Reaction System

Event-driven AI reactions - parry, dodge, counter - that respond to game events in real time.

The flip side of actions. Actions are polled - reactions are event-driven.

When to Use This

• AI that parries incoming attacks
• Dodge-rolls triggered by projectile detection
• Counter-attacks after blocking
• Any "respond to this stimulus" behavior

The system doesn't decide when to react - you do. It decides what to react with and whether the reaction is allowed.

Core Concepts

Actions vs Reactions

How It Works

1. Something happens - your code detects a stimulus (damage, projectile, proximity)
2. Ask the system - call EvaluateBestReaction() with an optional category filter
3. System evaluates - checks gates (enabled, cooldown, BlockReactions tag, RequiresTags, BlockTags), groups by priority, picks weighted random winner
4. You fire it - call ExecuteReaction() with the chosen reaction ID and a context payload
5. Lifecycle managed - the system handles ability activation, tag management, cooldowns, timeout, and completion

ReactionSet (Data Asset)

Identity

Execution

Tags

Scoring

Action Interaction

Selection Logic

EvaluateBestReaction(CategoryTag):
  1. Filter by CategoryTag (skip if empty)
  2. Gate checks per reaction:
     - Is enabled?
     - Off cooldown?
     - Under MaxConsecutiveUses?
     - SEC.Reaction.BlockReactions not on ASC?
     - All RequiresTags present?
     - No BlockTags present?
  3. Group survivors by Priority (highest first)
  4. Weighted random pick among top-priority group
  → FChosenReaction { ReactionId, AbilityTag, Priority, Weight }

ReactionEvaluationComponent

Core API

Delegates

Execution Flow

ExecuteReaction("Parry", Context)
  ├── 1. Validate gates (enabled, cooldown, BlockReactions, RequiresTags, BlockTags)
  ├── 2. Cancel current action (if bCancelCurrentAction)
  ├── 3. Apply AddTags to ASC
  ├── 4. Pack FSECExecutionContext into FGameplayEventData
  │      ├── Auto-populate InstigatorTags from instigator ASC
  │      ├── Auto-populate TargetTags from target ASC
  │      └── TargetData passed through directly
  ├── 5. Activate ability (ByEvent or ByTag)
  ├── 6. Start cooldown, record use
  ├── 7. Fire OnReactionStarted
  └── Listen for: AbilityEndTag event OR AbilityTimeout
       └── CompleteExecution
           ├── Remove AddTags from ASC
           └── Fire OnReactionCompleted

Reaction-to-Reaction Blocking

Reaction A (Parry):
  AddTags:   [SEC.State.ReactionActive]
  BlockTags: []

Reaction B (Dodge):
  AddTags:   [SEC.State.ReactionActive]
  BlockTags: [SEC.State.ReactionActive]   ← can't fire while Parry is active

FSECExecutionContext

SECReactionSetComponent (Pawn)

Resolution Priority

1. Runtime Override — SetRuntimeOverride(ReactionSet). Highest priority. Use for boss phase transitions.
2. Equipped Weapon — If the weapon actor implements ISECWeaponReactionSetProvider, calls GetWeaponReactionSetForRole(RoleTag).
3. Config Role — Role-specific ReactionSet from EnemyAIConfig → RoleReactionSets[CurrentRole].
4. Config Default — Fallback from EnemyAIConfig → DefaultReactionSet.
5. Component Default — SECReactionSetComponent → DefaultReactionSet property (set in Blueprint).
6. None — No ReactionSet found. AI cannot react.

This is the same resolution chain as SECActionSetComponent. Learn one, know both.

Swap Modes

Replicated State

Delegates

Weapon Reaction Set Provider

// Your weapon actor implements ISECWeaponReactionSetProvider
UReactionSet* GetWeaponReactionSetForRole(FGameplayTag RoleTag) override
{
    // Return a role-specific reaction set, or a single set for all roles
    return MyWeaponReactionSet;
}

// Tell the pawn about the weapon:
Pawn->ReactionSetComponent->SetEquippedWeapon(WeaponActor);
 
// Clear to revert to Config-based resolution:
Pawn->ReactionSetComponent->SetEquippedWeapon(nullptr);

Key API

Quick Setup

1. Create Asset: Right-click → Miscellaneous → Data Asset → ReactionSet
2. Define Reactions: Fill in ReactionId, AbilityClass, AbilityTag, AbilityEndTag
3. Assign to Config: Set DefaultReactionSet on your EnemyAIConfig, or add role-specific sets to RoleReactionSets
4. Pawn Component: If using EnemyCharacterBase, SECReactionSetComponent is already created. Otherwise, add it to your pawn.
5. Controller Component: Add ReactionEvaluationComponent to your AIController (already present on EnemyControllerBase)
6. Trigger from your logic:

// In your Blueprint or C++ (e.g., OnDamageReceived):
FSECExecutionContext Context;
Context.Target = DamageInstigator;
Context.Magnitude = DamageAmount;
Context.EventTag = SEC_Stimulus_MeleeHit;
Context.TargetData = USECTargetDataLibrary::MakeTargetDataFromDirection(HitDirection);
 
FChosenReaction Chosen = ReactionComp->EvaluateBestReaction(SEC_Reaction_Defensive);
if (Chosen.IsValid())
{
    ReactionComp->ExecuteReaction(Chosen.ReactionId, Context);
}

Gameplay Tags

Integration Points