AddOn programming tutorial

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← WoW AddOnWorld of Warcraft UI programming is simple and daunting at the same time.

It is simple in that it uses some very simple programming constructs and a well-defined method of creating visual elements.

It is daunting because there is so much to learn, so much detail available, but very little in the way of guides through the forest. It is also daunting because the inevitable errors which crop up can be so hard to find.

The goal of this AddOn programming tutorial is to teach by examples and by didactic instruction, leading the new AddOn programmer from the very simpliest AddOns to complex interactions with the XML user interface.

These pages have their genesis in the frustration of an experienced programmer in trying to make his way over the myriad of obstacles that others have already passed and forgot. Where to find this, how to do that. Some of the most basic stuff should be covered, but is left out of most other presentations encountered. As this author encounters the pits and barbs and finds solutions and resources they will be shared in this AddOn programming tutorial. So, let's get on with this.

Square One - Introduction[]

You have to start someplace, and the traditional place to start is with "Hello, World". To accomplish this first task you will need the following items:

WoW (World of Warcraft) installed
An editor that can work with and save pure text.

An AddOn lives in a very specific place. To find that place, first go to the WoW directory (also sometimes called a folder) using whatever file management software you have available. On a Windows platform, that would be Windows Explorer (not Internet Explorer). The program is usually installed on the C: drive of the computer, under the directory named "Program Files". From pretty much any address bar, simply type "C:\Program Files\World of Warcraft" and press enter.

On a Macintosh system, you would use Finder; and on Linux there is a similar mechanism. Whatever you use, you are looking for the installed location for WoW. This is usually at "/Applications/World of Warcraft/" on a Macintosh.

Once you find the WoW directory, there is another directory called Interface and within that is another called AddOns. AddOns is the home of all AddOns in WoW. Each AddOn has its own directory under the AddOns directory.

Now go ahead and do these steps:

Create a directory for your AddOn named "HelloWorld"
Create three files named: HelloWorld.toc, HelloWorld.lua, HelloWorld.xml

Note that the only difference in the names is the suffix. These denote, in order, Table of Contents, the Lua code file, and the XML user interface visual elements file. The name of your AddOn directory and the name on the .toc file must match.

Now to put something into each of these files, and this is where the editor that works with text files comes in. The files must be saved as text, not as some document format. Notepad is a pure text editor, but it is very limited. There are many out there, some of which can help with your programming efforts by understanding the syntax of the Lua language (more on that later). My personal favorite text editor is UltraEdit.

For additional Lua Editors check out the additional Lua editors page, with which Notepad++ is highly recommended; it comes with all the basic features of Notepad along with more advanced features for progressed programmers including syntax highlighting for LUA.

The .toc or Table of Contents[]

This file tells WoW about your AddOn: what files to load and what order to load them in. Later you will want to peruse the TOC format page for all of the gory details about what you could put into this file. For now we are just going to give you some basic stuff to include.

Using your trusted text file editor, place the following into the HelloWorld.toc file and save it:

## Interface: 30300
## Title: Hello, World!
## Notes: My first AddOn
HelloWorld.lua
HelloWorld.xml

See the line with ## Interface: 30300 in it? That value is obsolete, and you will need to put in the value that matches the client version. That value is:

## Interface: 11200

for vanilla WoW on a private server (pre-classic wow launch).

If you already have some AddOns installed, you can just look into their .toc files and see what they used. Another way is to visit http://wow.go-hero.net and look there. You will want the TOC Build number for the latest Public Server version listed.

What is this number? It is the user-interface (UI) version for the AddOn. The "30300" is version 3.03.00 (or 3.3.0). This number tells WoW that your AddOn is compatible with Blizzard UI level 3.3.0. If your UI number does not match the Blizzard UI number, your AddOn will be considered out of date. This is to minimize problems caused by old UI modifications hosing Blizzard's UI.

Starting from the first line we are saying:

The compatible UI version for this addon is 30300 (or 3.3.0)
The title of the addon to be displayed in game is "Hello, World!"
The description of the addon to be displayed in game are the notes
HelloWorld.lua file will be loaded.
HelloWorld.xml file will be loaded.

For more details on stuff you can put in here, please visit the TOC format page.

The .lua or Lua code file[]

The .lua files are where the main "what to do" instructions for the add-on reside. You will see a variety of terms for this such as logic and executable code (or simply "code"). Lua logic, or executable code or a script, does its thing in response to something that happens in the game. Things that happen in the game are called Events.

Events[]

There are two basic kinds of events. The first is when something happens in the game. This might be somebody saying something, something happening to your character, or another character's stats changing. Nearly everything that happens in the game causes events.

The second kind of event is when you do something to a UI Element (a UI Element is something on the screen and is affectionately called a widget. We'll get to that more in the next section). This second kind of event might be clicking on something in your bags or button bar. There is a technical difference between the two types of events, and we will discuss that as the tutorial progresses. For more detail on the first kind of event see Events (API) and for the second see Widget handlers.

These concepts are extremely important because absolutely nothing happens in the game except in response to an event. Furthermore, should you happen to write a piece of code that runs for an extended time (perhaps forever), absolutely nothing new will happen in the game. Your screen will be frozen and nothing will move. That would be classified as "not good".

So, how do you tell WoW that you are interested in a particular event? There are two ways: first, you can tell WoW which code to run when a particular event happens. This is called registering your event. Second, you can tell the XML to run a piece of code when a UI Element is manipulated (such as clicking on it or moving your mouse over it). Pieces of code that run in response to events are called functions.

Functions[]

Functions are groupings of code that accomplish a specific purpose. There are numerous predefined functions provided by WoW (called API functions), or you can make your own user-defined functions. While there are multiple ways to create functions in Lua, the easiest to understand looks like this:

<local> function function_name(<zero or more arguments>)
   ... code ...
end

The <local> is an optional keyword that limits the visibility of the function to a particular scope. Scope will be covered in more depth shortly.
The function_name is simply a name you make up so you can reference your function from other parts of your AddOn.
The <zero or more arguments> are ways to pass information into the function. This is what gives functions their power. Each time you call the function, you can supply a different set of arguments and get different results based upon them.
The ... code ... is where the work gets done in a function. Here is where you do calculations, comparisons, call other functions, etc. to get the task of the function done.
The end simply marks the end of the definition of the function.

Note that this only defines or declares the function. The function is not actually run until some other piece of code references (or calls) it.

For more information on functions, see the Lua 5.1 Reference Manual or Programming in Lua (first edition). Also see the Lua page which lists more Lua resources.

HelloWorld.lua[]

Now to continue with our Hello, World code example. Place the following into your HelloWorld.lua file and save it:

function HelloWorld() 
  print("Hello, World!"); 
end

You should understand everything in here by now. This function is named HelloWorld and it has zero arguments. The code part is simply the 'print("Hello, World!");' portion. And it ends with "end".

This is a fine piece of code, but by itself is useless unless something calls the function. Onward to UI Elements (aka Widgets).

The .xml or XML visual elements file[]

UI Elements, or Widgets, are all of the tiny bits of graphics that make up the User Interface. WoW uses XML to layout everything that you see on the screen. Additionally, when things happen (called Events, remember?) to the widgets on the screen, Widget Handlers can be called to perform whatever action you want. We will see shortly how we tell WoW which widgets we are interested in and which Events we want handled by which Widget Handler.

Blizzard XML format declared[]

For those of you who don't know, XML stands for eXtensible Markup Language and is a means of tagging content with identifiers. What the identifiers are and how they are organized can be defined in something called a Schema. In our case, we want to create XML documents that WoW will understand, so we will use the Schema provided by Blizzard for the Wow User Interface.

We declare that our document conforms to the Blizzard schema with the following bit of magic:

Inside the frame, one of the many things we can define are Scripts. Scripts are nothing more than small pieces of Lua code. Where we place the script determines when it will be invoked. Because Scripts live within a Frame, we include the 'Scripts' tag inside the 'Frame' tag. Notice the difference, in the 'Scripts' tag, the 's' sets it apart from the 'Script' tag.

 <Frame name="HelloWorldFrame">
  <Scripts>

  </Scripts>
 </Frame>

The various widgets have several Events that can occur; and if we want to declare a Widget Handler to process the event, we include the event name under the Scripts tag of the widget we are interested in. Not every widget has the same set of events. In this example, we are interested in an event named 'OnLoad'. The OnLoad event happens when the widget is loaded into the UI. For this example, we want the OnLoad event to run the script named HelloWorld. This script was defined in the HelloWorld.lua as a function.

  <Scripts>
   <OnLoad>
    HelloWorld();
   </OnLoad>
  </Scripts>

Take a look at the Widget handlers page for a list of widgets and the events you can write widget handlers for.

The complete HelloWorld.xml file should look like this:

<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="HelloWorld.lua"/>
  <Frame name="HelloWorldFrame">
   <Scripts>
    <OnLoad>
     HelloWorld();
    </OnLoad>
   </Scripts>
  </Frame>
</Ui>

There is an important item you should note in the above: the HelloWorld(); is the only piece which is NOT a tag or an attribute. It is important to note that content in a WoW .xml UI document is always a piece of code if it is not another set of tags and their associated attributes. The only valid place for a piece of code is under the tag for an event.

Having gotten this far, it is time to run your new add-on.

Running your HelloWorld AddOn[]

You should have a directory under your AddOns directory named "HelloWorld" and in that directory should be three files named HelloWorld.toc, HelloWorld.lua, and HelloWorld.xml. The contents of these three files should be EXACTLY as described above.

Now start WoW and log into your account, but don't select your character yet. Click the red 'AddOns' button on the lower left of the character-selection screen to see all of the AddOns WoW has detected; there will be one for every folder in your AddOns directory except for the AddOns starting with Blizzard_.

You should see your new HelloWorld in this list. The name should be yellow, and the checkbox to the left should be checked.

If the name is red and you see an Out of date message to the right, you probably didn't change the ## Interface: 30300 value as described above under The .toc or Table of Contents. Review that section and make the appropriate change. I do not recommend running your AddOns with the 'Load out of date AddOns' checkbox checked. That's asking for trouble as an AddOn that attempts to use old UI features can corrupt a new UI. Nearly every patch that changes the UI level has had problems with old AddOns that have not been updated to conform to the new UI standards.

If you don't see the name at all, make sure that you have placed the HelloWorld directory in the AddOns directory, which is in the Interface directory under the WoW installation location. On my system (Windows) it is 'C:\Program Files\World of Warcraft\Interface\AddOns\HelloWorld'. The files inside that directory should all start with 'HelloWorld' and have the .toc, .lua, and .xml endings.

A note for Windows users; make sure that you have not saved the files as HelloWorld.toc.txt, etc., as an option in Windows Explorer will hide the .txt on the end.

Please note the CaSe of the names. While Windows is insensitive to case for directory and file names, the case is important to other systems (e.g. Mac, Linux). Also, inside the game itself, WoW is sensitive to the case for the names of its variables and filenames. Keep the case the same to avoid problems.

Now you have a yellow Hello, World! showing up in your AddOn list. Note the ! in the name. The name of the AddOn shown in this list is taken from the ## Title: Hello, World! line in the HelloWorld.toc file. In the future, we will see how to change colors and languages.

If you move your mouse cursor over the Hello, World! name, you should see a tool-tip pop up with two lines in it. The first line is the same as the Title, and the second line is taken from the ## Notes: My first AddOn line in the .toc file. This can also be customized for color and language.

Cancel out of the AddOns display and enter the world with any of your characters. Once your character loads, you should see a message in the default chat window that says "Hello, World!".

Success!!

Review[]

You have created your first AddOn and have successfully run it. Now let's review a bit about what was accomplished.

The 'Hello, World!' text is taken from the line in the HelloWorld.lua file that reads 'print("Hello, World!");'. Wrapped around that is a function named "print" which is responsible for displaying the text in the default chat window.

The print function is inside a function we created called "HelloWorld" that had no parameters. Our function will do the same thing every time it is called.

The name of our function was then placed in the HelloWorld.xml file as the action to be taken when the Onload event of the Scripts tag of the Frame we created. We placed the name of our function in this specific place because we wanted our function to be executed (run, called, processed) when our Frame was fully loaded.

WoW knew that it should create our Frame because we placed the name of the .xml file into the .toc file. The .toc file must have the same name as the AddOn's directory and is the first of the files in our AddOn that Blizzard processes.

Inside the .toc file is where we tell WoW about our AddOn (the ## statements) and what files that need to be loaded. Every line that does not start with a ## is a file to be processed by WoW. The order in which the file names appear is important because WoW processes the files in the order listed.

Knowing this, we place the .lua file before the .xml file because we want the function HelloWorld declared (or defined) before we try to call it in the .xml file.

Even though this is a trivial example of an AddOn, important concepts and information have been covered. Further pages in this tutorial will cover other areas and expand upon topics initially presented here.

External links[]

Lua 5.1 Reference Manual - The definitive LUA
Programming in Lua (first edition)
Extensible Markup Language (XML) - The definitive XML
XML Schema Validation