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Drag and Drop

2.1 Overview

Drag and Drop is a direct manipulation gesture found in many Graphical User Interface systems that provides a mechanism to transfer information between two entities logically associated with presentation elements in the GUI. Normally driven by a physical gesture of a human user using an appropriate input device, Drag and Drop provides both a mechanism to enable continuous feedback regarding the possible outcome of any subsequent data transfer to the user during navigation over the presentation elements in the GUI, and the facilities to provide for any subsequent data negotiation and transfer.

A typical Drag and Drop operation can be decomposed into the following states (not entirely sequentially):

• A DragSource comes into existence, associated with some presentation element (Component) in the GUI, to initiate a Drag and Drop of some potentially Transferable data.
• 1 or more DropTarget(s) come into/go out of existence, associated with presentation elements in the GUI (Components), potentially capable of consuming Transferable data types.
• A DragGestureRecognizer is obtained from the DragSource and is associated with a Component in order to track and identify any Drag initiating gesture by the user over the Component.
• A Human user makes a Drag gesture over the Component, which the registered DragGestureRecognizer detects, and notifies its DragGestureListener of.

Note: Although the body of this document consistently refers to the stimulus for a drag and drop operation being a physical gesture by a human user this does not preclude a programmatically driven DnD operation given the appropriate implementation of a DragSource.

• The DragGestureListener causes the DragSource to initiate the Drag and Drop operation on behalf of the user, perhaps animating the GUI Cursor and/or rendering an Image of the item(s) that are the subject of the operation.
• As the user gestures navigate over Component(s) in the GUI with associated DropTarget(s), the DragSource receives notifications in order to provide "Drag Over" feedback effects, and the DropTarget(s) receive notifications in order to provide "Drag Under" feedback effects based upon the operation(s) supported and the data type(s) involved.

The gesture itself moves a logical cursor across the GUI hierarchy, intersecting the geometry of GUI Component(s), possibly resulting in the logical "Drag" cursor entering, crossing, and subsequently leaving Component(s) and associated DropTarget(s).

The DragSource object manifests "Drag Over" feedback to the user, in the typical case by animating the GUI Cursor associated with the logical cursor.

DropTarget objects manifest "Drag Under" feedback to the user, in the typical case, by rendering animations into their associated GUI Component(s) under the GUI Cursor.

• The determination of the feedback effects, and the ultimate success or failure of the data transfer, should one occur, is parameterized as follows:
• By the transfer "operation" selected by the user, and supported by both the DragSource and DropTarget: Copy, Move or Reference(link).
• By the intersection of the set of data types provided by the DragSource and the set of data types comprehensible by the DropTarget.
• When the user terminates the drag operation, normally resulting in a successful Drop, both the DragSource and DropTarget receive notifications that include, and result in the type negotiation and transfer of, the information associated with the DragSource via a Transferable object.

The remainder of this document details the proposed API changes to support this model.