Version: 2022.2
Language : English
Create a drag-and-drop UI to drag between Editor windows
Create looping transitions

Create a transition event

Version: 2021.3+

This example demonstrates the lifecycle of a transition event.

Example overview

The example creates a custom Editor window with a button and color palette. If you click the button, the following appears:

  • A transition starts to change the color palette from blue to green.
  • The transition events that are sent during the transition.
  • The duration of the events.

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


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 the example

  1. Create a project in Unity with any template.

  2. From the menu, select Assets > Create > UI Toolkit > Editor Window.

  3. In the UI Toolkit Editor Window Creator window, enter TransitionExample.

  4. Save your changes. This creates three files as TransitionExample.cs, TransitionExample.uss, and TransitionExample.uxml.

  5. Replace TransitionExample.cs with the following content:

    using System;
    using UnityEditor;
    using UnityEngine;
    using UnityEngine.UIElements;
    public class TransitionExample : EditorWindow
        [SerializeField] private VisualTreeAsset m_VisualTreeAsset = default;
        private Button clickMeButton;
        private VisualElement colorChanger;
        private Label eventLabel;
        private Label timeLabel;
        private DateTime lastEvent;
        private static readonly TimeSpan NearlyInstantaneousThreshold = TimeSpan.FromMilliseconds(10);
        private static readonly string ClickMeButtonClass = "click-me";
        private static readonly string ColorChangerClass = "color-changer";
        private static readonly string ColorChangerTransitionClass = "color-transition";
        private static readonly string EventLabelName = "eventLabel";
        private static readonly string TimeLabelName = "timeLabel";
        private static readonly string TimeBelowThresholdText = "Almost instantaneous.";
        [MenuItem("Window/UI Toolkit/Transition Example")]
        public static void ShowExample()
            TransitionExample wnd = GetWindow<TransitionExample>();
            wnd.titleContent = new GUIContent("TransitionExample");
            wnd.minSize = new Vector2(500f, 400f);
        public void CreateGUI()
            lastEvent = DateTime.Now;
            // Each editor window contains a root VisualElement object
            VisualElement root = rootVisualElement;
            // Instantiate UXML
            VisualElement uxmlAsset = m_VisualTreeAsset.Instantiate();
            // Get the relevant elements by querying the root element
            clickMeButton = root.Q<Button>(className: ClickMeButtonClass);
            colorChanger = root.Q<VisualElement>(className: ColorChangerClass);
            eventLabel = root.Q<Label>(name: EventLabelName);
            timeLabel = root.Q<Label>(name: TimeLabelName);
            // Add callbacks for clicking on the button and monitoring the color changing element.
        private void OnDisable()
        private void OnClickEvent(ClickEvent evt)
        private void OnTransitionRun(TransitionRunEvent evt)
            DisplayLatestEvent("TransitionRunEvent", DateTime.Now);
        private void OnTransitionStart(TransitionStartEvent evt)
            DisplayLatestEvent("TransitionStartEvent", DateTime.Now);
        private void OnTransitionEnd(TransitionEndEvent evt)
            DisplayLatestEvent("TransitionEndEvent", DateTime.Now);
        private void OnTransitionCancel(TransitionCancelEvent evt)
            DisplayLatestEvent("TransitionCancelEvent", DateTime.Now);
        private void DisplayLatestEvent(string eventType, DateTime timestamp)
            // If two events are sent too close together, add both to the Latest event line.
            // This happens if the delay is set to 0 and the TransitionRun and TransitionStart
            // are sent at the same time, or if the button was pressed before the transition
            // was over, thus sending TransitionCancel and TransitionRun (and potentially
            // TransitionStart) events close together.
            var elapsed = timestamp - lastEvent;
            if (elapsed <= NearlyInstantaneousThreshold)
                timeLabel.text = TimeBelowThresholdText;
                eventLabel.text += eventType;
                timeLabel.text = $"{elapsed:s\\.ff} s";
                eventLabel.text = eventType;
            lastEvent = timestamp;
  6. Replace TransitionExample.uxml with the following content:

    <ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements"
            xsi="" engine="UnityEngine.UIElements" editor="UnityEditor.UIElements"
            noNamespaceSchemaLocation="../../UIElementsSchema/UIElements.xsd" editor-extension-mode="False">
        <Style src="TransitionExample.uss"/>
        <ui:VisualElement class="main-container">
            <ui:Button text="Click Me!" class="click-me"/>
            <ui:VisualElement class="color-changer"/>
            <ui:VisualElement class="label-section">
                <ui:VisualElement class="label-line">
                    <ui:Label text="Latest event(s) :  "/>
                    <ui:Label name="eventLabel"/>
                <ui:VisualElement class="label-line">
                    <ui:Label text="Time since last event :  "/>
                    <ui:Label name="timeLabel"/>
  7. Replace TransitionExample.uss with the following content:

    .click-me {
        width: 250px;
        height: 50px;
        font-size: 40px;
        -unity-font-style: bold-and-italic;
        margin: 30px;
    .color-changer {
        margin: 10px;
        width: 150px;
        height: 150px;
        border-width: 10px;
        border-radius: 75px;
        background-color: rgb(0, 31, 138);
        transition: background-color 3s ease-in-out 1s;
    .main-container {
        align-items: center;
        justify-content: space-between;
        flex-grow: 1;
        background-color: rgb(60, 60, 60);
    .label-section {
        margin: 10px;
        border-width: 2px;
        width: 95%;
        align-items: center;
    .label-line {
        flex-direction: row;
        margin: 5px;
        flex-grow: 1;
        align-items: center;
        width: 90%;
        height: 25px;
        font-size: 14px;
        padding: 0;
    .color-transition {
        background-color: rgb(177, 221, 111);
  8. Select TransitionExample.cs and drag TransitionExample.uxml to the 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
    field 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
    if it’s not already there.

  9. To try the example, from the menu, select Window > UI Toolkit > Transition Example.

  10. Click the button to view the color transition and the description of the events sent. There is a long delay between the TransitionRunEvent and the TransitionStartEvent. During the long duration, you can click the button again to interrupt the transition.

Additional resources

Create a drag-and-drop UI to drag between Editor windows
Create looping transitions