another batch of topic overviews reviewing (letters e,f)

[wxWidgets.git] / docs / doxygen / overviews / dataobject.h

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// Name:        dataobject.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/*!

 @page overview_dataobject wxDataObject overview

 Classes: #wxDataObject, #wxClipboard, #wxDataFormat, #wxDropSource, #wxDropTarget

 See also: @ref overview_dnd and @ref overview_samplednd

 This overview discusses data transfer through clipboard or drag and drop.
 In wxWidgets, these two ways to transfer data (either between different
 applications or inside one and the same) are very similar which allows to
 implement both of them using almost the same code - or, in other
 words, if you implement drag and drop support for your application, you get
 clipboard support for free and vice versa.

 At the heart of both clipboard and drag and drop operations lies the
 #wxDataObject class. The objects of this class (or, to
 be precise, classes derived from it) represent the data which is being carried
 by the mouse during drag and drop operation or copied to or pasted from the
 clipboard. wxDataObject is a "smart" piece of data because it knows which
 formats it supports (see GetFormatCount and GetAllFormats) and knows how to
 render itself in any of them (see GetDataHere). It can also receive its value
 from the outside in a format it supports if it implements the SetData method.
 Please see the documentation of this class for more details.

 Both clipboard and drag and drop operations have two sides: the source and
 target, the data provider and the data receiver. These which may be in the same
 application and even the same window when, for example, you drag some text from
 one position to another in a word processor. Let us describe what each of them
 should do.

 @li The data provider (source) duties
 @li The data receiver (target) duties


 @section overview_dataobject_source The data provider (source) duties

 The data provider is responsible for creating a #wxDataObject containing the 
 data to be transferred. Then it should either pass it to the clipboard using
 #SetData function or to #wxDropSource and call #DoDragDrop function.

 The only (but important) difference is that the object for the clipboard
 transfer must always be created on the heap (i.e. using @c new) and it will
 be freed by the clipboard when it is no longer needed (indeed, it is not known
 in advance when, if ever, the data will be pasted from the clipboard). On the
 other hand, the object for drag and drop operation must only exist while
 #DoDragDrop executes and may be safely deleted afterwards and so can be 
 created either on heap or on stack (i.e. as a local variable).

 Another small difference is that in the case of clipboard operation, the
 application usually knows in advance whether it copies or cuts (i.e. copies and
 deletes) data - in fact, this usually depends on which menu item the user
 chose. But for drag and drop it can only know it after
 #DoDragDrop returns (from its return value).


 @section overview_dataobject_target The data receiver (target) duties

 To receive (paste in usual terminology) data from the clipboard, you should
 create a #wxDataObject derived class which supports the data formats you need 
 and pass it as argument to wxClipboard::GetData. If it returns @false,

 no data in (any of) the supported format(s) is available. If it returns @true, 
 the data has been successfully transferred to wxDataObject.

 For drag and drop case, the wxDropTarget::OnData virtual function will be called 
 when a data object is dropped, from which the data itself may be requested by calling
 wxDropTarget::GetData method which fills the data object.

*/