View source for API:RegisterEventHandler

{{LuaApi|group=Interface}}
Registers a Lua function to be called whenever a UI event is raised.

<pre>
RegisterEventHandler(eventId, functionName)
</pre>

== Arguments ==
;eventId : <code>number</code> – Id of the event, usually a value from <code>SystemData.Events</code>.
;functionName : <code>string</code> – Name of the Lua function to call, as a dotted path from the global table, for example <code>"MyAddon.OnLoadingEnd"</code>. It is a name, not a function value.

== Returns ==
Nothing.

== Details ==
* Each time the event is raised, the client looks up <code>functionName</code> and calls it. The handler receives the arguments carried by the event. Events raised with [[API:BroadcastEvent|BroadcastEvent]] carry none.
* One event can have any number of handlers, and one function can handle several events. Registering the same function for the same event a second time is refused with an error.
* The client remembers which UI module was being loaded or initialized when the handler was registered, and makes that module current while the handler runs.
* Handlers can register or unregister handlers while an event is being delivered. Each handler is called at most once per delivery.
* Handlers stay registered until [[API:UnregisterEventHandler|UnregisterEventHandler]] is called with the same event id and the same name.
* Handlers that belong to a window are registered with [[API:WindowRegisterEventHandler|WindowRegisterEventHandler]] instead; they are stored on the window, not in this list.

== Errors ==
These are written to the UI log; the call returns normally.
* <code>Error in function call 'RegisterEventHandler': Invalid Event Id</code> (event id 0)
* <code>Error in function call 'RegisterEventHandler': Invalid Script Name</code> (empty name)
* <code>Error in function call 'RegisterEventHandler': Script %s is already registered for event #%d</code>

== Example ==
From <code>easystem_actionbarclustermanager/source/actionbarclustermanager.lua</code>:
<pre>
RegisterEventHandler (SystemData.Events.PLAYER_CAREER_LINE_UPDATED, "ActionBarClusterManager.SpawnCluster")
RegisterEventHandler (SystemData.Events.LOADING_END,                "ActionBarClusterManager.SpawnCluster")
</pre>

From <code>interfacecore/interfacecore.lua</code>:
<pre>
RegisterEventHandler( SystemData.Events.RESOLUTION_CHANGED, "InterfaceCore.OnResolutionChanged")
</pre>

== See also ==
* [[API:UnregisterEventHandler|UnregisterEventHandler]]
* [[API:BroadcastEvent|BroadcastEvent]]
* [[API:WindowRegisterEventHandler|WindowRegisterEventHandler]]