property BytesTransferred : LongInt
The number of bytes transferred so far in the current file.
When transmitting, this number is sometimes only an estimate. The uncertainty comes from the fact that the protocol doesn't know when a particular byte has actually been transferred. BytesTransferred is the number of bytes the protocol has transferred to the output buffer of the communications device, minus the number of bytes that the driver reports are currently in the buffer.
Unfortunately, this calculation is still imperfect because it's impossible to know how much of the output buffer holds actual file data and how much holds overhead characters needed by the protocol. Each protocol has a few simple rules it uses to estimate this proportion, which in practice yield good estimates.
This property is most useful within an OnProtocolStatus event handler. See "Protocol status" in the printed documentation for more information.
See also: BytesRemaining