fc2171bd4c
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@27090 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
73 lines
3.8 KiB
TeX
73 lines
3.8 KiB
TeX
\section{wxDataObject overview}\label{wxdataobjectoverview}
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Classes: \helpref{wxDataObject}{wxdataobject},
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\helpref{wxClipboard}{wxclipboard},
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\helpref{wxDataFormat}{wxdataformat},
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\helpref{wxDropSource}{wxdropsource},
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\helpref{wxDropTarget}{wxdroptarget}
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See also: \helpref{Drag and drop overview}{wxdndoverview} and \helpref{DnD sample}{samplednd}
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This overview discusses data transfer through clipboard or drag and drop. In
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wxWidgets, these two ways to transfer data (either between different
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applications or inside one and the same) are very similar which allows to
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implement both of them using almost the same code - or, in other
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words, if you implement drag and drop support for your application, you get
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clipboard support for free and vice versa.
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At the heart of both clipboard and drag and drop operations lies the
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\helpref{wxDataObject}{wxdataobject} class. The objects of this class (or, to
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be precise, classes derived from it) represent the data which is being carried
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by the mouse during drag and drop operation or copied to or pasted from the
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clipboard. wxDataObject is a "smart" piece of data because it knows which
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formats it supports (see GetFormatCount and GetAllFormats) and knows how to
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render itself in any of them (see GetDataHere). It can also receive its value
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from the outside in a format it supports if it implements the SetData method.
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Please see the documentation of this class for more details.
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Both clipboard and drag and drop operations have two sides: the source and
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target, the data provider and the data receiver. These which may be in the same
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application and even the same window when, for example, you drag some text from
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one position to another in a word processor. Let us describe what each of them
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should do.
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\subsection{The data provider (source) duties}\label{wxdataobjectsource}
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The data provider is responsible for creating a
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\helpref{wxDataObject}{wxdataobject} containing the data to be
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transferred. Then it should either pass it to the clipboard using
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\helpref{SetData}{wxclipboardsetdata} function or to
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\helpref{wxDropSource}{wxdropsource} and call
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\helpref{DoDragDrop}{wxdropsourcedodragdrop} function.
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The only (but important) difference is that the object for the clipboard
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transfer must always be created on the heap (i.e. using {\tt new}) and it will
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be freed by the clipboard when it is no longer needed (indeed, it is not known
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in advance when, if ever, the data will be pasted from the clipboard). On the
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other hand, the object for drag and drop operation must only exist while
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\helpref{DoDragDrop}{wxdropsourcedodragdrop} executes and may be safely deleted
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afterwards and so can be created either on heap or on stack (i.e. as a local
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variable).
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Another small difference is that in the case of clipboard operation, the
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application usually knows in advance whether it copies or cuts (i.e. copies and
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deletes) data - in fact, this usually depends on which menu item the user
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chose. But for drag and drop it can only know it after
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\helpref{DoDragDrop}{wxdropsourcedodragdrop} returns (from its return value).
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\subsection{The data receiver (target) duties}\label{wxdataobjecttarget}
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To receive (paste in usual terminology) data from the clipboard, you should
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create a \helpref{wxDataObject}{wxdataobject} derived class which supports the
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data formats you need and pass it as argument to
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\helpref{wxClipboard::GetData}{wxclipboardgetdata}. If it returns {\tt false},
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no data in (any of) the supported format(s) is available. If it returns {\tt
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true}, the data has been successfully transferred to wxDataObject.
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For drag and drop case, the \helpref{wxDropTarget::OnData}{wxdroptargetondata}
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virtual function will be called when a data object is dropped, from which the
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data itself may be requested by calling
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\helpref{wxDropTarget::GetData}{wxdroptargetwxdroptarget} method which fills
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the data object.
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