# Reactions

Reactions are code objects that run when a message is fired - essentially a message handler attached to an instance of data.

Reactions can be defined only for messages that have a definition (a *MsgDef* instance).
Some data definitions inherit `MsgReactionsPreset` and can have *reactions* (message handlers) attached to any of their instances.
Ractions can also have a *target* which can filter out messages.

# Message definitions

Message definitions (or *MsgDef*) are presets which describe a message or event.
Their *Id* must match the message or event they describe, as well as their *Params*.
Finally the message or event can be associated with a specific *Target* they are concerned with.

# Message reaction presets

Message reaction presets (or *MsgReactionPreset*) are presets that contain code which can react to messages being fired, similar to global message handlers ([see *OnMsg*](LuaMessages.md.html)).
This message handler is associated with the preset instance and it's lifecycle.
When the reaction function is called, the first parameter is always *preset*.

# Unit reaction presets

This type of data can contain event handlers that are attached to units (their *Target* is *Unit*), while the preset is associated with the unit in some way.
Instead of reacting to global [Lua messages](LuaMessages.md.html), these handlers react to events the units can fire themselves by calling `self:CallReactions(event_id, ...)`.
When the reaction function is called, the first two parameters are always *preset, unit*.

## Reference

Note that *Unit* inherits *ReactionObject*.

**function ReactionObject:AddReactions(instance, list, insert_locations)**
:	Registers all handlers from the reaction objects in *list*. These reactions are associated with the object *instance*.

**function ReactionObject:RemoveReactions(instance)**
:	Removes the reactions associated with the object *instance*.

**function ReactionObject:AddEventReaction(event_id, instance, handler)**
:	Registers a single event *handler* (as opposed to a *Reaction* object). The handler will react to the event *event_id*.

**function ReactionObject:RemoveEventReactions(event_id, instance)**
:	Removes all event handlers associated with this *event_id* and *instance*.

**ReactionObject:CallReactions(event_id, ...)**
:	Emits an event, which will call all registered reactions/handlers.

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