ScriptReference/UIElements.CallbackEventHandler.RegisterCallback

[giscus[bot]]

ScriptReference/UIElements.CallbackEventHandler.RegisterCallback

https://docs.unity3d.com/ScriptReference/UIElements.CallbackEventHandler.RegisterCallback.html

[vertxxyz]

You can see a curated list of events here.
I have compiled the list for reference purposes:

Events in UnityEngine and UnityEditor as of 2023.2:

UnityEngine.UIElements

Type	Description
AttachToPanelEvent	When an element is added to an element that is a descendent of a panel.
BlurEvent	Sent immediately after an element has lost focus. This event trickles down, it does not bubble up, and it cannot be cancelled.
ChangeEvent<T0>	When a value in a field changes.
ClickEvent	When the left mouse button is clicked.
ContextClickEvent	When clicking the right mouse button.
ContextualMenuPopulateEvent	When a contextual menu requires menu items.
CustomStyleResolvedEvent	When the custom style properties of a VisualElement have been resolved.
DetachFromPanelEvent	Sent just before an element is detached from its parent if the parent is the descendant of a panel.
DragEnterEvent	Used to manage events that occur when dragging enters an element or one of its descendants. The DragEnterEvent does not trickle down and does not bubble up.
DragExitedEvent	Sent to a dragged element when the drag and drop process ends.
DragLeaveEvent	Used to manage events sent when dragging leaves an element or one of its descendants. The DragLeaveEvent does not trickle down and does not bubble up.
DragPerformEvent	When another element is dragged and dropped on the element.
DragUpdatedEvent	When the element being dragged enters a possible drop target.
ExecuteCommandEvent	Sent by the Editor when an element in the panel should execute a command. (See also command events)
FocusEvent	Immediately after an element has gained focus. This event trickles down and does not bubbles up.
FocusInEvent	Immediately before an element gains focus. This event trickles down and bubbles up.
FocusOutEvent	Immediately before an element loses focus. This event trickles down and bubbles up.
GeometryChangedEvent	Sent after layout calculations, when the position or the dimension of an element changes.
IMGUIEvent	Used to send a IMGUI event that has no equivalent UIElements event.
InputEvent	When text from a TextField changes.
KeyDownEvent	When a key is pressed.
KeyUpEvent	When a pressed key is released.
MouseCaptureEvent	After a handler starts capturing the mouse.
MouseCaptureOutEvent	Before a handler stops capturing the mouse.
MouseDownEvent	When a mouse button is pressed.
MouseEnterEvent	When the mouse pointer enters an element or one of its descendent elements. The event trickles down but does not bubble up.
MouseEnterWindowEvent	When the mouse pointer enters a window. The event bubbles up but does not trickle down.
MouseLeaveEvent	When the mouse pointer exits an element and all its descendent elements. The event trickles down but does not bubble up.
MouseLeaveWindowEvent	When the mouse pointer exits a window. The event bubbles up but does not trickle down.
MouseMoveEvent	When the mouse moves.
MouseOutEvent	When the mouse pointer exits an element. The event trickles down and bubbles up.
MouseOverEvent	When the mouse pointer enters an element. The event trickles down and bubbles up.
MouseUpEvent	When a mouse button is released.
NavigationCancelEvent	When the user presses the cancel button.
NavigationMoveEvent	Typically when the user presses the D-pad, moves a joystick or presses the arrow keys.
NavigationSubmitEvent	When the user presses the submit button.
PointerCancelEvent	When pointer interaction is cancelled.
PointerCaptureEvent	When a pointer is captured by a VisualElement.
PointerCaptureOutEvent	When a VisualElement releases a pointer.
PointerDownEvent	When a pointer is pressed.
PointerEnterEvent	When a pointer enters a VisualElement or one of its descendants. The event does not trickle down and does not bubble up.
PointerLeaveEvent	Sent when a pointer exits an element and all of its descendants. The event does not trickle down and does not bubble up.
PointerMoveEvent	When a pointer changes state. For example if its position or pressure change, or a different button is pressed.
PointerOutEvent	When a pointer exits an element. The event trickles down and bubbles up.
PointerOverEvent	When a pointer enters an element. The event trickles down and bubbles up.
PointerStationaryEvent	When a pointer does not change for a set amount of time, determined by the operating system.
PointerUpEvent	When a pointer's last pressed button is released.
TooltipEvent	Sent to find the first VisualElement that displays a tooltip.
TransitionCancelEvent	When a transition is canceled.
TransitionEndEvent	The completion of a transition. In the case where a transition is removed before completion then the event will not fire.
TransitionRunEvent	When a transition is created (i.e. added to the set of running transitions).
TransitionStartEvent	When a transition's delay phase ends.
ValidateCommandEvent	Sent by the Editor while it determines whether the command will be handled by an element in the panel.
WheelEvent	When the mouse wheel moves.

UnityEditor.UIElements

Type	Description
SerializedObjectChangeEvent	When any value in a SerializedObject changes.
SerializedPropertyChangeEvent	When a value in a PropertyField changes.
SerializedObjectBindEvent (internal)	When a SerializedObject is bound to the element.
SerializedPropertyBindEvent (internal)	When a SerializedProperty is bound to the element.

[vertxxyz]

There exists a rather large trap when using the callback that takes userArgs that means only one event is registered on a target despite registering two:

Handle events
You can register multiple callbacks for an event. However, you can only register the same callback function on the same event and propagation phase once.

"The same callback function" is comparing delegates, and multiple calls to an event like target.RegisterCallback<Event, This>((evt, args) => args.Callback(evt), this); will be the same delegate, and subsequent calls will just overwrite the old ones.
However multiple calls to target.RegisterCallback<Event>(evt => Callback(evt)); involves different delegates, as an argument is captured.
So, use of the userArgs-taking method like this (one that isn't still involving a capture) will only allow a single event of the same type to register on one target.