chan postevent, introduced in Tcl 8.5, indicates to the given channel, which must have been created with chan create, that the channel is now ready for a read and/or write operation.
EventSpec is a (non-empty) list of event types to post. Supported event types are read and write.
Prior to Tcl 8.7, chan postevent should be scheduled using after rather than called directly because calling it from within one of the refchan api procedures could result in the channel becoming unresponsive because an event handler might already have been in progress. As of Tcl 8.7, this scheduling is automatically done internally.