wxDataObject

wxDataObject

A wxDataObject represents data that can be copied to or from the clipboard, or dragged and dropped. The important thing about wxDataObject is that this is a 'smart' piece of data unlike 'dumb' data containers such as memory buffers or files. Being 'smart' here means that the data object itself should know what data formats it supports and how to render itself in each of its supported formats.

A supported format, incidentally, is exactly the format in which the data can be requested from a data object or from which the data object may be set. In the general case, an object may support different formats on 'input' and 'output', i.e. it may be able to render itself in a given format but not be created from data on this format or vice versa. wxDataObject defines an enumeration type

enum Direction
{
    Get  = 0x01,    // format is supported by GetDataHere()
    Set  = 0x02     // format is supported by SetData()
};

which distinguishes between them. See wxDataFormat documentation for more about formats.

Not surprisingly, being 'smart' comes at a price of added complexity. This is reasonable for the situations when you really need to support multiple formats, but may be annoying if you only want to do something simple like cut and paste text.

To provide a solution for both cases, wxWidgets has two predefined classes which derive from wxDataObject: wxDataObjectSimple and wxDataObjectComposite. wxDataObjectSimple is the simplest wxDataObject possible and only holds data in a single format (such as HTML or text) and wxDataObjectComposite is the simplest way to implement a wxDataObject that does support multiple formats because it achieves this by simply holding several wxDataObjectSimple objects.

So, you have several solutions when you need a wxDataObject class (and you need one as soon as you want to transfer data via the clipboard or drag and drop):

1. Use one of the built-in classes You may use wxTextDataObject, wxBitmapDataObject or wxFileDataObject in the simplest cases when you only need to support one format and your data is either text, bitmap or list of files.

2. Use wxDataObjectSimple Deriving from wxDataObjectSimple is the simplest solution for custom data - you will only support one format and so probably won't be able to communicate with other programs, but data transfer will work in your program (or between different copies of it).

3. Use wxDataObjectComposite This is a simple but powerful solution which allows you to support any number of formats (either standard or custom if you combine it with the previous solution).

4. Use wxDataObject directly This is the solution for maximal flexibility and efficiency, but it is also the most difficult to implement.

Please note that the easiest way to use drag and drop and the clipboard with multiple formats is by using wxDataObjectComposite, but it is not the most efficient one as each wxDataObjectSimple would contain the whole data in its respective formats. Now imagine that you want to paste 200 pages of text in your proprietary format, as well as Word, RTF, HTML, Unicode and plain text to the clipboard and even today's computers are in trouble. For this case, you will have to derive from wxDataObject directly and make it enumerate its formats and provide the data in the requested format on demand.

Note that neither the GTK+ data transfer mechanisms for clipboard and drag and drop, nor OLE data transfer, copy any data until another application actually requests the data. This is in contrast to the 'feel' offered to the user of a program who would normally think that the data resides in the clipboard after having pressed 'Copy' - in reality it is only declared to be available.

There are several predefined data object classes derived from wxDataObjectSimple: wxFileDataObject, wxTextDataObject and wxBitmapDataObject which can be used without change.

You may also derive your own data object classes from wxCustomDataObject for user-defined types. The format of user-defined data is given as a mime-type string literal, such as "application/word" or "image/png". These strings are used as they are under Unix (so far only GTK+) to identify a format and are translated into their Windows equivalent under Win32 (using the OLE IDataObject for data exchange to and from the clipboard and for drag and drop). Note that the format string translation under Windows is not yet finished.

wxPython note: At this time this class is not directly usable from wxPython. Derive a class from wxPyDataObjectSimple instead.

Virtual functions to override

Each class derived directly from wxDataObject must override and implement all of its functions which are pure virtual in the base class.

The data objects which only render their data or only set it (i.e. work in only one direction), should return 0 from GetFormatCount.

Derived from

None

Include files

Members

wxDataObject::wxDataObject
wxDataObject::~wxDataObject
wxDataObject::GetAllFormats
wxDataObject::GetDataHere
wxDataObject::GetDataSize
wxDataObject::GetFormatCount
wxDataObject::GetPreferredFormat
wxDataObject::SetData

wxDataObject::wxDataObject

wxDataObject()

Constructor.

wxDataObject::~wxDataObject

~wxDataObject()

Destructor.

wxDataObject::GetAllFormats

virtual void GetAllFormats( wxDataFormat *formats, Direction dir = Get) const

Copy all supported formats in the given direction to the array pointed to by formats. There is enough space for GetFormatCount(dir) formats in it.

wxDataObject::GetDataHere

virtual bool GetDataHere(const wxDataFormat& format, void *buf ) const

The method will write the data of the format format in the buffer buf and return true on success, false on failure.

wxDataObject::GetDataSize

virtual size_t GetDataSize(const wxDataFormat& format ) const

Returns the data size of the given format format.

wxDataObject::GetFormatCount

virtual size_t GetFormatCount(Direction dir = Get) const

Returns the number of available formats for rendering or setting the data.

wxDataObject::GetPreferredFormat

virtual wxDataFormat GetPreferredFormat(Direction dir = Get) const

Returns the preferred format for either rendering the data (if dir is Get, its default value) or for setting it. Usually this will be the native format of the wxDataObject.

wxDataObject::SetData

virtual bool SetData( const wxDataFormat& format, size_t len, const void *buf )

Set the data in the format format of the length len provided in the buffer buf.

Returns true on success, false on failure.

1. Use one of the built-in classes	You may use wxTextDataObject, wxBitmapDataObject or wxFileDataObject in the simplest cases when you only need to support one format and your data is either text, bitmap or list of files.
2. Use wxDataObjectSimple	Deriving from wxDataObjectSimple is the simplest solution for custom data - you will only support one format and so probably won't be able to communicate with other programs, but data transfer will work in your program (or between different copies of it).
3. Use wxDataObjectComposite	This is a simple but powerful solution which allows you to support any number of formats (either standard or custom if you combine it with the previous solution).
4. Use wxDataObject directly	This is the solution for maximal flexibility and efficiency, but it is also the most difficult to implement.