Avalonia/docs/spec/styles.md

Styling in Avalonia

The main difference between Avalonia and existing XAML toolkits such as WPF and UWP is in styling. Styling in Avalonia uses a CSS-like system that aims to be more powerful and flexible than existing XAML styling systems. For convenience for the rest of this document we'll refer to existing XAML toolkit's styling as "WPF styling" as that's where it originated.

Basics

• Styles are defined on the Control.Styles collection (as opposed to in ResourceDictionaries in WPF).
• Styles have a Selector and a collection of Setters
• Selector works like a CSS selector.
• Setters function like WPF's setters.
• Styles are applied to a control and all its descendants, depending on whether the selector matches.

Simple example

Make all Buttons in a StackPanel have a blue Background:

<StackPanel>
  <StackPanel.Styles>
    <Style Selector="Button">
      <Setter Property="Button.Background" Value="Blue"/>
    </Style>
  </StackPanel.Styles>
  <Button>I will have a blue background.</Button>
</StackPanel>

This is very similar to WPF, except TargetType is replaced by Selector.

Note that currently (as of Alpha 2) you always need to specify the fully qualified property name (i.e. Button.Background instead of simply Background). This restriction will be lifted in future.

Style Classes

As in CSS, controls can be given style classes which can be used in selectors:

<StackPanel>
  <StackPanel.Styles>
    <Style Selector="Button.blue">
      <Setter Property="Button.Background" Value="Blue"/>
    </Style>
  </StackPanel.Styles>
  <Button Classes="blue">I will have a blue background.</Button>
  <Button>I will not.</Button>
</StackPanel>

Each control can be given 0 or more style classes. This is different to WPF where only a single style can be applied to a control: in Avalonia any number of separate styles can be applied to a control. If more than one style affects a particular property, the style closest to the control will take precedence.

Style classes can also be manipulated in code using the Classes collection:

control.Classes.Add("blue");
control.Classes.Remove("red");

Pseudoclasses

Also as in CSS, controls can have pseudoclasses; these are classes that are defined by the control itself rather than by the user. Pseudoclasses start with a : character.

One example of a pseudoclass is the :pointerover pseudoclass (:hover in CSS - we may change to that in future).

Pseudoclasses provide the functionality of Triggers in WPF and VisualStateManager in UWP:

<StackPanel>
  <StackPanel.Styles>
    <Style Selector="Button:pointerover">
      <Setter Property="Button.Foreground" Value="Red"/>
    </Style>
  </StackPanel.Styles>
  <Button>I will have red text when hovered.</Button>
</StackPanel>

Other pseudoclasses include :focus, :disabled, :pressed for buttons, :checked for checkboxes etc.

Named Controls

Named controls can be selected using # as in CSS, e.g. Button#Name.

Children

As with CSS, you can select children and descendants:

• StackPanel > Button#Foo selects a Button named "Foo" that is the child of a StackPanel.
• StackPanel Button.foo selects all Buttons with the foo class that are descendants of a StackPanel.

Templates

You can select controls in the template of a lookless control by using the /template/ selector, so Button /template/ Border#outline selects Border controls named "outline" in the template of a Button.