Saving variables between game sessions

An Addon may need to save settings and data between game sessions - that is, some information may need to persist through a user log out. To enable this, the addons may specify a number of variables to be saved to disk when the player's character logs out of the game, and restored when the character logs back in. Variables that are saved and restored by the client are called SavedVariables.

Summary: to save a global variable FOOBAR, add  or   to an addon's .toc file.

Specifying which variables to save
To tell the WoW client that you want a variable to persist through log out, you need to add it to your addon's .toc file. There are two directives you may add to your .toc file, both should be followed by a colon and a comma-delimited list of variable names in the global environment (for most addons, this means variables that haven't been defined using the local keyword) that the addon wants to persist.
 * : Variables listed after this directive are saved on a per-account basis: if any of the characters on that account logs in, those variables will be restored. This may be more useful for global addon settings, or addons that implement profiles one can freely switch between.
 * : Variables listed after this directive are saved on a per-character basis: a separate copy of the variable is stored and restored for each character. This may be more useful for simple per-character options or history data.

The loading process
The variables saved by those directives are not immediately available when your addon loads; instead, they're loaded at a later point. The client fires events to let addons know that their saved variables were loaded.
 * 1) WoW FrameXML code is loaded and executed.
 * 2) Addon code is loaded and executed.
 * 3) Saved variables for one addon a time are loaded and executed, then ADDON_LOADED event is fired for that addon.
 * 4) VARIABLES_LOADED event is fired to let addons know that all saved variables were loaded.

Addons should generally use ADDON_LOADED to determine when their saved variables are accessible; the first argument of the event is the name of the addon for which it is being fired; VARIABLES_LOADED should be avoided as it is will not be fired for load-on-demand addons.

Saving to disk
The client saves variables to disk automatically when you log out, disconnect, quit the game, or reload your user interface. If an addon needs to make last-minute updates before the variables are saved, the PLAYER_LOGOUT event can be used: it notifies addons that the character is about to log out, and lets you alter your saved variables before they are serialized.

Code Example
To illustrate the concepts, let's consider a simple addon, HaveWeMet, shown below. The greets your characters when you log on: if it's seen you log into that character before, it outputs "Hello again, ", and it if has not, it outputs "Hi; what is your name?" to the chat frame. When its slash command, /hwm, is used, it tells the player how many characters it has met before.

There are two pieces of information that need to persist between sessions: the number of characters the addon has met, and whether it has met any particular character. To save the count, a global variable, HaveWeMetCount is used (and saved on a per-account basis through #SavedVariables); while HaveWeMetBool is saved per-character and used to determine whether the addon has seen this character before.

Common Pitfalls
There are a few common issues beginners may experience:
 * Variables are saved and loaded in the global environment : If you want to save a local value, you have to first read it from the global environment (_G table) on ADDON_LOADED, then return it into the global environment before the player logs out
 * Saved variables are loaded after the addon code is executed : They cannot be accessed immediately, and will overwrite any "defaults" the addon may place in the global environment during its loading process.
 * Only some variable types may be saved : Strings, booleans, numbers and tables are the only variable types that will be saved (functions, userdata and coroutines will not). Circular references in tables may not be preserved.
 * Saving tables : Tables are a great way to avoid having to use a large number of names in the global namespace. However, they may be more difficult to initialize to default values when your addon is updated and you add or remove a key. Multiple saved variables that reference the same table will each create a separate (but identical) instance of the table, and as such will no longer point to the same table when they are loaded again.

Storage
Saved variables are stored on a per-account basis in three file classes:
 * WTF\Account\ACCOUNTNAME\SavedVariables.lua - Blizzard's saved variables.
 * WTF\Account\ACCOUNTNAME\SavedVariables\AddOnName.lua - Per-account settings for each individual AddOn.
 * WTF\Account\ACCOUNTNAME\RealmName\CharacterName\AddOnName.lua - Per-character settings for each individual AddOn.

Deleting the WTF folder, or simply moving its contents will therefore reset the settings for all of your addons.