Pseudo Classes

Pseudo classes in Avalonia, similar to those in CSS, are keywords added to a selector that specify a special state of the selected element(s). They can be used to style controls differently based on certain conditions. For example, a button could be styled differently when it's being pressed, or a TextBox can have a different style when it is disabled.

Avalonia supports a number of built-in pseudo classes, and controls can define their own.

Usage

To use pseudo classes, you append a colon (:) and the pseudo class to the selector. Here's an example:

<Button Content="Click Me!">
  <Button.Styles>
    <Style Selector="Button:pointerover">
      <Setter Property="Background" Value="Red"/>
    </Style>
  </Button.Styles>
</Button>

In this example, the button's background will change to red when the pointer is over it, thanks to the pointerover pseudo class.

## Built-in Pseudo Classes Some of the built-in pseudo classes include:

• :pointerover: The mouse pointer is over the control.
• :pressed: The control is being pressed.
• :disabled: The control is disabled.
• :focus: The control has input focus.
• :watermark: For TextBox control, when it's displaying a watermark.
• :checked: For checkable controls, like CheckBox or MenuItem, when it's checked.
• :indeterminate: For controls like CheckBox, when it's in the indeterminate state.
• :valid: For input controls, when the input is valid.
• :invalid: For input controls, when the input is invalid.

You can combine pseudo classes with type selectors and class selectors to create a wide range of effects.

Custom Pseudo Classes

Controls can define their own pseudo classes for specific behaviors. To define a pseudo class, a control typically creates a static readonly field of type PseudoClass and calls PseudoClasses.Set() to enable the pseudo class and PseudoClasses.Remove() to disable it.

For example, a custom :custom pseudo class might be defined as follows:

public static readonly PseudoClass CustomPseudoClass = PseudoClass.Parse(":custom");

// to enable
PseudoClasses.Set(CustomPseudoClass);

// to disable
PseudoClasses.Remove(CustomPseudoClass);

This allows developers to add more expressiveness and control to their styles, tailoring styles to very specific control states.