Data Objects and Data Sources: Manipulation
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After a data object or data source has been created, you can perform a number of common operations on the data, such as inserting and removing data, enumerating the formats the data is in, and more. This article describes the techniques necessary to complete the most common operations. Topics include:
Inserting data into a data source
Determining the formats available in a data object
Retrieving data from a data object
Inserting Data into a Data Source
How data is inserted into a data source depends on whether the data is supplied immediately or on demand, and in which medium it is supplied. The possibilities are as follows.
Supplying Data Immediately (Immediate Rendering)
Call COleDataSource::CacheGlobalData repeatedly for every Clipboard format in which you are supplying data. Pass the Clipboard format to be used, a handle to the memory containing the data and, optionally, a FORMATETC structure describing the data.
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If you want to work directly with STGMEDIUM structures, you call COleDataSource::CacheData instead of COleDataSource::CacheGlobalData in the option above.
Supplying Data on Demand (Delayed Rendering)
This is an advanced topic.
Call COleDataSource::DelayRenderData repeatedly for every Clipboard format in which you are supplying data. Pass the Clipboard format to be used and, optionally, a FORMATETC structure describing the data. When the data is requested, the framework will call COleDataSource::OnRenderData, which you must override.
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If you use a CFile object to supply the data, call COleDataSource::DelayRenderFileData instead of COleDataSource::DelayRenderData in the option above. When the data is requested, the framework will call COleDataSource::OnRenderFileData, which you must override.
Determining the Formats Available in a Data Object
Before an application allows the user to paste data into it, it needs to know if there are formats on the Clipboard that it can handle. To do this, your application should do the following:
Create a COleDataObject object and a FORMATETC structure.
Call the data object’s AttachClipboard member function to associate the data object with the data on the Clipboard.
Do one of the following:
Call the data object’s IsDataAvailable member function if there are only one or two formats you need. This will save you time in cases where the data on the Clipboard supports significantly more formats than your application.
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Call the data object’s BeginEnumFormats member function to start enumerating the formats available on the Clipboard. Then call GetNextFormat until the Clipboard returns a format your application supports or there are no more formats.
If you are using ON_UPDATE_COMMAND_UI, you can now enable the Paste and, possibly, Paste Special items on the Edit menu. To do this, call either CMenu::EnableMenuItem or CCmdUI::Enable. For more information on what container applications should do with menu items and when, see the Menus and Resources: Container Additions article.
Retrieving Data from a Data Object
Once you have decided on a data format, all that’s left is to retrieve the data from the data object. To do this, the user decides where he or she wants to put the data, and the application calls the appropriate function. The data will be available in one of the following mediums:
| Medium | Function to call |
| Global Memory (HGLOBAL) | COleDataObject::GetGlobalData |
| File (CFile) | COleDataObject::GetFileData |
| STGMEDIUM structure (IStorage) | COleDataObject::GetData |
Commonly, the medium will be specified along with its Clipboard format. For example, a CF_EMBEDDEDSTRUCT object is always in an IStorage medium that requires an STGMEDIUM structure. Therefore, you would use GetData because it is the only one of these functions that can accept an STGMEDIUM structure.
For cases where the Clipboard format is in an IStream or HGLOBAL medium, the framework can provide a CFile pointer that references the data. The application can then use file read to get the data in much the same way as it might import data from a file. Essentially, this is the client-side interface to the OnRenderData and OnRenderFileData routines in the data source.
The user can now insert data into the document just like for any other data in the same format.