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Unity Engine

Change events

Events that occur when the user changes the state of an element.
Read time 5 minutesLast updated 21 days ago
When the value of an element changes, a 
ChangeEvent
is sent. This is typically sent when a value in a field of a control changes. For example, when the user toggles a checkbox.
The 
ChangeEvent
is a typed event, and contains both the previous and the new value of the visual element.
The event triggers after a change assigns a new value to a visual element. You can't cancel change events to prevent a value change on a visual element.
The base class for 
ChangeEvent
is the EventBase class.
The following table describes the change event and its propagation phases:

Event

Description

Trickles down

Bubbles up

Cancellable

ChangeEventA generic event sent when the value of an element changes.YesYesNo

Unique properties

previousValue
: The previous value of the target control.
newValue
: The new value of the target control.

Event List

ChangeEvent

ChangeEvent
is a notification event that allows you to react to the value change of a visual element. For example, when you toggle a checkbox to mute music in a game, the game should stop all music.
This event applies to all controls that implement the 
INotifyValueChanged<T>
, where 
<T>
is the type of the 
ChangeEvent
. This event is also used internally to update properties within objects linked to the UI via Bindings.
It fires even when the value of a control is set by code. You can modify the value on a control without firing the 
ChangeEvent
by calling 
SetValueWithoutNotify
in the 
INotifyValueChange<T>
interface.
You can register a callback function to receive a 
ChangeEvent
in two ways:
  1. Call 
    RegisterCallback<>()
    on a visual element
  2. Call 
    RegisterValueChangedCallback()
    on a visual element deriving from 
    INotifyValueChange<T>
Registering a callback via RegisterCallback works on all visual elements, regardless of whether they store internal values or not. If you want to listen to any changes that happen in the child controls of a parent element, this is the method to use.
Because 
ChangeEvent
is a typed event, you must specify the type when registering the event. The code below demonstrates registering and receiving a 
ChangeEvent
of the type 
bool
.
// Registering the callbackrootVisualElement.RegisterCallback<ChangeEvent<bool>>(OnBoolChangedEvent);
// Event callbackprivate void OnBoolChangedEvent(ChangeEvent<bool> evt) {     // Handling code}
Elements that hold values, such as toggles and integer fields, implement the 
INotifyValueChange<T>
interface. It’s possible to register a callback on these elements directly, by calling RegisterValueChangedCallback. This is a more convenient way to register a callback because the type is already built-in. You can unregister event handlers again by calling 
myElement.UnregisterValueChangedCallback
.
var newToggle = new Toggle("Test Toggle");newToggle.RegisterValueChangedCallback(OnTestToggleChanged);
private void OnTestToggleChanged(ChangeEvent<bool> evt){     // Handling code}

Target, propagation, and filtering

ChangeEvent
 participates in both the trickle-down and bubble-up phases. A callback registered on an ancestor element receives every matching 
ChangeEvent
that originates from a descendant control as the event bubbles up the tree.
Within a callback, use these properties to see how the event reached your handler:

Property

Meaning for 
ChangeEvent

EventBase.target
The visual element that dispatched the change (where the value actually changed).
EventBase.currentTarget
The visual element on which your callback was registered.
If you only care about one specific control, compare 
evt.target
to that element (or to a known child) before running your logic. 
RegisterValueChangedCallback
 calls 
RegisterCallback
 for 
ChangeEvent<T>
on that control; it does not stop other controls from dispatching their own 
ChangeEvent
, and a handler on a parent still receives bubbled events from siblings or nested fields when the event type matches.
Composite controls can involve several internal fields. Updating related UI (for example labels or nested fields) inside a value-changed handler can trigger additional 
ChangeEvent
s or nested dispatch. To avoid unintended work or feedback loops, filter with 
evt.target
, prefer 
SetValueWithoutNotify
 when programmatic updates must not notify, and keep handler side effects minimal.
For how 
target
and 
currentTarget
behave during dispatch, refer to Dispatch events.

Examples

The following examples demonstrate the usage of 
ChangeEvent
and how to set and get a control value.
To view an example, do the following:
  1. Under Assets >
    Scripts

    ?

    > Editor, create a C# script called ChangeEventTestWindow.
  2. Copy the example code into the C# script.
  3. From the Editor Toolbar, select Window > UI Toolkit > Change Events Test Window

Example 1: Registering callbacks to the receive change events

using UnityEditor;using UnityEngine;using UnityEngine.UIElements;public class ChangeEventTestWindow : EditorWindow{    private Toggle m_MyToggle;    [MenuItem("Window/UI Toolkit/Change Event Test Window")]    public static void ShowExample()    {        ChangeEventTestWindow wnd = GetWindow<ChangeEventTestWindow>();        wnd.titleContent = new GUIContent("Change Event Test Window");    }    public void CreateGUI()    {        // Create a toggle        m_MyToggle = new Toggle("Test Toggle") { name = "My Toggle" };        rootVisualElement.Add(m_MyToggle);        // Register a callback on the toggle        m_MyToggle.RegisterValueChangedCallback(OnTestToggleChanged);        // Register a callback on the parent        rootVisualElement.RegisterCallback<ChangeEvent<bool>>(OnBoolChangedEvent);    }    private void OnBoolChangedEvent(ChangeEvent<bool> evt)    {        // currentTarget is rootVisualElement; target is the control whose value changed (here, the toggle).        Debug.Log($"Parent callback: target={evt.target.name}, currentTarget={evt.currentTarget.name}; " +            $"old value: {evt.previousValue}, new value: {evt.newValue}");    }    private void OnTestToggleChanged(ChangeEvent<bool> evt)    {        // currentTarget and target are both the toggle when the callback is registered on the toggle.        Debug.Log($"Toggle callback: target={evt.target.name}, currentTarget={evt.currentTarget.name}; " +            $"old value: {evt.previousValue}, new value: {evt.newValue}");    }}

Example 2: Setting a control value through code

using UnityEditor;using UnityEngine;using UnityEngine.UIElements;public class ChangeEventTestWindow : EditorWindow{    private Toggle m_MyToggle;    [MenuItem("Window/UI Toolkit/Change Event Test Window")]    public static void ShowExample()    {        GetWindow<ChangeEventTestWindow>().titleContent = new GUIContent("Change Event Test Window");    }    public void CreateGUI()    {        // Create a toggle and register callback         m_MyToggle = new Toggle("Test Toggle") { name = "My Toggle" };        m_MyToggle.RegisterValueChangedCallback((evt) => { Debug.Log("Change Event received"); });        rootVisualElement.Add(m_MyToggle);        // Create button to toggle the toggle's value        Button button01 = new Button() { text = "Toggle" };        button01.clicked += () =>         {            m_MyToggle.value = !m_MyToggle.value;        };        rootVisualElement.Add(button01);        // Create button to toggle the toggle's value without triggering a ChangeEvent        Button button02 = new Button() { text = "Toggle without notification" };        button02.clicked += () =>        {            m_MyToggle.SetValueWithoutNotify(!m_MyToggle.value);        };        rootVisualElement.Add(button02);    }}