Event Dispatcher

A typed broadcast — bind one or many listeners, fire once, all run. Use for loose coupling (e.g. health changed → UI update).

Delegate

A type-safe function pointer used by Event Dispatchers and bindings. Bind, broadcast, unbind.

Multicast

An RPC the server calls that runs on every connected client. Use for one-shot effects (explosions, sounds).

LineTraceSingle — raycast single hit by channel

UKismetSystemLibrary::LineTraceSingle. WorldContextObject is hidden. End pin is world position, not direction.

Blueprint communication: 4 methods ranked by coupling

1. DIRECT REFERENCE (tightest coupling): Get reference → CastTo<BP_Other> → call function. Use when: actor A always knows about actor B (e.g. Player → HUD). 2. CAST via GetAllActorsOfClass (loose): find → cast → call. Use when: you need to find actors dynamically. 3. EVENT DISPATCHER (loose): Owner calls dispatcher → listeners receive event. Use when: one actor notifies many others (e.g. death notification). 4. BLUEPRINT INTERFACE (loosest): Call interface message → any implementing actor responds. Use when: multiple different actor types should respond to same action (e.g. 'Interact'). 5. GAME INSTANCE (persistent): Store shared data in GameInstance → GetGameInstance → Cast. Use when: data must survive level loads.

PlayAnimation on widget — play UMG widget animation

Line trace raycast for interaction system

Pattern: Input trigger → LineTraceSingle(CameraLocation, CameraLocation + ForwardVector * Distance, Visibility, false, MakeArray(Self), None) → Branch(ReturnValue) → True: BreakHitResult → HitActor → CastTo InteractableInterface → call Interact. End pin is world POSITION not direction. Use DrawDebugType ForOneFrame during dev.

Multiplayer: Server RPC for damage, Multicast RPC for effects

Pattern for shooting in multiplayer: ON CLIENT: InputAction Shoot → ServerRPC_Shoot (Custom Event, Run On Server, Reliable). ServerRPC_Shoot (runs on SERVER): LineTraceByChannel → Branch(hit) → ApplyDamage → MulticastRPC_PlayEffect(ImpactPoint). MulticastRPC_PlayEffect (Custom Event, Multicast, Unreliable): SpawnNiagaraSystemAtLocation(NS_BulletImpact, ImpactPoint). Key rules: - Server RPC: Custom Event → Replicates='Run On Server', Reliable=true. - Multicast: Custom Event → Replicates='Multicast', Reliable=false (VFX can be dropped). - Client RPC: Custom Event → Replicates='Run On Owning Client'. - Never directly modify replicated vars on clients — always go through Server RPC.

Event dispatcher pattern for decoupled communication between actors

Pattern: Actor A declares EventDispatcher OnScoreChanged(int NewScore). Actor B (listener) BeginPlay: GetActorOfClass(A) → BindEvent(OnScoreChanged) → custom event handler. Actor A calls OnScoreChanged.Broadcast(Score) when score changes. Bind in BeginPlay, Unbind in EndPlay to avoid dangling references.

SpawnEmitterAtLocation — spawn particle effect at world location

UGameplayStatics::SpawnEmitterAtLocation. For Niagara, use SpawnSystemAtLocation.