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.
This package defines the classes and interfaces necessary to perform Drag
and Drop operations in Java. It
defines classes for the drag-source and the drop-target, as well as
events for transferring the data being dragged. This package also provides
a means for giving visual feedback to the user throughout the
duration of the Drag and Drop operation.
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 user makes a Drag gesture over the
Component,
which the registered
DragGestureRecognizer detects, and notifies its
DragGestureListener of.
Note: Although this API 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. This package
contains the abstract class MouseDragGestureRecognizer for
recognizing mouse device gestures. Other abstract subclasses may be
provided by the platform to support other input devices or
particular Component class semantics.
- 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.
