The most basic building block in UI Toolkit is a visual element. The visual elements are ordered into a hierarchy tree with parent-child relationships. This is called the visual tree.

The diagram below displays a simplified example of the visual tree, and the rendered result in UI Toolkit.

The Root in the diagram represents the

EditorWindow.rootVisualElement

in the Editor UI and the

UIDocument.rootVisualElement

in a runtime UI.

The

VisualElement

class is the base for all nodes in the visual tree. The base class contains properties such as styles, layout data, and event handlers. Visual elements can have children and descendant visual elements. For example, in the diagram above, the first visual element has three child visual elements: , , and .

You can customize the appearance of visual elements through inline styles and stylesheets. You can also use event callbacks to modify the behavior of a visual element.

VisualElement

derives into subclasses that define additional behavior and functionality, such as controls. UI Toolkit includes a variety of built-in controls with specialized behavior. A control is an element of a graphical user interface. For example, the following items are available as built-in controls:

A control includes the visual of the control, and the scripted logic to operate and interact with the control. It can consist of a single visual element, or a combination of multiple visual elements. For example, the Toggle control contains three elements:

The implementation of the Toggle control defines the behavior of the control. It has an internal value of whether the toggle state is true or false. This logic alternates the visibility of the checkmark image when the value changes.

You can also combine visual elements together and modify their behavior to create custom controls.

To use a control in a UI, add it to the visual tree.