Broadcasting Events to Clients via Multicast RPCs

In this lesson, you will learn how to use Multicast RPCs to trigger non-persistent visual and auditory feedback across all connected clients simultaneously.

The Role of Multicast RPCs

When you need to play a sound effect, spawn a particle burst, or trigger a camera shake that doesn't affect the game's state (meaning it doesn't need to be saved or tracked), using ReplicatedVariables is overkill and inefficient. Instead, we use Multicast RPCs.

A Multicast RPC is a function called on the server that instructs every connected client to execute that same function locally. It is "fire-and-forget"—if a client isn't connected at the exact moment the server fires the event, they will miss the effect entirely. This makes it perfect for "cosmetic" events that are transient in nature.

Implementation Strategy

To implement a multicast, you define a function in your Actor's header file with the NetMulticast keyword. Note that while you can call a Multicast RPC from a client, it is best practice to have the Server trigger the event. This ensures the server acts as the authoritative "source of truth" to orchestrate the effect for everyone.

// In your header file
UFUNCTION(NetMulticast, Reliable)
void Multicast_PlayImpactEffect(FVector Location);

// In your implementation file
void AMyActor::Multicast_PlayImpactEffect_Implementation(FVector Location)
{
    // This runs on the Server and every Client
    UGameplayStatics::SpawnEmitterAtLocation(this, ImpactParticleTemplate, Location);
    UGameplayStatics::PlaySoundAtLocation(this, ImpactSound, Location);
}

Critical Considerations

• Reliable vs. Unreliable: Use Reliable for critical feedback, but be cautious; heavy use of reliable multicasts can saturate bandwidth. For simple visual polish like particle effects, the Unreliable keyword is preferred, as losing a single frame of an effect is often imperceptible to the player.
• Authority: Always call the Multicast from an authoritative source (the Server). If a client calls a Multicast, it will only execute locally, failing to propagate to others. By routing your effect through a Server RPC first, you ensure consistent synchronization across the network.

Key Takeaway: Use Multicast RPCs for transient visual and audio events that don’t change the game state, ensuring they are triggered by the Server to guarantee synchronization for all clients.