About CallbackHandler

[Layback_]

Hi all,

I'm not sure whether this question is okay to be asked at this forum, so if I'm violating rules, please let me know.

So, I have been studying about addons for about a week and noticed that many of (probably most of) the addons utilizes a library called 'CallbackHandler'.

According to CallbackHandler overview page, it says...

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.

With my lack of understanding, I still don't get how this works and what it actually does.

Could anyone please provide me some brief explanation with an example?

Thank you.

[p3lim]

In short, fake events.

When a library does something and the addon using the library wants to know when that finished or had results, it can assign a callback function using CallbackHandler (if the library has support for it).

For example, the library LibResInfo will fire callbacks whenever it thinks that a resurrect is in progress or has finished in some way, and addons can use CallbackHandler to assign a function to respond to such an event.

More information: https://en.wikipedia.org/wiki/Callba...programming%29

[Layback_]

Originally Posted by p3lim In short, fake events. When a library does something and the addon using the library wants to know when that finished or had results, it can assign a callback function using CallbackHandler (if the library has support for it). For example, the library LibResInfo will fire callbacks whenever it thinks that a resurrect is in progress or has finished in some way, and addons can use CallbackHandler to assign a function to respond to such an event. More information: https://en.wikipedia.org/wiki/Callba...programming%29

Hi p3lim,

So, is it like a particular function can accept a callback function as one of its parameter (or argument) and this callback function can be triggered(?) based on which event has been fired?

[Phanx]

It works almost exactly like the built-in RegisterEvent/OnEvent system:

local MyFrame = CreateFrame("Frame")
 
MyFrame:RegisterEvent("SOME_EVENT")
 
MyFrame:SetScript("OnEvent", function(self, event, arg1, arg2)
    print(event .. " fired with arguments " .. arg1 .. " and " .. arg2)
end)

versus:

local MyFrame = CreateFrame("Frame")
 
local LibSomething = LibStub("LibSomething-1.0")
LibSomething.RegisterCallback(MyFrame, "SomethingHappened")
 
function MyAddon:SomethingHappened(event, arg1, arg2)
    print(event .. " fired with arguments " .. arg1 .. " and " ... arg2)
end

If you're asking how to use CallbackHandler directly, eg. in a library:

local LibSomething = LibStub:NewLibrary("LibSomething-1.0", 1)
 
-- Set up the callback system:
local CallbackHandler = LibStub("CallbackHandler-1.0")
LibSomething.callbacks = CallbackHandler:New(LibSomething)
 
-- Fire an event named "SomethingHappened" with arguments 52 and "lolcats":
LibSomething.callbacks:Fire("SomethingHappened", 52, "lolcats")

If you want more details about how it works internally, just read the code for CallbackHandler-1.0 and/or any library that uses it. It's pretty straightforward.

[Seerah]

Even addons use it, not just libraries. Especially addons with a plug-in system. You can look at my sStats addon, too, if you wish.

[Layback_]

Thank you guys!

I honestly didn't get 100%, but I got the concept of it.

Should do some practice by myself and see how I go

[Lombra]

You can also think of it as a simple function call, but instead of calling a specific function, it calls all functions that are named appropriately.

Fire("doStuff") will call all functions that you registered using RegisterCallback("doStuff").