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.

Multicast

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

Multicast RPC — call a function on server AND all clients

Custom Event → Replicates = 'Multicast'. When called on server, executes on server and all connected clients. Use for visual effects, sounds, animations that all players should see.

RPC

Remote Procedure Call — a function call sent across the network. Server / Client / Multicast versions exist.

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.

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.

MultiGate — distribute execution across multiple outputs

Object pooling pattern for frequently spawned actors like projectiles

Pattern: PoolManager with Array<AActor*> AvailablePool. Init: loop SpawnActor N times → SetActorHiddenInGame(true) → SetActorEnableCollision(false) → Array_Add to pool. GetFromPool: Array_Length > 0 → remove last → SetHidden(false) → SetCollision(true) → SetActorLocation → return. ReturnToPool: SetHidden(true) → SetCollision(false) → Array_Add back.

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.

LineTraceMulti — raycast returning multiple hits