[Lazarus] Generalized DragDrop

[zeljko]

On Saturday 22 of January 2011 16:04:34 Michael Van Canneyt wrote:
> 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.

Files dropping is already implemented, and I'm pretty sure that Qt supports 
DnD across different apps on all platforms. Haven't studied gtk2 in this case, 
but your suggestion looks good.

zeljko