docs/latex/wx/tdnd.tex

   1 \section{Drag-and-drop and clipboard overview}\label{wxdndoverview}
   2
   3 Classes: \helpref{wxDataObject}{wxdataobject},
   4 \helpref{wxTextDataObject}{wxtextdataobject},
   5 \helpref{wxDropSource}{wxdropsource},
   6 \helpref{wxDropTarget}{wxdroptarget},
   7 \helpref{wxTextDropTarget}{wxtextdroptarget},
   8 \helpref{wxFileDropTarget}{wxfiledroptarget}
   9
  10 It has to be noted that the API for drag and drop in wxWindows is not
  11 yet finished which is mostly due to the fact that DnD support under
  12 GTK 1.0 is very rudimentary and entirely different from the XDnD
  13 protocol used by GTK 1.2. This also entails that not all of the documentation
  14 concerning DnD might be correct and some of the code might get broken
  15 in the future. The next release of wxWindows will be based on GTK 1.2
  16 and will hopefully include a much improved DnD support. The general
  17 design on the wxDropSource side will be the same but especially the
  18 wxDropTarget is almost certain to change.
  19
  20 Note that wxUSE\_DRAG\_AND\_DROP must be defined in setup.h in order
  21 to use Drag'n'Drop in wxWindows.
  22
  23 This overview describes wxWindows support for drag and drop and clipboard
  24 operations. Both of these topics are discussed here because, in fact, they're
  25 quite related. Drag and drop and clipboard are just two ways of passing the
  26 data around and so the code required to implement both types of the operations
  27 is almost the same.
  28
  29 Both operations involve passing some data from one program to another,
  30 although the data can be received in the same program as the source. In the case
  31 of clipboard transfer, the data is first placed on the clipboard and then
  32 pasted into the destination program, while for a drag-and-drop operation the
  33 data object is not stored anywhere but is created when the user starts
  34 dragging and is destroyed as soon as he ends it, whether the operation was
  35 ended successfully or cancelled.
  36
  37 To be a {\it drag source}, i.e. to provide the data which may be dragged by
  38 user elsewhere, you should implement the following steps:
  39
  40 \begin{itemize}\itemsep=0pt
  41 \item {\bf Preparation:} First of all, the data object must be created and
  42 initialized with the data you wish to drag. For example:
  43
  44 \begin{verbatim}
  45         wxDataObject *my_data = new wxTextDataObject data("This string will be dragged.");
  46 \end{verbatim}
  47 \item{\bf Drag start:} To start dragging process (typically in response to a
  48 mouse click) you must call \helpref{DoDragDrop}{wxdropsourcedodragdrop} function
  49 of wxDropSource object which should be constructed like this:
  50
  51 \begin{verbatim}
  52         wxDropSource dragSource( this );
  53         dragSource.SetData( my_data );
  54 \end{verbatim}
  55 \item {\bf Dragging:} The call to DoDragDrop() blocks until the user release the
  56 mouse button (unless you override \helpref{GiveFeedback}{wxdropsourcegivefeedback} function
  57 to do something special). When the mouse moves in a window of a program which understands the
  58 same drag-and-drop protocol (any program under Windows or any program supporting GTK 1.0
  59 DnD protocol under X Windows), the corresponding \helpref{wxDropTarget}{wxdroptarget} methods
  60 are called - see below.
  61 \item {\bf Processing the result:} DoDragDrop() returns an {\it effect code} which
  62 is one of the values of \helpref{wxDragResult}{wxdropsource} enum. Codes
  63 of wxDragError, wxDragNone and wxDragCancel have the obvious meaning and mean
  64 that there is nothing to do on the sending end (except of possibly logging the
  65 error in the first case). wxDragCopy means that the data has been successfully
  66 copied and doesn't require any specific actions neither. But wxDragMove is
  67 special because it means that the data must be deleted from where it was
  68 copied. If it doesn't make sense (dragging selected text from a read-only
  69 file) you should pass FALSE as parameter to DoDragDrop() in the previous step.
  70 \end{itemize}
  71
  72 To be a {\it drop target}, i.e. to receive the data dropped by user you should
  73 follow the instructions below:
  74
  75 \begin{itemize}\itemsep=0pt
  76 \item {\bf Initialization:} For a window to be drop target, it needs to have
  77 an associated \helpref{wxDropTarget}{wxdroptarget} object. Normally, you will
  78 call \helpref{wxWindow::SetDropTarget}{wxwindowsetdroptarget} during window
  79 creation associating you drop target with it. You must derive a class from
  80 wxDropTarget and override its pure virtual methods. Alternatively, you may
  81 derive from \helpref{wxTextDropTarget}{wxtextdroptarget} or
  82 \helpref{wxFileDropTarget}{wxfiledroptarget} and override their OnDropText()
  83 or OnDropFiles() method.
  84 \item {\bf Drop:} When the user releases the mouse over a window, wxWindows
  85 queries the associated wxDropTarget object if it accepts the data. For
  86 this, \helpref{GetFormatCount}{wxdroptargetgetformatcount} and \helpref{GetFormat}{wxdroptargetgetformat} are
  87 used and if the format is
  88 supported (i.e. is one of returned by GetFormat()),
  89 then \helpref{OnDrop}{wxdroptargetondrop} is called.
  90 Otherwise, wxDragNone is returned by DoDragDrop() and
  91 nothing happens.
  92 \item {\bf The end:} After processing the data, DoDragDrop() returns either
  93 wxDragCopy or wxDragMove depending on the state of the keys (<Ctrl>, <Shift>
  94 and <Alt>) at the moment of drop. There is currently no way for the drop
  95 target to change this return code.
  96 \end{itemize}
  97