thickness:

<Style TargetType="{x:Type nav:NavigationMenu}">
    <Setter Property="toolkit:Derive.BorderThickness" Value="{x:Static joufflu:Dimensions.Thickness}" />
    <Setter Property="toolkit:Derive.BorderThicknessFactor" Value="0,0,1,0" />
</Style>

In a style, the fed property stops being overridable. The derived value is pushed with SetCurrentValue, which outranks a style setter — and even a local value. So do not also set the property it feeds (here BorderThickness) in the same style, move any Style.Triggers that change it onto the Derive property, and expect that a consumer can no longer override it. When the fed property has to stay overridable — a control’s Padding, say — give it a plain keyed default instead of deriving it.

Derive.CornerRadius

Same shape, with Derive.CornerRadiusFactor in the TopLeft,TopRight,BottomRight,BottomLeft order.

<!-- Top corners only, matching the border it sits in -->
<Border toolkit:Derive.CornerRadius="{x:Static joufflu:Dimensions.Radius}"
        toolkit:Derive.CornerRadiusFactor="1,1,0,0" />

Derive.Margin

Same shape again, on any FrameworkElement, with Derive.MarginFactor.

<!-- Spaced everywhere but the top -->
<Border toolkit:Derive.Margin="{x:Static joufflu:Dimensions.Spacing}"
        toolkit:Derive.MarginFactor="1,0,1,1" />

Notes

• The source resource may be a double (the usual case, spread over every side or corner before scaling) or an already built Thickness / CornerRadius, whose own sides are then scaled.
• Derive.BorderThickness applies to Border and to any Control; Derive.CornerRadius applies to Border; Derive.Margin to any FrameworkElement. Anything else throws.
• The derived value is written with SetCurrentValue, which outranks a style setter : a style deriving a value should not also set the property it feeds, and the Style.Triggers meant to change it have to move to the Derive property instead. An animation or a template trigger still takes over. So Derive fits a value the control owns — a Border inside a template — not one a consumer overrides : a property like a text input’s Padding carries a plain keyed default (Dimensions.InputPadding*) instead, which a local value or a style still beats.

Snippets use these XML namespaces:

xmlns:joufflu="clr-namespace:Joufflu;assembly=Joufflu"
xmlns:toolkit="clr-namespace:Joufflu.Toolkit;assembly=Joufflu"