The visual tree holds all the visual elements in a window. It is an object graph made of lightweight nodes refered to as visual elements.
These nodes are allocated on the C# heap, either manually or by loading UXML assets from a UXML template file.
Each node contains the layout information, its drawing and redrawing options, and how the node responds to events.
VisualElement
is the common base class for all nodes in the visual tree. The VisualElement
base class contains properties for styles, layout data, local transformations, event handlers, and so on.
VisualElement
has several subclasses that define additional behaviour and functionality, including specialized controls. VisualElement
may have child elements.
You are not required to derive from the VisualElement
base class to work with UIElements. You can customize the look and behavior of a VisualElement
through stylesheets and event callbacks.
The root object of the visual tree is referred to as the panel. A new element is ignored until it is connected to the panel. You can add elements to an existing element to attach your user interface to the panel.
To verify if a VisualElement
is connected to a panel, you can test the panel
property of this element. When the visual element is not connected, the test returns null
.
You add a new element to the tree with the rootVisualElement
element of a container object in the UnityEditor.UIElements
namespace.
The elements in the visual tree are drawn in the following order:
The only way to change their drawing order is to reorder VisualElement
objects in their parents.
The different coordinate systems are defined as follow:
The layout system computes the VisualElement.layout
property (type Rect
) for each element.
The layout.position
is expressed as pixels relative to the coordinate space of its parent. Although you can assign a value to layout.position
directly, it is recommended that you use style sheets and the layout system to position elements.
Each VisualElement
also has a layout.transform
property (typeITransform
) that positions an element relative to its parent. By default, the transform
is the identity.
The VisualElement.layout.position
and VisualElement.layout.transform
properties define how to transform between the local coordinate system and the parent coordinate system.
The VisualElementExtensions
static class provides the following extension methods that transform points and rectangles between coordinate systems:
WorldToLocal
transforms a Vector2
or Rect
from Panel
space to the referential within the element.LocalToWorld
transforms a Vector2
or Rect
to Panel
space referentialChangeCoordinatesTo
transforms Vector2
or Rect
from the local space of one element to the local space of another.For example, in the image above, the tree is arranged as follows:
Panel
DockArea
and labelled “Coordinates”)VisualElement
acts as the root (refered to as “rootVisualContainer”)
VisualElement
acts as a parent of the button (“red container”)
Button
From the point of view of the panel:
position
property (defined in layout
property) is set as (100, 100) because it is relative to its parent: the root container.position
property (defined in layout
property) is set as (0, 0) because it is relative to its parent: the red container.The origin of an element is its top left corner.
Use the worldBound
property to retrieve the window space coordinates of the VisualElement
, taking into account the transforms and positions of its ancestry.