How to receive data dragged from other applications (part 3 of 6)

Unserstanding data formats

Understanding Data Formats

In this section we will investigate various data formats used to transfer data from IDataObject instances. Data formats can vary in four different ways:

  1. The data format
    Just like the clipboard a data object can store data in numerous different formats. In fact, data objects use the same way of describing data formats as that used by the clipboard. A format may be one of the built-in ones such as CF_TEXT or an application defined format such as "Rich Text Format".
  2. The storage medium used to transfer the data
    The data can be delivered via various different mechanisms. These mechanisms are described by the TYMED_* enumeration and are:
    The data is transferred via global memory accessed via a global memory handle of type HGLOBAL.
    The data is transferred via a disk file.
    The data is transferred as a stream accessed through an IStream interface.
    The data is transferred in a Windows storage object accessed through an IStorage interface.
    The data is specified by a GDI component accessed via a HBITMAP handle.
    The data is a metafile accessed via a HMETAFILEPICT handle.
    The data is an enhanced metafile accessed via a HENHMETAFILE handle.
    The way we access the data depends on the storage medium. By far the most common storage medium is the global memory handle and we will concentrate mainly on that mechanism in this article.
  3. The "Aspect", or view of the data
    This indicates how much detail should be provided when rendering the data. The possible aspects are: a full representation as an embedded object, a thumbnail view, an icon view or a print preview. The first of these is the norm and the only one considered in this article.
  4. The target device
    The type of device the data is targetted at can also be specified. However, the usual case is that the data is device independent and this is the only case considered by this article.

Windows encapsulates all this information inside a TFormatEtc structure, defined in the ActiveX unit. Listing 3 reproduces the definitions.

  tagFORMATETC = record
    cfFormat: TClipFormat;
    ptd: PDVTargetDevice;
    dwAspect: Longint;
    lindex: Longint;
    tymed: Longint;
  TFormatEtc = tagFORMATETC;

The fields are discussed briefly in Table 3 below. See the Windows API help file for more detailed information.

TFormatEtc methods
cfFormat Tells us about the data format and is a clipboard format identifier.
ptd Informs about the target device and is a structure of type DVTARGETDEVICE. The value of this field is nil if the data is device independent. We will always set this value to nil in this article.
dwAspect The view aspect. In all the cases we will consider, this value will be DVASPECT_CONTENT meaning we should represent the data as an embedded object. Other possible values are DVASPECT_THUMBNAIL, DVASPECT_ICON and DVASPECT_DOCPRINT.
lindex Tells us when the view aspect must be split across page boundaries. Often, and in all cases considered here, lindex is -1 indicating that all the data should be displayed.
tymed Describes the media type of the storage medium. This is one of the TYMED_* enumeration values discussed above.

We will use TFormatEtc structures in one of two ways:

  1. We will examine structures filled in by Windows to learn about a data object's format.
  2. We will set up our own structures to request objects in a required format.

That concludes the discussion of data objects. Armed with this knowledge we can move on to the next section where we discuss how to interogate, and extract data from, data objects.