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Unity Engine

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Unity Engine

Drag-and-drop events

Events that occur when the user drags and drops an element.
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Drag events are sent during operations where visual elements have drag-and-drop behavior. This is an Editor-only event.
To implement drag-and-drop functionality, make sure that visual elements register callbacks for specific events.
Visual elements that support drag operations separate into two types:
  • Draggable visual elements
  • Droppable visual elements
You can select a draggable visual element, drag it to a droppable visual element, and release the element to drop it.
The base class for all drag-and-drop events is DragAndDropEventBase.
The following table describes the drag-and-drop events and their propagation phases:

Event

Description

Trickles down

Bubbles up

Cancellable

DragExitedEventSent when the drag-and-drop process ends.YesYesNo
DragUpdatedEventSent when the dragged element moves over a drop target.YesYesYes
DragPerformEventSent when the dragged element drops onto a target.YesYesYes
DragEnterEventSent when the dragged element enters a new 
VisualElement
.		YesNoNo
DragLeaveEventSent when the dragged element exits the current drop target.YesNoNo

Make visual elements draggable

To make a visual element draggable, you need to register callbacks on the following three event types:
Use the following steps for a drag operation:
  1. Set its state to "being dragged".
  2. Add the appropriate data to 
    DragAndDrop
    .
  3. Call 
    DragAndDrop.StartDrag()
    .
  4. Provide a visual cue to the drag operation. The drop area visual element should remove this feedback when it receives a 
    DragPerformEvent
    or a 
    DragExitedEvent
    .

Event list

DragExitedEvent

The 
DragExitedEvent
 is sent when the user drags any draggable object over a visual element and releases the mouse pointer. When a drop area visual element receives a 
DragExitedEvent
, it needs to remove all feedback from drag operations.

DragUpdatedEvent

The 
DragUpdatedEvent
 is sent when the pointer moves over a visual element as the user moves a draggable object.
When a drop area visual element receives a 
DragUpdatedEvent
, it needs to update the drop feedback. For example, you can move the "ghost" of the dragged object so it stays under the mouse pointer.
The drop area visual element should also examine 
DragAndDrop
 properties and set 
DragAndDrop.visualMode
to indicate the effect of a drop operation. For example, a drop operation could create a new object, move an existing object, or reject the drop operation.

DragPerformEvent

The 
DragPerformEvent
 is sent when the user drags any draggable object and releases the mouse pointer over a visual element. This only occurs if a visual element sets 
DragAndDrop.visualMode
 to something other than 
DragAndDropVisualMode.None
or 
DragAndDropVisualMode.Rejected
to indicate that it can accept dragged objects.
When a drop area visual element receives a 
DragPerformEvent
, it needs to act on the dragged objects stored in 
DragAndDrop.objectReferences
, 
DragAndDrop.paths
or 
DragAndDrop.GetGenericData()
.
For example, it might add new visual elements at the location where the user drops the objects.

DragEnterEvent

The 
DragEnterEvent
 is sent when the pointer enters a visual element during a drag operation.
When a drop area visual element receives a 
DragEnterEvent
, it needs to provide feedback that lets the user know that it, or one of its children, is a target for a potential drop operation. For example, you can add a USS class to the target element and display a "ghost" of the dragged object under the mouse pointer.

DragLeaveEvent

The 
DragLeaveEvent
 is sent when the pointer exits a visual element as the user moves a draggable object.
When a drop area visual element receives a 
DragLeaveEvent
, it needs to stop providing drop feedback. For example, you can remove the USS class that you added when the target element received a 
DragEnterEvent
, and no longer display the “ghost” of the dragged object.

Examples