Customize UXML tag names and attributes

You can customize a UXML tag name, override the default value of a base class attribute, or restrict the accepted attributes for your custom control.

Customize a UXML tag name

By default, the tag name in UXML for your custom control is the C# class name. It’s not recommended that a tag has a different name than the C# class. However, you can customize the tag name if there are name conflicts.

To customize a UXML tag name, override its IUxmlFactory.uxmlName and IUXmlFactory.uxmlQualifiedName properties. Make sure of the following:

• uxmlName is unique in your namespace
• uxmlQualifiedName is unique in your project

If both names aren’t unique, Unity throws an exception when it loads the assembly.

The following code example overrides and customizes the UXML tag name:

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

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

Select a factory for an element

By default, IUxmlFactory instantiates an element and selects the element by the name of the element. To consider attribute values on the element for the selection, override IUXmlFactory.AcceptsAttributeBag(). The factory examines the element attributes to decide if it can instantiate an object for the UXML element.

If your VisualElement class is generic, it can be useful to have the factory examine element attributes. In this case, the class factory for a specialization of your class could examine the value of a UXML type attribute. Depending on the value, instantiation can be accepted or refused.

If more than one factory can instantiate an element, the first registered factory is selected.

Override the default value of a base class attribute

To change the default value of an attribute declared in a base class, set its defaultValue in the derived UxmlTraits class.

The following code example changes the default value of m_TabIndex:

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

Restrict accepted attributes

By default, the generated UXML schema states that an element can have any attribute. This is in contrast to XML validators that check that the value of a declared attribute matches its declaration.

The IUxmlAttributes bag includes additional attributes that are passed to the IUxmlFactory.AcceptsAttributeBag() and IUxmlFactory.Init() functions. The factory implementation decided whether to use these additional attributes. The default behavior is to discard additional attributes. These additional attributes aren’t attached to the instantiated VisualElement and you can’t query these attributes with UQuery.

To restrict the accepted attributes to those explicitly declared in the IUxmlAttributes bag when you define a new element, set the UxmlTraits.canHaveAnyAttribute property to false in your UxmlTraits constructor.

Additional resources

• Create custom controls
• Expose custom controls to UXML and UI Builder