samples/dnd/d_and_d.txt

   1                         Drag-and-Drop Support in wxWindows
   2                         ==================================
   3
   4 1. Overview
   5    --------
   6
   7  a) What is it?
   8
   9     We're calling drag-and-drop (or d&d for short) the OLE mechanism of data
  10 transfer. Please note that it's not the same thing as the file oriented d&d
  11 of Windows 3.1 "File Manager" which is designed for and limited to the file
  12 names only.
  13
  14     OLE d&d allows application to transfer data of any type to the same or
  15 another process.
  16
  17
  18  b) How is it done? (user's point of view)
  19
  20     To start a d&d operation the user presses the mouse button 1 (left) and
  21 drags the selected object to another window (which must be at least partially
  22 visible on the screen) or to an icon on the taskbar in which case the
  23 corresponding window will be automatically restored. To finish the operation,
  24 the user releases the button. Default d&d operation is "move", but several key
  25 act as modifiers: keeping down the <Ctrl> key at the moment of drop does
  26 "copy", while <Shift> or <Alt> force the "move" (makes sense if default isn't
  27 "move").
  28
  29
  30  c) How is it done? (programmer's point of view)
  31
  32     There are several objects participating in a d&d operation. First of all,
  33 there is the data object itself. Second, there is the drop source which is
  34 responsible for creating the data object (if it doesn't exist yet) and starting
  35 the d&d operation. Finally, the drop target receives the notification when
  36 the data is dropped onto the associated window (see below) and is responsible
  37 for pasting the data and returning the result code (copy, move or failure).
  38 There is one class for each one of these roles in wxWindows d&d implementation,
  39 plese see their descriptions below for details.
  40
  41
  42
  43 2. Drop Target
  44    -----------
  45
  46  a) Being a drop target
  47
  48     ... is as easy as deriving your window class from wxDropTarget and
  49 associating it with a wxWindow object (or perhaps some wxWindow-derived class,
  50 such as wxFrame). The pure virtual function wxDropTarget::OnDrop() must be
  51 implemented in your application and will be called whenever the mouse button
  52 is released over the window in question. Other virtual functions that will be
  53 called in the process of the d&d operation are OnEnter and OnLeave.
  54
  55 @@ should OnDragOver() be user overridable also?
  56
  57   You should associate wxDropTarget and wxWindow calling SetDropTarget:
  58     wxWindow *pWindow = GetTopWindow();
  59     pWindow->SetDropTarget(new MyDropTarget);
  60
  61 The object created passed to SetDropTarget becomes the propriety of wxWindow
  62 and will be deleted with the window (or when you call SetDropTarget next
  63 time). You can always break the association by calling SetDropTarget(NULL).
  64
  65   When some data is dragged over a window, the program must decide if it's
  66 going to accept this data or not. The virtual function IsAcceptedData() is
  67 called to do it. The default implementation takes care of OLE interface
  68 pointer manipulations and only requires you to override GetCountFormats()
  69 and GetFormat(n) functions to let it know what data formats you support.
  70 If it's not flexible enough for your application (i.e. the set of supported
  71 formats changes over time...), you should override IsAcceptedData(). In 99%
  72 of cases the default implementation is ok and you only have to return count
  73 of supported formats (CF_xxx constants or one of your custom formats which
  74 must have been registered) and their values.
  75
  76  b) OnDrop(long x, long y, const void *pData)
  77
  78     (x, y) are drop point (client) coordinates, pData is the pointer to data
  79     (whatever it is).
  80
  81     If 'true' is returned from OnDrop, the operation is considered to be
  82     successful and the corresponding code (MOVE or COPY depending on the
  83     keyboard control keys) is returned. Otherwise, the operation is cancelled.
  84
  85     Please remember that returning 'true' here may mean 'move' and so the
  86     drop source will delete the corresponding data - which would lead to
  87     data loss if you didn't paste it properly.
  88
  89  c) OnEnter()
  90
  91     called when the mouse enters the window: you might use this function to
  92     give some additional visual feedback.
  93
  94  d) OnLeave()
  95
  96     called when the mouse leaves the window; might be a good place to clean
  97 up things allocated in OnEnter.
  98
  99  e) Simple wxDropTarget specializations
 100
 101     Two (very simple) wxDropTarget-derived classes are provided for two most
 102 common situations: d&d of text and file d&d. To use them you only need to
 103 override one virtual function OnDropText in wxTextDropTarget's case and
 104 OnDropFiles for wxFileDropTarget.
 105
 106     The (x, y) are the same as for OnDrop() function. OnDropText's last
 107 parameter points to a (always ANSI, not Unicode) text string, while
 108 OnDropFiles() parameter is the array of file names just dropped (and the
 109 count of them is passed in the 3rd parameter).
 110
 111 3. Data Object
 112    -----------
 113
 114  a) Drag and drop and clipboard
 115
 116     The effect of a d&d operation is the same as using the clipboard to
 117 cut/copy and paste data and it would be nice to use the same code to implement
 118 these two data transfer mechanisms. The wxDataObject allows you to do exactly
 119 this. It encapsulates the data which can be passed either through the clipboard
 120 or d&d.
 121
 122
 123  b) Data format
 124
 125     There are several standard clipboard formats, such as text, bitmap or
 126 metafile picture. All of them are defined in wxDataObject::StdFormats
 127 enumeration. Of course, it's not always enough and you'll often need your
 128 own format for data transfer. The simple helper class wxDataFormat may help
 129 you: when you create an object of this class, it registers a new clipboard
 130 data format identified by the string passed to its ctor.
 131
 132     After your new format is registered, you may use it as any other one.
 133
 134 4. Drop Source
 135    -----------
 136
 137  a) Starting the d&d operation
 138
 139     In order to start the d&d operation you should call the DoDragDrop function
 140 (typically in reply to a "mouse button press" message). NB: DoDragDrop() is a
 141 blocking function which enters into its own message loop and may return after
 142 an arbitrarily long time interval. During it, the QueryContinueDrag() is called
 143 whenever the mouse or keyboard state changes. The default behaviour is quite
 144 reasonable for 99% of cases: the drag operation is cancelled if the <Esc> key
 145 is preessed and the drop is initiated if the mouse button is released.
 146
 147  b) After the end of d&d
 148
 149     The drop source behaviour depends on DoDragDrop() return code. If it
 150 returns wxDropSource::None or wxDropSource::Copy there is normally nothing to
 151 do, but you shouldn't forget to delete your data if it returns the
 152 wxDropSource::Move code.
 153
 154  c) DoDragDrop
 155
 156  d) QueryContinueDrag
 157
 158
 159 5. Remarks
 160    -------
 161
 162
 163 @@@@ TODO: support tymed != TYMED_HGLOBAL;
 164            better support of CF_BMP, CF_METAFILE
 165            scrolling support!! (how?)
 166            sample demonstrating use of user-defined formats
 167            sample which really does something useful