Version: 2019.4
The Layout Engine
Writing UXML Templates

The UXML format

UXML files are text files that define the logical structure of the user interface. The format used in UXML files is inspired by HTML (HyperText Markup Language), XAML (eXtensible Application Markup Language), and XML (eXtensible Markup Language). If you are familiar with these recognized formats, you should notice lots of similarities in UXML. However, the UXML format includes small differences to provide an efficient way to work with Unity.

This section describes the UXML format that Unity supports and provides details on writing, loading, and defining UXML templates. It also includes information on defining new elements, and how to use UQuery.

UXML makes it is easier for less technical users to build a user interface within Unity. In UXML you can:

  • define the structure of the user interface (UI) in XML.
  • define the UI layout with USS stylesheets.

This leaves developers to do technical tasks, such as importing assets, defining logic, and processing data.

Defining new elements

UIElements is extensible. You can define your own user interface components and elements.

Before you can use UXML files to define new elements, you must derive a new class from VisualElement or one of its subclasses, then implement the appropriate functionality within this new class. Your new class must implement a default constructor.

For example, the following code derives the new StatusBar class and implements its default constructor:

class StatusBar : VisualElement
    public StatusBar()
        m_Status = String.Empty;

    string m_Status;
    public string status { get; set; }

In order for the UIElements to instantiate a new class when reading a UXML file, you must define a factory for your class. Unless your factory needs to do something special, you can derive the factory from UxmlFactoy<T>. It’s recommended that you put the factory class within your component class.

For example, the following code demonstrates how to define a factory for the StatusBar class by deriving its factory from UxmlFactory<T>. The factory is named UxmlFactory:

class StatusBar : VisualElement
    public new class UxmlFactory : UxmlFactory<StatusBar> {}

    // ...

With this factory defined, you can use the <StatusBar> element in UXML files.

Defining attributes on elements

You can define UXML traits for a new class and set its factory to use these traits.

For example, the following code demonstrates how to define a UXML traits class to initialize the status property as a property of the StatusBar class. The status property is initialized from XML data.

class StatusBar : VisualElement
    public new class UxmlFactory : UxmlFactory<StatusBar, UxmlTraits> {}

    public new class UxmlTraits : VisualElement.UxmlTraits
        UxmlStringAttributeDescription m_Status = new UxmlStringAttributeDescription { name = "status" };
        public override IEnumerable<UxmlChildElementDescription> uxmlChildElementsDescription
            get { yield break; }

        public override void Init(VisualElement ve, IUxmlAttributes bag, CreationContext cc)
            base.Init(ve, bag, cc);
            ((StatusBar)ve).status = m_Status.GetValueFromBag(bag, cc);

    // ...

The UxmlTraits serves two purposes:

  • It is used by the factory to initialize the newly created object.

  • It is analyzed by the schema generation process to get information about the element. This information is translated into XML schema directives.

The code example above does the following:

  • The declaration of m_Status defines an XML attribute named status.
  • The uxmlChildElementsDescription returns an empty IEnumerable which indicates that StatusBar element has no child.
  • The Init() member reads the value of the status attribute in a property bag from the XML parser and sets the StatusBar.status property to this value.
  • By placing the UxmlTraits class inside the StatusBar class allows the Init() method to access the private members of StatusBar.
  • The new UxmlTraits class inherits from the base class UxmlTraits, so it shares the attributes of the base class.
  • Init() calls base.Init() to initialize the base class properties.

The code example above declares a string attribute with the UxmlStringAttributeDescription class. UIElements supports the following types of attributes and each links a C# type to an XML type:

Attribute Attribute value
UxmlStringAttributeDescription A string
UxmlFloatAttributeDescription A single precision floating point value in the range of the C# float type.
UxmlDoubleAttributeDescription A double precision floating point value in the range of the C# double type.
UxmlIntAttributeDescription An integer value in the range of the C# int type.
UxmlLongAttributeDescription A long integer value in the range of the C# long type.
UxmlBoolAttributeDescription true or false
UxmlColorAttributeDescription A string representing a color (#FFFFFF)
UxmlEnumAttributeDescription<T> A string representing one of the values for the Enum type T.

In the code example above, the uxmlChildElementsDescription returns an empty IEnumerable which indicates that the StatusBar element does not accept children.

To have an element accept children of any type, you must override the uxmlChildElementsDescription property. For example, for the StatusBar element to accept children of any type, the uxmlChildElementsDescription property must be specified as follows:

public override IEnumerable<UxmlChildElementDescription> uxmlChildElementsDescription
        yield return new UxmlChildElementDescription(typeof(VisualElement));

Defining a namespace prefix

Once you have defined a new element in C#, you can start using the element in your UXML files. If your new element is defined in a new namespace, you should define a prefix for the namespace. Namespace prefixes are declared as attributes to the root <UXML> element and replace the full namespace name when scoping elements.

To define a namespace prefix, add a UxmlNamespacePrefix attribute to your assembly for each namespace prefix you want to define.

[assembly: UxmlNamespacePrefix("My.First.Namespace", "first")]
[assembly: UxmlNamespacePrefix("My.Second.Namespace", "second")]

This can be done at the root level (outside any namespace) of any C# file of the assembly.

The schema generation system does the following:

  • checks for these attributes and uses them to generate the schema.
  • adds the namespace prefix definition as an attribute of the <UXML> element in newly created UXML files
  • includes the schema file location for the namespace in its xsi:schemaLocation attribute.

You should update the UXML schema of your project. Select Assets > Update UIElements Schema to ensure that your text editor recognizes the new element.

The defined prefix is available in the newly created UXML by selecting Create > UIElements Editor Window in the Project/Assets/Editor folder.

Advanced usage

Customizing a UXML name

You can customize a UXML name by overriding its IUxmlFactory.uxmlName and IUXmlFactory.uxmlQualifiedName properties. Make sure the uxmlName is unique within your namespace and that the uxmlQualifiedName is unique in your project.

If both names are not unique, an exception is thrown when you attempt to load your assembly.

The following code example demonstrates how to override and custom the UXML name:

public class FactoryWithCustomName : UxmlFactory<..., ...>
    public override string uxmlName
        get { return "UniqueName"; }

    public override string uxmlQualifiedName
        get { return uxmlNamespace + "." + uxmlName; }

Selecting a factory for an element

By default, the IUxmlFactory instantiates an element and selects the element using the name of the element.

You can make the selection process consider attribute values on the element by overriding IUXmlFactory.AcceptsAttributeBag. The factory will then examine the element attributes to decide if it can instantiate an object for the UXML element.

This is useful if, for example, your VisualElement class is generic. In this case, the class factory for a specialization of your class could examine the value of a XML type attribute. Depending on the value, instantiation could be accepted or refused. See the implemenatation of PropertyControl<T> for an example.

In the case where more than one factory can instantiate an element, the first registered factory is selected.

Overriding the default value of a base class attribute

You can change the default value of an attribute declared in a base class by setting its defaultValue in the derived UxmlTraits class.

For example, the following code shows how to change the default value of m_TabIndex:

class MyElementTraits : VisualElement.UxmlTraits
        public MyElementTraits()
            m_TabIndex.defaultValue = 0;

Accepting any attribute

By default, the generated XML schema states that an element can have any attribute.

Values of attributes, other than those declared in the UxmlTraits class, are not restricted. This is in contrast to XML validators that check that the value of a declared attribute matches its declaration.

Additional attributes are included in the IUxmlAttributes bag that is passed to the IUxmlFactory.AcceptsAttributBag() and IUxmlFactory.Init() functions. It is up to the factory implementation whether to use these additional attributes. The default behavior is to discard additonal attributes.

This means that these additional attributes are not attached to the instantiated VisualElement and these atttibutes are not queryable with UQuery.

When defining a new element, you can restrict the accepted attributes to those explicitly declared by setting the UxmlTraits.canHaveAnyAttribute property to false in your UxmlTraits constructor.

Using Schema definitions

Schema definition files specify the attributes and which child elements each UXML element can contain. Use schema definition files as a guide for writing correct documents and to validate your documents.

In the UXML template file, the xsi:noNamespaceSchemaLocation and xsi:schemaLocation attributes of the <UXML> root element specify where the schema definition files are located.

Select Assets > Create > UIElements Editor Window to automatically update your schema definition with the latest information from the VisualElement sub-classes used by your project. To force an update of the UXML schema files, select Assets > Update UIElements Schema.

Note: Some text editors do not recognize the xsi:noNamespaceSchemaLocation attribute. If your text editor cannot find the schema definition files, you should also specify the xsi:schemaLocation attribute.

  • 2019–05–23 Page amended
The Layout Engine
Writing UXML Templates