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Note: fully updated ruby documentation for DataObject classes is needed. For the time being, the test case in test/test_clipboard.rb and the dnd sample demonstrate how to use data object classes in some detail. For many purposes the ready-to-use classes TextDataObject, FiletDataObject, and BitmapDataObject will suffice.

A DataObject represents data that can be copied to or from the clipboard, or dragged and dropped. The important thing about DataObject 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. DataObject defines an enumeration type

Get  = 0x01    // format is supported by GetDataHere()
Set  = 0x02     // format is supported by SetData()

which distinguishes between them. See DataFormat 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, Widgets has two predefined classes which derive from DataObject: DataObjectSimple and DataObjectComposite. DataObjectSimple is the simplest DataObject possible and only holds data in a single format (such as HTML or text) and DataObjectComposite is the simplest way to implement a DataObject that does support multiple formats because it achieves this by simply holding several DataObjectSimple objects.

So, you have several solutions when you need a DataObject 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 TextDataObject,BitmapDataObject or FileDataObject in the simplest cases when you only needto support one format and your data is either text, bitmap or list of files.
2. Use DataObjectSimple Deriving from DataObjectSimple is the simplestsolution for custom data – you will only support one format and so probablywon’t be able to communicate with other programs, but data transfer will workin your program (or between different copies of it).
3. Use DataObjectComposite This is a simple but powerfulsolution which allows you to support any number of formats (eitherstandard or custom if you combine it with the previous solution).
4. Use DataObject directly This is the solution formaximal flexibility and efficiency, but it is also the most difficult toimplement.

Please note that the easiest way to use drag and drop and the clipboard with multiple formats is by using DataObjectComposite, but it is not the most efficient one as each DataObjectSimple 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 DataObject 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 DataObjectSimple: FileDataObject, TextDataObject and BitmapDataObject which can be used without change.

You may also derive your own data object classes from CustomDataObject 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.

Virtual functions to override

Each class derived directly from DataObject 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 get_format_count.

Derived from


See also

Clipboard and drag and drop overview, DnD sample, FileDataObject, TextDataObject, BitmapDataObject, CustomDataObject, DropTarget, DropSource, TextDropTarget, FileDropTarget




Array get_all_formats( Integer dir = Get)

Should return an Array of DataFormat objects supported in the given direction by this class.


String get_data_here(DataFormat format )

Should return a String containing the object’s data, or nil on failure.


Integer get_data_size(DataFormat format )

Returns the data size of the given format format; may be overridden if desired.


Integer get_format_count(Integer direction = 1)

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


DataFormat get_preferred_format(Integer direction = 1)

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 DataObject.

By default, the first member of the list returned by get_all_formats is the preferred format, but this method may be overridden.


Boolean set_data( DataFormat format,
String data )

Should store and accept the data data for the format format.

Should returns true on success, false on failure.

[This page automatically generated from the Textile source at Wed Sep 09 02:21:04 +0100 2009]