Bind a custom control

Version: 2021.3+

This example demonstrates how to bind a custom control to a native Unity type.

Example overview

The example creates a custom control that displays a 2D image. It uses the custom control in a custom Editor for a new asset type, and it binds the custom control to a field in an asset of the new type.

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. You are recommended to have a basic understanding of the following:

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
UXML
USS
Custom UXML elements
BindableElement
INotifyValueChanged

Create a serialized object

Create a serialized object that contains a Texture2D.

Create a Unity project with any template.
In the Project windowA window that shows the contents of your Assets folder (Project tab) More info
See in Glossary, create a folder named bind-custom-control to store the files.

Create a C# script named TextureAsset.cs and replace its contents with the following:

using UnityEngine;
    
namespace UIToolkitExamples
{
    [CreateAssetMenu(menuName = "UIToolkitExamples/TextureAsset")]
    public class TextureAsset : ScriptableObject
    {
        public Texture2D texture;
    
        public void Reset()
        {
            texture = null;
        }
    }
}

Create and style a custom control

Create a custom control with C# that represents a reference to a 2D texture asset, and style it with USS.

In that folder, create a folder named Editor.
In the Editor folder, create a C# script named TexturePreviewElement.cs.
Replace the contents of TexturePreviewElement.cs with the following:

using System;
using UnityEditor.UIElements;
using UnityEngine;
using UnityEngine.UIElements;
using Object = UnityEngine.Object;

namespace UIToolkitExamples
{
    public class TexturePreviewElement : BindableElement, INotifyValueChanged<Object>
    {
        public new class UxmlTraits : BindableElement.UxmlTraits { }
        public new class UxmlFactory : UxmlFactory<TexturePreviewElement, UxmlTraits> { }

        public static readonly string ussClassName = "texture-preview-element";

        Image m_Preview;
        ObjectField m_ObjectField;
        Texture2D m_Value;

        public TexturePreviewElement()
        {
            AddToClassList(ussClassName);

            // Create a preview image.
            m_Preview = new Image();
            Add(m_Preview);

            // Create an ObjectField, set its object type, and register a callback when its value changes.
            m_ObjectField = new ObjectField();
            m_ObjectField.objectType = typeof(Texture2D);
            m_ObjectField.RegisterValueChangedCallback(OnObjectFieldValueChanged);
            Add(m_ObjectField);

            styleSheets.Add(Resources.Load<StyleSheet>("texture_preview_element"));
        }
        
        void OnObjectFieldValueChanged(ChangeEvent<Object> evt)
        {
            value = evt.newValue;
        }

        public void SetValueWithoutNotify(Object newValue)
        {
            if (newValue == null || newValue is Texture2D)
            {
                // Update the preview Image and update the ObjectField.
                m_Value = newValue as Texture2D;
                m_Preview.image = m_Value;
                // Notice that this line calls the ObjectField's SetValueWithoutNotify() method instead of just setting
                // m_ObjectField.value. This is very important; you don't want m_ObjectField to send a ChangeEvent.
                m_ObjectField.SetValueWithoutNotify(m_Value);
            }
            else throw new ArgumentException($"Expected object of type {typeof(Texture2D)}");
        }

        public Object value
        {
            get => m_Value;
            // The setter is called when the user changes the value of the ObjectField, which calls
            // OnObjectFieldValueChanged(), which calls this.
            set
            {
                if (value == this.value)
                    return;

                var previous = this.value;
                SetValueWithoutNotify(value);

                using (var evt = ChangeEvent<Object>.GetPooled(previous, value))
                {
                    evt.target = this;
                    SendEvent(evt);
                }
            }
        }
    }
}

In the Editor folder, create a folder named Resources.

In the Resources folder, create a StyleSheet named texture_preview_element.uss and replace its contents with the following:

.texture-preview-element {
    width: 200px;
    height: 200px;
}

.texture-preview-element > .unity-image {
    flex-grow: 1;
}

Create the Inspector UI

Create 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 UI, and bind the custom control to the Texture2D object.

In the Editor folder, create a C# script named TextureAssetEditor.cs and replace its contents with the following:

using UnityEditor;
using UnityEngine.UIElements;
using UnityEngine;

namespace UIToolkitExamples
{
    [CustomEditor(typeof(TextureAsset))]
    public class TextureAssetEditor : Editor
    {
        [SerializeField]
        VisualTreeAsset m_VisualTree;

        public override VisualElement CreateInspectorGUI()
        {
            return m_VisualTree.CloneTree();
        }
    }
}

Set the binding with UXML

Create a UXML file with a TexturePreviewElement, and set the binding-path property to texture. This binds the TexturePreviewElement to TextureAsset.texture.

In the Editor folder, create a UI Document named texture_asset_editor.uxml and replace its contents with the following:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:example="UIToolkitExamples" editor-extension-mode="True">
    <example:TexturePreviewElement binding-path="texture" />
</ui:UXML>

In the Project window, select TextureAssetEditor.cs.
Drag texture_asset_editor.uxml to Visual Tree in the Inspector.

Test the binding

From the menu, select Assets > Create > UIToolkitExamples > TextureAsset. This creates an instance of a TextureAsset.
Select the new TextureAsset object. In the Inspector, you can see the Texture Preview Element. If you put a texture asset reference into the field, you can see a preview above the field. If you change the asset reference in the Texture Preview Element in the UI, the TextureAsset.texture changes.

Additional resources

Bind to a list without ListView

Bind a custom control to custom data type