The OnProgress event is automatically called from a Read operation. The event occurs after a block of data has been read from the input stream, and before the data has been decompressed. 

The OnProgress event is of type TNotifyEvent. When your procedure is called, the Sender parameter will be set to the object itself. You can use the OnProgress event to update a user interface control, such as a progress indicator. You can use the Position property, inherited from TStream, to determine the number of raw (uncompressed) bytes read from the input stream so far.