Create a custom binding to bind USS selectors

Version: 2023.2+

This example shows how to create a custom binding that binds USS selectors to a 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.

Example overview

This example creates a custom binding that assigns USS class selectors to each child element of a visual element based on the order of the child elements in the hierarchy. The first element always has the left round corner edge, while the last element always has the right round corner edge.

You can find the completed files that this example creates in this GitHub repository.

Prerequisites

This guide is for developers familiar with the Unity Editor, UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary Toolkit, and C# scripting. Before you start, get familiar with the following:

Create a custom binding type

Create a custom binding type that assigns USS classes based on the sibling index. The binding updates only when the sibling index changes.

Create a project in Unity with any template.

In the Assets folder of your project, create a C# script named AddMenuUSSClass.cs with the following content:

using UnityEngine.UIElements;
    
[UxmlObject]
public partial class AddMenuUssClass : CustomBinding
{
    protected override BindingResult Update(in BindingContext context)
    {
        // Assign USS classes based on the sibling index. The binding updates when the sibling index changes.
        var element = context.targetElement;
        var index = element.parent.IndexOf(element);
        element.EnableInClassList("menu-button--first", index == 0);
        element.EnableInClassList("menu-button--last", index == element.parent.childCount - 1);                
            
        return new BindingResult(BindingStatus.Success);
    }
}

Create the UI and the binding

Generally speaking, you create a binding that binds to an existing property of the element. For demonstration purposes, this example creates a custom binding that binds to a property that doesn’t exist in the element. You must use UXML or C# to set up the binding that binds to a non-existing property. This example uses UXML to set up the binding.

In the Assets folder of your project, create a USS file named CustomBinding.uss with the following contents:

.menu-button {
    flex-direction: row;
    height: 64px;
    align-items: center; 
    justify-content: center;
}
    
.menu-button--first {
    border-top-left-radius: 15px;
    border-bottom-left-radius: 15px;
    border-left-width: 2px;
}
    
.menu-button--last {
    border-top-right-radius: 15px;
    border-bottom-right-radius: 15px;
    border-right-width: 2px;
}
    
Button {
    margin: 0px;
    border-color: red;
}

In the Assets folder of your project, create a UXML file named CustomBinding.uxml with the following contents:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements" editor-extension-mode="True">
    <ui:VisualElement class="menu-button">
        <ui:Button text="Bloub">
            <Bindings>
                <AddMenuUssClass property="add-menu-button-class" />
            </Bindings>
        </ui:Button>
        <ui:Button text="Bip">
            <Bindings>
                <AddMenuUssClass property="add-menu-button-class" />
            </Bindings>
        </ui:Button>
        <ui:Button text="Boop">
            <Bindings>
                <AddMenuUssClass property="add-menu-button-class" />
            </Bindings>
        </ui:Button>
    </ui:VisualElement>
</ui:UXML>

Double-click the CustomBinding.uxml file to open it in UI Builder.
In the StyleSheet panel, select + > Add Existing USS, and then select the CustomBinding.uss file.
Save your changes.

Test the binding

To test the binding, change the order of the button elements in the UI Builder and observe the changes in the UI Builder ViewportThe user’s visible area of an app on their screen.
See in Glossary.

Double-click the CustomBinding.uxml file to open it in UI Builder.
In the Hierarchy panel, expand the root VisualElement to display the child elements, which are buttons.
Drag and drop the buttons to change their order. The first button always has the left round corner edge, while the last button always has the right round corner edge.

Additional resources

Create custom binding types

SerializedObject data binding