Updating availability of an instant messaging contact

摘要

A contact is determined to be unavailable if a predetermined period of time has elapsed since an instant message was sent to the contact and an expected notification for the instant message was not received during that predetermined period of time by the device that sent the instant message. If the expected notification is a delivered notification, then the contact is deemed unreachable. If the expected notification is a read notification and a delivered notification for the instant message has been received by the device that sent the instant message, then the contact is deemed busy.