Managing Movement Modes and State Transitions

In this lesson, you will learn how to synchronize complex movement modes like swimming, flying, and falling across the network to ensure your character’s physics and animations remain consistent for all players.

The Challenge of Distributed Physics

In a networked environment, the CharacterMovementComponent (CMC) manages a state machine that tracks movement modes (e.g., MOVE_Walking, MOVE_Falling, MOVE_Flying). When a player transitions between these states—such as jumping off a ledge into a swim volume—the transition must be authoritative. If the client predicts the transition to MOVE_Swimming but the server disagrees (due to latency or collision logic), the character will jitter or "rubber-band" as the server forces a correction.

Synchronizing State Transitions

To handle these transitions effectively, we rely on the CMC’s built-in replication logic, which tracks the MovementMode variable. However, when building custom modes or complex logic, you must ensure that state changes originate on the Server.

When you want to trigger a transition (e.g., entering a custom MOVE_CustomSliding state), you should never update the variable directly on the client. Instead, follow this pattern:

1. Request: The client sends a Server RPC to notify the server of the intent to change states.
2. Validate: The server performs its own check (e.g., checking if the character is touching the floor) to ensure the state change is legitimate.
3. Apply: The server calls SetMovementMode(), which internally handles the replication of the new mode to all clients.

// Within your Character class
void AMyCharacter::RequestSwimTransition()
{
    // Always validate on the server to prevent cheating/desync
    if (GetLocalRole() < ROLE_Authority)
    {
        Server_SetMovementMode(MOVE_Swimming);
    }
}

void AMyCharacter::Server_SetMovementMode_Implementation(EMovementMode NewMode)
{
    GetCharacterMovement()->SetMovementMode(NewMode);
}

By allowing the server to define the state, the CharacterMovementComponent automatically handles the synchronization of position, velocity, and animation parameters. When the server updates the MovementMode, the state is sent via the replicated movement packet, allowing proxies to interpolate smoothly between the physics states without conflicting with the server’s authoritative simulation.

Key Takeaway: Always rely on the server to authorize state transitions and let the CharacterMovementComponent’s native replication handle the broadcast of mode changes to ensure physics consistency across all clients.