Transferring Data Contained in Property Sets
Storing information about the contents of a document is useful, and a standard
that would allow all files to share the same information with all applications
would be even better. However, this is only one example of how property sets
can be used.
Another use for them is in the transfer of data between OLE objects or
applications. To better understand the process of transferring data between two OLE
objects, consider the following example. A trader on Wall Street relies on large
amounts of data to make important financial decisions regarding trades of
securities. This market data must be delivered to the trader's workstation in real
time (or at least very soon after it was generated).
Assume that, at any given moment during an open day on Wall Street, the trader
is interested in the opening, high, low, current, and last closing price of an
instrument (a specific stock), and the volume of trading on that stock so far
that day. The symbol for the stock will also be needed.
As noted earlier, a property in a property set has three attributes: the
property identifier, a value type indicator, and a value. In this example, the
following properties can be defined:
Property Identifier
| Type Indicator
| Value Represented
|
PID_SYMBOL
| VT_LPSTR
| Zero-terminated string
|
PID_OPEN
| VT_CY
| Currency value
|
PID_CLOSE
| VT_CY
| Currency value
|
PID_HIGH
| VT_CY
| Currency value
|
PID_LOW
| VT_CY
| Currency value
|
PID_LAST
| VT_CY
| Currency value
|
PID_VOLUME
| VT_I4
| 32-bit unsigned integer
|
Without worrying about how the data is transferred between the data server and
the trader's application, think about the data transferred and its format.
Every property set must have a FMTID associated with it.
A single data element representing the stock of XYZ during the day might look
like this:
PID_SYMBOL contains "XYZ"
PID_OPEN contains 42-3/4
PID_CLOSE contains 42-3/4
PID_HIGH contains 49-1/8
PID_LOW contains 40-7/8
PID_LAST contains 47-3/8
PID_VOLUME contains 123,032
During a trading day on Wall Street only a few of the properties of a single
instrument change. The opening price doesn't change all day, and the low price
may go unchanged for several hours. Using property sets as the data transfer
format allows applications to transfer only the data that has changed. For
example, at some point during the day the Data Server may update the Traders
Application by transferring the following StockQuote property set:
PID_SYMBOL contains "XYZ"
PID_HIGH contains 49-1/8
PID_LAST contains 49-1/8
PID_VOLUME contains 23,321
This example demonstrates how property sets can be used as a data transfer
format, allowing sparse data representation. Transferring only the changed data
enhances the overall performance of the trader's workstation.
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