On Sat, 22 Jan 2011, Hans-Peter Diettrich wrote:
In fpGUI Graeme implemented an interface for inter-process dragging. It would
be nice to have a similar interace in the LCL, that extends the current
intra-process dragging.
While it is correct that Graeme did this, I still have not seen in action.
i.e. I have used the application-specific DnD, but I wouldn't know how to
handle the intra-app DnD.
Why that?
- A more general approach could solve the known problem with docking forms,
by including dragging by the window manager.
- Drag-drop of files or other objects as an alternative to an OpenDialog.
- Embedding of shared objects (COM, CORBA...)
What's required?
- A generalization of the DragManager, so that visual (DragImage...) and
application feedback (events) are not restricted to application-internal drag
sources and implementations. This is not hard to do, all that can be handled
in the already existing or added specialized DragObjects, instead of the
(superfluous) DragPerformers.
- A DragObject should include a SourceType property, so that the targets can
handle more source types, e.g. filenames or COM/CORBA objects, in addition to
controls. Mime types (strings or enum) can be used for that purpose. Further
information for inter-process dragging can be added (source PID, HWND...), so
that an application can distinguish internal references (to controls...) from
references to objects in other processes.
- The event handlers must have access to the extended source information.
This is also no problem, since only one drag operation can be active at any
time, so that a global DragObject reference can be used everywhere. A
predefined DragObject can be used whenever an external object is being
dragged over an application form.
- More drag and drop types should be defined, in addition to "drop" and
"dock", e.g. "open files", "link" or "embed". Eventually specialized drag
targets must register themselves, for e.g. accepting files or embedding
objects, in addition do DockSite registration.
I'm all for it.
Although I'm not sure that this can be implemented in a cross-platform way.
On Windows, the intra-app drag/drop is handled using OLE/DDE.
On Linux, there is no such thing, and I think it depends on the used desktop software (KDE vs Gnome).
I don't know how it is done in Mac.
But your suggestion is at least a step in the right direction.
I would implement support in 2 objects:
TFileDragObject (external files, as dropped by file managers)
TExternalDragObject (external app DnD)
Both are created whenever an external DnD is found.
I don't like the idea of SourceType as a general property. I would make it an
enumerated
(stInternal,stFiles,stExternal) and the TExternalDragObject object can have a
MimeTypes property.
The TFileDragObject can have a 'FileNames' property of type TStrings; it's all
it needs.
Putting everything in the base class is IMHO not necessary, and only
complicates things.
Michael.
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