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"Message receipts" -- plugin for Tkabber.


I. The Idea

This plugin implements XEP-0184 "Message receipts": in
one-to-one chat sessions it is able to request confirmations of
reception of outgoing messages and reply to such requests coming
from remote party.
Reception status is tracked for each message placed in the chat
log window of a chat session using special icon: when the
message is sent, the icon depicts "unconfirmed" status, when the
confirmation arrives for this message, the icon associated with
this message is changed to depict "confirmed" status.

Receipts are only sent when all these conditions hold:
* Receipt is requested (attached to a message).
* Replying receipt requests isn't explicitly prohibited using
  the "reply" Customize option (see below).
* The receipt request is attached to a message which is either
  received in a MUC's private chat or is originated from a user
  which is subscribed to our presence.
In all other cases receipt requests are silently ignored.
This is done in order not to leak presence information.

Note that unlike some other IM clients Tkabber doesn't wait for
receipt to arrive -- it just changes the status icon
associated with particular message when its receipt is received.


II. Configuration

This plugin can be configured using Tkabber's Customize
mechanism. Two configuration options are available under the
"Plugins -> Message Receipts" group of settings (also available
under "Chat -> Message Receipts"):

* ::plugins::options::receipts(request) -- Attach request for
  receipt to each outgoing chat message.
  Default is true.

* ::plugins::options::receipts(reply) -- Issue message receipts
  for messages that request them.
  Default is true.
  Note that issuing of receipts is further restricted based on
  privacy considerations (see above).


III. Limitations

Some functionality is missing. See TODO for details.


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