WowAce

What is the enhancement in mind? How should it look and feel?

The idea is to have AceEvent-3.0 send messages when message are used on unused (based on CallbackHandler :OnUsed and :OnUnused hooks).

This would allow addons providing optional messages to setup appropriately.

Please provide any additional information below.

This could be implemented like this in AceEvent-3.0.lua :

function AceEvent.messages:OnUsed(_, message)
    self:Fire('AceEvent30_OnMessageUsed', message)
end

function AceEvent.messages:OnUnused(_, message)
    self:Fire('AceEvent30_OnMessageUnused', message)
end