AddOn loading process

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This technical detail describes the order in which addon code is loaded when the client first starts up, or reloads the interface, as well as what information is available to the Lua environment at various points in the process.

The general outline:

  1. When the client first starts, Interface\AddOns directory is scanned, and a list of files and addon dependencies are built.
  2. Addon code is executed after the player selects a character and presses 'Enter World'.
  3. After all addon code has been loaded, the saved variables are executed. ADDON_LOADED fires after each addon's saved variables have been loaded.

Order of addon loading

The client scans through the Interface\addons directory, storing a list of present files, and loading all .toc files into memory; files that were not found during this step cannot be loaded by the game later. This step only happens on client start-up, which makes it impossible to install additional addons / add additional graphics and/or data files to addons without restarting the client.

The alphabetical name of the addon and dependency information in the addon's .toc files are used to compute the order in which addons will be loaded when the player logs in. In most cases, an individual addon may assume that all of the addons that it depends on will be loaded first. The order begins alphabetically, but it will branch off to follow the dependencies of loading addons as needed. The load order can therefore branch many times if addons depend on addons which in turn have their own dependencies. This makes alphabetical order a rather unreliable method of knowing when your addon will be loaded relative to other addons (which can be important for hooking functionality). A few addons, such as "_NPCScan", prefix the true name of the addon with a symbol that Blizzard ranks 'higher' than the letter 'A'. This almost completely ensures that that addon will be one of the first addons loaded.

Order of files loaded within an addon

Files within an addon are loaded in the order they're listed in the addon's .toc file. Files included through XML <Include file="src.xml" /> or <Script file="src.lua" /> are loaded at the time the tag is encountered while parsing the XML file. XML OnLoad script handlers execute when all of a widget's children have been created.

When the addon code is first loaded during this step, only basic information about the player -- name, class, race and realm -- is available.

To illustrate the loading order, consider the following addon code example:

LoadingOrder.toc

##Interface: 70200
##Title: Loading Order Demo
file1.lua
file2.xml
file3.lua

file1.lua

print("This loads first")

file2.xml

<Ui>
  <Frame name="TemplateFrame" virtual="true">
    <Frames><Frame><Scripts><OnLoad>print("Inherited elements of the frame are processed first."); </OnLoad></Scripts></Frame></Frames>
  </Frame>
  <Frame inherits="TemplateFrame">
   <Frames>
    <Frame>
     <Scripts><OnLoad>print("First child frame's OnLoad fires first")</OnLoad></Scripts>
    </Frame>
    <Frame>
     <Scripts><OnLoad>print("Second child frame's OnLoad fires second")</OnLoad></Scripts>
    </Frame>
    <Scripts><OnLoad>print("Parent frame's OnLoad fires after its children's.")</OnLoad></Scripts>
   </Frames>
  </Frame>
  <Script file="file2.5.lua"/>
</Ui>

file2.5.lua

print("Files included in XML are executed as they are encountered");

file3.lua

print("This concludes this presentation");

Order of events fired during loading

After the addon code has been loaded, the loading process can be followed by registering for various events, listed here in order of firing. This information is very important because many addons rely on information that is not available when addons first load, such as buffs, spells, talents, quests, pets, pvp information, etc. By monitoring one of the following events with a blank frame, you can trigger the appropriate "OnEvent" handler and execute code that is dependent on that information as soon as it is available.

  1. ADDON_LOADED
    • This event fires whenever an addon has finished loading and the SavedVariables for that addon have been loaded from their file.
  2. SAVED_VARIABLES_TOO_LARGE (Error Condition)
    • Generally will not fire. This event indicates an error state where the SavedVariables of an addon failed to load due to an out-of-memory error. (The old error state was a client crash!)
    • The upshot here is that your addon could be in a state where the saved variables did not load. This event's purpose is to indicate that you are in this error state.
    • If you are in this state your addon's SavedVariables will NOT be saved back to disk at the next logout. This was done with the reasoning that it will prevent valid data from accidentally being wiped by defaults.
    • It is possible for an addon's account wide SavedVariables to load, but for the character specific SavedVariables to fail, or vice versa. There is no way to detect the difference between no variables loaded and some.
  3. SPELLS_CHANGED
    • This event fires shortly before the PLAYER_LOGIN event and signals that information on the user's spells has been loaded and is available to the UI.
  4. PLAYER_LOGIN
    • This event fires immediately before PLAYER_ENTERING_WORLD.
    • Most information about the game world should now be available to the UI.
    • All sizing and positioning of frames is supposed to be completed before this event fires.
    • Addons that want to do one-time initialization procedures once the player has "entered the world" should use this event instead of PLAYER_ENTERING_WORLD.
  5. PLAYER_ENTERING_WORLD
    • This event fires immediately after PLAYER_LOGIN
    • Most information about the game world should now be available to the UI. If this is an interface reload rather than a fresh log in, talent information should also be available.
    • All sizing and positioning of frames is supposed to be completed before this event fires.
    • This event also fires whenever the player enters/leaves an instance and generally whenever the player sees a loading screen

Since Patch 3.0.2, VARIABLES_LOADED has not been a reliable part of the addon loading process. It is now fired only in response to CVars, Keybindings and other associated "Blizzard" variables being loaded, and may therefore be delayed until after PLAYER_ENTERING_WORLD. The event may still be useful to override positioning data stored in layout-cache.txt.

Somewhere around Patch 5.4.0, PLAYER_ALIVE stopped being fired on login. It now only fires when a player is resurrected (before releasing spirit) or when a player releases spirit. Previously, PLAYER_ALIVE was used to by addons to signal that quest and talent information were available because it was the last event to fire (fired after PLAYER_ENTERING_WORLD), but this is no longer accurate.

Load On Demand behavior

Load on Demand addons cannot rely on most of the event sequence being fired for them; only ADDON_LOADED is a reliable indication that the saved variables for your LoD addon have been loaded.

See also

  • Handling events for information on how to sign up to receive event notifications