trigger

• Halo: ReachYes
• Halo 4Yes
• Halo 2: AnniversaryYes

The core executable element. Each trigger is an event handler: the engine evaluates its conditions, and if they all pass, it executes the trigger's actions. When any condition fails, execution halts for the current evaluation — subsequent conditions and actions in the same trigger are skipped.

Compile-time limits including trigger count are listed under Limits on each megalo version page — for example 49, 73, 107, or 107 (MCC). trigger is full-script-only — it cannot be added in base-derived scripts.

Syntax

trigger <kind> [<name>]
	[<conditions and actions>]
end

• kind — determines the execution context (see Trigger kinds)
• name — optional; unique within the script. When omitted, the trigger is anonymous.

trigger initialization
	; runs once when the variant loads
end

trigger player
	condition player_died current_player enemy
	action set_score add 1 player killing_player
end

trigger bro_spawn_location
	action object_set_invincibility current_object true
end

Trigger kinds

The kind controls which implicit context variables are available and how often the trigger runs.

Kind	Context variable	Runs…
general	(none)	Once per tick, globally
player	current_player	Once per player, per tick
random_player	current_player	Once per tick, for a random player
team	current_team	Once per team, per tick
object	current_object	Once per object, per tick
initialization	(none)	Once when the variant loads
local_initialization	(none)	Once per machine at load
host_migration	(none)	After host migration
double_migration	(none)	After double host migration
object_death	current_object	When a bound object is destroyed
local	(none)	Local-only scope
pregame	(none)	During lobby / pre-game phase

Object-filter triggers

When the trigger kind is a map_object label name, the trigger runs once per matching object on the map, with current_object set to that object:

map_object invasion_respawn_phase_1
	label "inv_res_p1"
end

trigger invasion_respawn_phase_1
	action set_respawn_filter current_object no_one
	action object_set_invincibility current_object true
end

This is equivalent to an object-kind trigger filtered to objects matching that label or type.

Action scope

Inside trigger blocks and action for_each sub-triggers, the body is an action scope. Only these keywords are valid:

Element	Purpose
condition	A predicate; halts the block if false
or	Chains the previous and next condition with logical OR
action	An imperative statement
temporary	Declares a temporary variable for this block
begin	Opens an explicit sub-block (rare)
end	Closes the current scope or sub-block

trigger player
	condition player_died current_player enemy or
	condition if current_player.is_leader equal_to true
	action set_score add 1 player killing_player
end

or chaining — or must appear immediately after a condition, and must be followed by another condition. Only conditions can follow or; actions or other keywords produce a compile error.

Capacity — A script may contain at most 512 conditions and 1024 actions across all triggers.

temporary in pregame — temporary declarations are not allowed inside pregame triggers.

begin / end — Explicit sub-blocks implement if/elseif-style branching. See begin.

Execution model

Triggers execute in the order they are defined in the script, on the same tick. For player, team, and object triggers, the engine iterates over every instance and runs the trigger body once per instance.

Within a trigger, conditions and actions are evaluated top to bottom:

1. Each condition is tested in order.
2. If a condition evaluates to false, the trigger halts — remaining conditions and actions are skipped for this evaluation.
3. If all conditions pass (or there are no conditions), actions run in order.

Actions can appear before conditions. Conditions after an action still halt the trigger if they fail — actions that already ran are not undone.

trigger player
	action timer_set_rate current_player.game_start_vo 1

	condition if current_player.heard_game_start == 0
	condition timer_expired current_player.game_start_vo

	action submit_incident game_start_slayer player current_player player none
	action set current_player.heard_game_start = 1
end

Interval

By default, triggers run every tick. An interval line limits execution frequency:

trigger general
	interval 5
	; runs once every 5 seconds instead of every tick
end

for_each sub-triggers

The action for_each statement embeds a nested trigger body that runs once per target:

action for_each player
	condition if current_player.team equal_to attackers
	action set current_player.fireteam set_to 0
end

action for_each invasion_objective
	condition if current_object.user_data equal_to k_territory_id_main_switch
	action navpoint_set_visible current_object everyone
end

Target	Iterates over…
player	Every player
team	Every team
object	Every object
general	Runs once (no iteration)
<map_object label>	Every object matching that filter

Inside a for_each block, conditions that evaluate to false skip the current iteration (like continue in C) rather than halting the entire trigger.

action for_each invasion_objective
	temporary object buy_zone current_object

	action for_each player
		condition object_in_area current_player current_object
		action set total_in_area add 1
	end
end

Temporary variables

Temporary variables can be declared at the top of a trigger or inside a for_each or begin block:

trigger player
	temporary number temp 0
	temporary object my_body current_player

	condition player_died current_player enemy
	temporary player killer none
	action player_death_get_killing_player current_player killer
end

See Variable model — Temporary variables.

Special triggers

• initialization — runs once when the gametype loads. Used for setup before gameplay (objectives, timers, intro sounds).
• host_migration — runs after the host changes mid-game. Used to restore state lost during migration.
• pregame — runs during the lobby phase. Only a whitelisted subset of conditions and actions is valid; temporary is not allowed.

trigger initialization
	action play_sound team defenders bone_cv_ph1_intro
	action timer_set_rate buy_zone_timer 1
end

trigger host_migration
	action for_each invasion_gates
		condition if current_object.user_data equal_to buy_zone.user_data
		action delete_object current_object
	end
end

See also

• condition — condition syntax and operators
• action — action syntax and families
• begin — explicit sub-blocks
• Example scripts — annotated trigger walkthroughs