[Twisted-Python] A question about ITransport.write()

[Lian Cheng]

Hi, all

As a Twisted newbie, I read the Twisted documentation, and found that:

   1. ITransport.write() doesn't return a deferred.
   2. IProtocol doesn't have a dataSent() callback or something alike.

Since ITransport.write() is a non-blocking call, how can I be informed 
when data sent by ITransport.write() is actually sent successfully?

Cheers
Cheng