Custom controls

UI Toolkit allows you to create custom controls. This makes it possible to create complex controls, or implement custom logic for user interface elements. A good custom control is abstract, self-contained, and recurring.

A Slide Toggle is a good example of a custom control:

• It’s abstract: you use it to switch between one setting and another.
• It’s self-contained: you provide it with a label and an initial value; it triggers an event when its state changes.
• It’s recurring: you can use it in multiple places within an application.

The menu bar of your application isn’t a good example of a custom control:

• It’s not abstract: it’s specific to your application.
• It’s not self-contained: it probably has dependencies to other parts of your application.
• It’s not recurring: there is probably only one menu in your application.

After you have created a custom control, you can style it with USS, create logic to handle events, and use it in UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary Builder.

Create a custom control

To create a custom control, you must create a custom control C# class and initialize it. You can expose it for use in UXML and UI Builder. You can also bind it to data.

Create a custom control class

Create a C# class derived from the VisualElement class or a subclass of the VisualElement class. For example, you can create a bindable custom control derived from the BaseField generic base class instead of BindableElement. This provides the following advantages:

• Implements the INotifyValueChanged interface for the generic parameter type that you specify
• Makes the control focusable by default
• Provides a horizontal layout with a label element on the left and input on the right

Note: It’s possible to create custom controls derived from built-in UI controls, if you understand their internal hierarchy and existing USS classes. Unity discourages this practice because your custom controls might be dependent on their internal structure, which is subject to change in the future.

Initialize a custom control

Custom controls inherit from VisualElement. A VisualElement is not bound to the lifetime of a GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary
and it does not receive Awake(), OnEnable(), OnDisable(), or OnDestroy() callbacks.

You can initialize a custom control in its constructor. However, if your application needs, you can delay initialization until the custom control is added to the UI. To do this, register a callback for an AttachToPanelEvent. To detect that your custom control has been removed from the UI, use the DetachFromPanelEvent callback.

void OnEnable()
{
    var myCustomElement = rootVisualElement.Q(className: "my-custom-element");
    myCustomElement.RegisterCallback<AttachToPanelEvent>(e =>
        { /* do something here when element is added to UI */ });
    myCustomElement.RegisterCallback<DetachFromPanelEvent>(e =>
        { /* do something here when element is removed from UI */ });
}

UI Toolkit dispatches these two events for all elements and doesn’t need a custom VisualElement subclass. For more information, see Panel events.

Expose custom controls to UXML and UI Builder

To use your custom controls with UXML and UI Builder, you must expose them. To do so, define a factory class derived from UxmlFactory<T> as described in Define new elements.

You can add new attributes to a custom control for use in UXML. To do this, define a traits class derived from UxmlTraits and override the Init() method as described in Define attributes on elements. The UI Builder relies on this to ensure that the visual elementA node of a visual tree that instantiates or derives from the C# VisualElement class. You can style the look, define the behaviour, and display it on screen as part of the UI. More info
See in Glossary
created for the ViewportThe user’s visible area of an app on their screen.
See in Glossary updates when you change the value of a UXML property in the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary.

To use your custom controls’ UXML properties in the Inspector of the UI Builder, you must match property names between UXML and C# script. Use standard kebab case notation for UXML, and camel case or Pascal case for C#. For example, a property named MyFloat or myFloat in C# should be my-float in UXML.

Note: You can’t create logic in your C# script to detect whether a visual element is in the UI Builder preview mode. Therefore, Unity recommends you to design your custom control to be self-contained without side effects that you’d need to disable inside the UI Builder. For more information about the UI Builder’s Preview Mode, see Testing inside the UI Builder.

Bind custom controls to data

You bind custom controls to serialized properties to synchronize values between the control and the property.

To bind custom controls to data:

• Implement the INotifyValueChanged interface and listen for ChangeEvents as needed.
• Inherit from the BindableElement class or implement the IBindable interface.

See SerializedObject data binding for more details.

For a bindable custom control example, see Create a bindable custom control.

Style custom controls with USS

Use USS to customize the look of a custom control the same way as a built-in control. You can also create USS custom properties to style a custom control.

Note: The Inspector window in the UI Builder doesn’t show USS custom properties. To edit USS custom properties, use a text editor to edit your USS file directly.

To interact with custom USS properties for a custom control in C#, use the CustomStyleProperty structure and the CustomStylesResolvedEvent event.

CustomStyleProperty describes the name and type of the property you read from the stylesheet.

UI Toolkit dispatches CustomStylesResolvedEvent for any element that directly receives a custom USS property. It dispatches the event for any element that a selector matches, for selectors for which the rule contains the value of the custom property. UI Toolkit doesn’t dispatch the event for elements that inherit the value. The event holds a reference to an ICustomStyle object. You must use its TryGetValue() method to read the effective value of a CustomStyleProperty. This method has overloads for different types of CustomStyleProperty.

For a custom style with a custom control example, see Create custom style for a custom control.

Note: You can’t define transitions for custom style properties.

Handle event for custom controls

For detailed information on how to handle events for custom controls, see Handle events.

Note:

• Unity dispatches keyboard events to the currently focused element. To handle keyboard events for a custom control, set properties related to focus.
• To handle touch and mouse input events, register callbacks for the relevant event types, such as pointer events and mouse events, in the constructor.

Use custom controls in UI Builder

To add a custom control to a visual treeAn object graph, made of lightweight nodes, that holds all the elements in a window or panel. It defines every UI you build with the UI Toolkit.
See in Glossary in UI Builder:

1. Select Library > Project > Custom Controls (C#).
2. Drag the custom control into the Hierarchy window.

Best practices and tips

Best practices:

• Expose properties that a custom control represents and other functional aspects of its behavior as UXML properties, and expose properties that affect the look of a custom control as USS properties.
• Use a namespace that’s unique, short, and readable to avoid name collisionsA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
  See in Glossary with other elements.
• Keep UXML attributes primitive. Data that you can specify in UXML is limited to the set of primitive data types. UXML doesn’t support complex data structures or collections. Pass complex data to your custom controls at runtime via C# scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
  See in Glossary or data binding, not in UXML.
• In C#, expose USS classes or names as constants. This allows you to locate elements by class or name using UQuery.
• Adopt the BEM standard for USS classes. This allows you to address every element with a class list selector.
• Use static callbacks for a lower memory footprint. When you register an instance method to be used as a callback, you might create unnecessary allocations. To avoid allocations, use anonymous static lambda functions that call into regular C# static methods. You can retrieve the context of the current element through the EventBase.currentTarget property.

Tips:

• Render custom geometry through the generateVisualContent callback for custom controls. For an example usage that renders a partially filled circle, see RadialProgress example.

• Custom controls are convenient. However, you might achieve the same outcomes with the following:

  • Assemble your UI from existing elements, and change their styles and properties.
  • Use UXML templates. Use regular C# MonoBehaviours to add logic that pertains to the specific UI Document that holds your UI. (To learn how to use MonoBehaviours to control UI in a UI Document, see Create your first runtime UI.) To achieve encapsulation, create properties and methods inside your MonoBehaviour that internally fetch VisualElements with UQuery and manipulate their properties.

Additional resources

• Controls
• Controls reference
• USS custom properties (variables)
• USS Variables in UI Builder