CallbackHandler is a back-end utility library that makes it easy for a library to fire its events to interested parties. It removes the need for addons to be aware of e.g. AceEvent.

The one remaining use for AceEvent Messages is messages that do not have a fixed source - ones that multiple libraries or addons can fire.

Including CallbackHandler-1.0 into your project

Are you a Library?

if using the WoWAce repositories

setup an external pointing to svn://svn.wowace.com/wow/callbackhandler/mainline/trunk/CallbackHandler-1.0 in the .pkgmeta file
disable nolib creation by adding enable-nolib-creation: no to the .pkgmeta file
set up your <library>.toc file to load CallbackHandler-1.0.xml (in case you support stand alone loading of the lib)
don't load CallbackHandler-1.0 via the .xml file ment for embedded loading of your lib
don't set CallbackHandler-1.0 as X-embeded or OptDep

otherwise

get a copy of the current version
copy CallbackHandler-1.0.lua into your library's folder
set up your <library>.toc file to load CallbackHandler-1.0.lua (in case you support stand alone loading of the lib)
don't load CallbackHandler-1.0 via the .xml file meant for embedded loading of your lib
don't set CallbackHandler-1.0 as X-embeded or OptDep

Are you an AddOn?

if using the WoWAce repositories

set up an external pointing to svn://svn.wowace.com/wow/callbackhandler/mainline/trunk/CallbackHandler-1.0 in the .pkgmeta file
set up your <addon>.toc or embeds.xml file to load CallbackHandler-1.0.xml
don't set CallbackHandler-1.0 as X-embeded or OptDep

otherwise

get a copy of the current version
copy CallbackHandler-1.0.lua into your addon's folder or a subfolder of it
set up your <addon>.toc or embeds.xml file to load CallbackHandler-1.0.lua
don't set CallbackHandler-1.0 as X-embeded or OptDep

Mixing in the CallbackHandler functions in your library or AddOn

MyLib.callbacks = MyLib.callbacks or 
    LibStub("CallbackHandler-1.0"):New(MyLib)

This adds 3 methods to your library:

MyLib.RegisterCallback(self, "eventName"[, method, [arg]])
MyLib.UnregisterCallback(self, "eventname")
MyLib.UnregisterAllCallbacks(self)

Make sure that the passed in self is your addon, and not the library itself, so the double-colon syntax will not work.

The MyLib.callbacks object is the callback registry itself, which you need to keep track of across in-game upgrades of your own library. (You do make sure that your library upgrades gracefully, right? ;-) )

Firing events

Assuming your callback registry is "MyLib.callbacks", firing named events is as easy as:

MyLib.callbacks:Fire("EventName", arg1, arg2, ...)

All arguments supplied to :Fire() are passed to the functions listening to the event.

Advanced uses

Renaming the methods

You can specify your own names for the methods installed by CallbackHandler:

MyLib.callbacks= MyLib.callbacks or 
    LibStub("CallbackHandler-1.0"):New(MyLib, 
        "MyRegisterFunc", 
        "MyUnregisterFunc", 
        "MyUnregisterAllFunc" or false
    )

Giving false as the name for "UnregisterAll" means that you do not want to give users access to that API at all - it will not be installed.

Tracking events being used

In some cases, it makes sense to know which events are being requested by your users. Perhaps to enable/disable code needed to track them.

CallbackHandler will always call callbacks:OnUsed() and :OnUnused() when an event starts/stops being used:

function MyLib.callbacks:OnUsed(target, eventname)
    -- "target" is == MyLib here
    print("Someone just registered for "..eventname.."!")
end

function MyLib.callbacks:OnUnused(target, eventname)
    print("Noone wants "..eventname.." any more")
end

"OnUsed" is only called if the event was previously unused. "OnUnused" is only called when the last user unregisters from an event. In other words, you won't see an "OnUnused" unless "OnUsed" has been called for an event. And you won't ever see two "OnUsed" in a row without "OnUnused" in between for an event.

Multiple event registries

As you may or may not know, CallbackHandler is the workhorse of AceEvent-3.0. It is used twice in AceEvent: once for in-game events, which cannot be fired by users, and once for "messages", which can be fired by users.

Providing multiple registries in AceEvent was as easy as:

AceEvent.events = AceEvent.events or 
    LibStub("CallbackHandler-1.0"):New(AceEvent, 
        "RegisterEvent", "UnregisterEvent", "UnregisterAllEvents"
    )

AceEvent.messages = AceEvent.messages or 
    LibStub("CallbackHandler-1.0"):New(AceEvent, 
        "RegisterMessage", "UnregisterMessage", "UnregisterAllMessages"
    )

AceEvent.SendMessage = AceEvent.messages.Fire

Of course, there is also some code in AceEvent to do the actual driving of in-game events (using OnUsed and OnUnused), but this is really the core of it.

Sorry

If the code examples above look mucked up, it's because CurseForge are still using the CSS style "white-space:normal;" when they should be using "white-space: pre-wrap;" for their <pre> HTML tags. /Mikk, november 2019