How to Write a WoW Addon

So you want to write a WoW addon? Well, you better get used to crappy docs, old websites, and skimpy examples. The fact that it’s all community driven and nothing official from Blizzard doesn’t make it any easier. I recently wrote my first addon for classic called Track Sales, so I know your pain. It’s not all doom and gloom though, there’s really only a few key concepts to understand and everything else falls into place. Though this guide isn’t specific to retail or classic, please use the retail version to follow along. The version I wrote this with is 80200.

First and foremost, if you’re struggling with your addon, the best advice I could give you is not to google, not to read the docs, not to read this post, but to read other addons! I’ve found hands down that has been the best way to learn how to do things. Generally, googling and reading docs are great tools but when developing Track Sales nothing was better than seeing how other people did things. Aim for smaller addons that will be easier to understand. Reading other people’s code is always an excellent way to learn (for anything). You’ll be amazed at the things you can learn just from reading someone else’s code. Clone your favorite open source project and check it out someday!

Hello Azeroth

We’ll start with a Hello World example. In _retail_/Interface/Addons create a folder named MyAddon, inside create a lua file named MyAddon.lua. Place this statement print("Hello Azeroth") in the file.

Next we need to tell WoW about our addon, this is done in the toc file. Create a new file named MyAddon.toc and add the below snippet.

# Your client version is probably different than 80200
# In game run "/run print((select(4, GetBuildInfo())))" and use that for the interface number

## Interface: 80200
## Version: 1.0.0
## Title: My Addon 
## Notes: My First Addon
## Author: You 

MyAddon.lua
# additional lua files

This tells WoW what client version our addon supports (if the current version is newer than 80200 you’ll see the “addon out of date” message), our addon name, and the files our addon needs. We’ll come back to the toc file later but for now load up WoW. On the character select screen check to see My Addon is listed in the addon menu. If it isn’t then double check that everything is correct. When you load into the game you should see “Hello Azeroth” appear in the chat.

Key Concepts

Building an addon can really be boiled down to these concepts. The rest is filling in the details of your addon.

Events
Hooks
Saved Variables
Ace (3rd party helpers, optional)
Console Commands
UI (sorry, I won’t be covering this, see Extended Character Stats for ideas)
Testing

Events

Events are fired when various actions happen in the game world ie open mail, learned a new spell, etc. We can register to events with the code below.

MyAddon = { }

local frame = CreateFrame("Frame")
frame:RegisterEvent("MAIL_SHOW")
frame:RegisterEvent("AUTOFOLLOW_BEGIN")

-- this syntax doesn't work in classic * 
frame:SetScript("OnEvent", function(this, event, ...)
    MyAddon[event](MyAddon, ...)
end)

function MyAddon:MAIL_SHOW()
    print("Mail Shown")
end

function MyAddon:AUTOFOLLOW_BEGIN(...)
    local player = ...
    print("Following "..player)
end

MyAddon = { } is essentially just a namespace. Lua doesn’t actually have namespaces but it’s the same concept as many languages such as c#, java, and c++. With this we can avoid using global variables.

Unfortunately, I can’t share much about CreateFrame other than “this is how you register events”. You’ll also use frames if you want to create a UI.

I have no idea what that crazy SetScript syntax is I just know that’ll dispatch all events efficiently. It works by convention so you’ll have to make sure the function name matches the event name. If you don’t want to be restriced to that convention AceEvent can help.

When you open the mailbox and start following a player you should see the messages appear. You can find other events here.

Hooks

Hooks are very similiar to events except that hooks fire before or after the original function (the built in WoW function). Hooks allow you to modify the parameters that get passed to the original function or if you wish, not call the original function at all…so you could really mess things up. “Secure Hooks” are the safe version to use. To create a hook we’ll use hooksecurefunc which is a “safe post-hook”, for your use case you may need pre or post hooks, docs.

MyAddon = { }

hooksecurefunc("PickupContainerItem", function(...) MyAddon:PickupContainerItem(...) end)
hooksecurefunc("TakeInboxMoney", function(mailId) MyAddon:TakeInboxMoney(mailId) end)

function MyAddon:PickupContainerItem(container, slot)    
    local _, _, _, _, _, _, link = GetContainerItemInfo(container, slot)

    print(link)
end

function MyAddon:TakeInboxMoney(mailId)   
    local _, _, sender, subject, money = GetInboxHeaderInfo(mailId)    
    
    print("Sender "..sender, "Subject "..subject, "Money "..money)
end

GetContainerItemInfo and GetInboxHeaderInfo are WoW apis, you can find other functions here. If you LEFT click an item in your inventory you should see it’s item link printed. To test TakeInboxMoney send some gold to an alt and loot it from mail. Don’t autoloot it! That would need the AutoLootMailItem hook. If it’s not working, try disabling other addons and try again. There can be some intricacies to hooks, see the notes section. Alternatively, try the AceHook package.

Saved Variables

SavedVariables are values saved to disk that can be used between game sessions. The values are stored in the WTF folder (Warcraft Text Files not WTF) upon logout. There are two types of saved variables, SavedVariables and SavedVariablesPerCharacter. They are saved on a per account and per character basis respectively. Don’t get tricked like I did, they’re nothing but global variables! You just have to add the saved variables to the toc file.

## SavedVariables: AccountDB
## SavedVariablesPerCharacter: CharacterDB

# you can use multiple variables comma delimited

In your addon you’ll usually want to initialize these the first time someone logs in with it. The PLAYER_LOGIN event (OnEnable if using Ace) is probably the best place to do this because I found some of the WoW apis aren’t ready at other times. Let’s add a login counter to demonstrate.

MyAddon = { }

local frame = CreateFrame("Frame")

-- trigger event with /reloadui or /rl
frame:RegisterEvent("PLAYER_LOGIN")

frame:SetScript("OnEvent", function(this, event, ...)
    MyAddon[event](MyAddon, ...)
end)

function MyAddon:PLAYER_LOGIN()

    self:SetDefaults()

    AccountDB.loginCount = AccountDB.loginCount + 1  
    CharacterDB.loginCount = CharacterDB.loginCount + 1

    print("You've logged in "..AccountDB.loginCount.." times")
    print(UnitName("Player").." logged in "..CharacterDB.loginCount.." times")
end

function MyAddon:SetDefaults()
    
    if not AccountDB then 

        AccountDB = {
            loginCount = 0
        }

        print("Global Initialized")
    end

    if not CharacterDB then 

        CharacterDB = {
            loginCount = 0
        }

        print("Character Initialized")
    end
end

Ace

If you’ve been reading about addons you’ve probably heard about Ace. At first I thought there was something special between WoW and Ace but nope, Ace is nothing but a 3rd party wrapper library over the WoW API. It’s designed to make some things simpler to do, and I say some because for your use case Ace might just get in the way (I ditched AceDB). Only use Ace if you really need it, some modules have a steep learning curve and it’s easy to get bogged down in the details of Ace instead of focusing on the important parts – writing your addon! In this post I’ll show how to use AceHook and AceConsole. Ace Console will be used for chat commands and also contains Ace’s Print function.

Download Ace from the site and extract the files somewhere. In your addon folder create a new folder named libs. Copy the folders LibStub, AceAddon-3.0, AceConsole-3.0, and AceHook-3.0 into libs. Next create a new file named embeds.xml in your addon’s root folder and add the following

<Ui xmlns="http://www.blizzard.com/wow/ui/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.blizzard.com/wow/ui/ ..\FrameXML\UI.xsd">
	<Script file="libs\LibStub\LibStub.lua"/>
	<Include file="libs\AceAddon-3.0\AceAddon-3.0.xml"/>
	<Include file="libs\AceConsole-3.0\AceConsole-3.0.xml"/>
	<Include file="libs\AceHook-3.0\AceHook-3.0.xml"/>
</Ui>

Now we’ll tell WoW about these libraries by adding the embeds.xml to the toc file

## Interface: 80200
## Version: 1.0.0
## Title: My Addon 
## Notes: My First Addon
## Author: You

MyAddon.lua
embeds.xml

With Ace embedded now we can aceify the lua file

--the ace way of initializing addons, add the ace modules 
--you want to use as trailing parameters in :NewAddon()
MyAddon = LibStub("AceAddon-3.0"):NewAddon("MyAddon", "AceConsole-3.0", "AceHook-3.0")

function MyAddon:OnInitialize()				

    self:SecureHook("PickupContainerItem")

    local frame = CreateFrame("Frame")
    frame:RegisterEvent("MAIL_SHOW")
    frame:RegisterEvent("AUTOFOLLOW_BEGIN")

    frame:SetScript("OnEvent", function(this, event, ...)
        MyAddon[event](MyAddon, ...)
    end)

    self:Print("Hello from Ace Console")
end

function MyAddon:OnEnable()
    --initialize Saved Variables and other start up tasks
end

function MyAddon:PickupContainerItem(container, slot)    
    _, _, _, _, _, _, link = GetContainerItemInfo(container, slot)

    self:Print(link)
end

function MyAddon:MAIL_SHOW()
    print("Mail Shown")
end

function MyAddon:AUTOFOLLOW_BEGIN(...)
    local player = ...
    print("Following "..player)
end

Now your addon is bootstrapped with Ace. What changed is how the addon is initialized and the new OnInitialize and OnEnable methods. OnInitialize can be used to register events, create hooks, and anything else you might need to do on startup.

If it all worked you should see Hello from Ace Console printed when you log in. Open the mailbox and follow random players, those should still work too.

Console Commands

To use slash commands in your addon you can use Ace Console to register commands and parse arguments.

MyAddon = LibStub("AceAddon-3.0"):NewAddon("MyAddon", "AceConsole-3.0")

function MyAddon:OnInitialize()				

    self:RegisterChatCommand("myaddon", "SlashCommands")
end

-- /myaddon 1 2 3
function MyAddon:SlashCommands(args)

    local arg1, arg2, arg3 = self:GetArgs(args, 3)	
    
    self:Print(arg1, arg2, arg3)
end

You can choose to handle arguments manually or use Ace Config, it all depends on your use case. I’d start with manual first.

UI

Sorry, I didn’t write a UI for Track Sales so I won’t be covering it. You can check out an early version of Extended Character Stats for some general ideas, visit curse for the latest. I advise you do this piece last and make sure the backend works first. Developing the UI and backend together is already hard enough no matter what environment you’re in.

Testing

The hardest part about developing an addon is testing it. Sadly, there’s no WoW sandbox or “Addon Dev Island” you can go to and have unlimited ability to test. Everything will have to be done manually in game. One nice thing is that you can call your addon’s functions any time with /run MyAddon:FunctionName(). With this it’s usually best to isolate the WoW api calls as best you can and have most of your addon’s logic in functions that you can easily test with /run. You’re also going to have to sprinkle Print statements everywhere. There’s a couple debug commands that may be useful as well. To have code changes take effect run /reloadui or /rl.

What Tripped Me Up

A couple things that weren’t immediately obvious to me was how to call functions in other files and how to reference Ace modules in other files. This is done by variables that get passed in from WoW to every lua file. You can access them by placing local addonName, ns = ... at the top of any file. addonName will be a string containing your addon name and ns acts as a namespace. This thread details it well.

To see an example add two new files MyAddonModule.lua and MyAddonHelper.lua Don’t forget to add them to the toc file!

-- MyAddon.lua

MyAddon = LibStub("AceAddon-3.0"):NewAddon("MyAddon")

function MyAddon:Test()				
    
    MyAddonModule:SayHello()
end

-- MyAddonModule.lua

MyAddonModule = MyAddon:NewModule("Module", "AceConsole-3.0")

local addonName, ns = ...

function MyAddonModule:SayHello()				

    self:Print("Hello from Module")
    ns:SayHelloHelper()
end

-- MyAddonHelper.lua

local addonName, ns = ...

function ns:SayHelloHelper()				

    print("Hello from Helper")
end

Test with /run MyAddon:Test()

With these key concepts you’ll be able to build any addon. The tricky part is figuring out which hooks and events you need. ctrl + f is going to be your best friend here, becoming familiar with the naming convention helps as well. There’s always forums like the WoW addon reddit if you have questions.

Joe Gannon