drag and drop overview

classes: wxdataobject, wxtextdataobject, wxdropsource, wxdroptarget, wxtextdroptarget, wxfiledroptarget

note that wxuse_drag_and_drop must be defined in setup.h in order to use drag and drop in wxwidgets.

see also: wxdataobject overview and dnd sample

it may be noted that data transfer to and from the clipboard is quite similar to data transfer with drag and drop and the code to implement these two types is almost the same. in particular, both data transfer mechanisms store data in some kind of wxdataobject and identify its format(s) using the wxdataformat class.

to be a drag source, i.e. to provide the data which may be dragged by the user elsewhere, you should implement the following steps:

preparation: first of all, a data object must be created and initialized with the data you wish to drag. for example:
```
	wxtextdataobject my_data("this text will be dragged.");
```
drag start: to start the dragging process (typically in response to a mouse click) you must call wxdropsource::dodragdrop like this:
```
	wxdropsource dragsource( this );
	dragsource.setdata( my_data );
	wxdragresult result = dragsource.dodragdrop( true );
```
dragging: the call to dodragdrop() blocks the program until the user releases the mouse button (unless you override the givefeedback function to do something special). when the mouse moves in a window of a program which understands the same drag-and-drop protocol (any program under windows or any program supporting the xdnd protocol under x windows), the corresponding wxdroptarget methods are called - see below.

processing the result: dodragdrop() returns an effect code which is one of the values of wxdragresult enum (explained here):

	switch (result)
	{
	    case wxdragcopy: /* copy the data */ break;
	    case wxdragmove: /* move the data */ break;
	    default:         /* do nothing */ break;
	}

to be a drop target, i.e. to receive the data dropped by the user you should follow the instructions below:

initialization: for a window to be a drop target, it needs to have an associated wxdroptarget object. normally, you will call wxwindow::setdroptarget during window creation associating your drop target with it. you must derive a class from wxdroptarget and override its pure virtual methods. alternatively, you may derive from wxtextdroptarget or wxfiledroptarget and override their ondroptext() or ondropfiles() method.
drop: when the user releases the mouse over a window, wxwidgets asks the associated wxdroptarget object if it accepts the data. for this, a wxdataobject must be associated with the drop target and this data object will be responsible for the format negotiation between the drag source and the drop target. if all goes well, then ondata will get called and the wxdataobject belonging to the drop target can get filled with data.
the end: after processing the data, dodragdrop() returns either wxdragcopy or wxdragmove depending on the state of the keys <ctrl>, <shift> and <alt> at the moment of the drop. there is currently no way for the drop target to change this return code.