Ue Gameplay Framework

UE Gameplay Framework

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Context Check

Read .agents/ue-project-context.md before proceeding. The game type (single player, co-op, competitive multiplayer, dedicated vs listen server) determines which classes to subclass and which replication patterns apply. Resolve: single-player or multiplayer? Dedicated or listen server? What are you implementing?

Class Responsibility Map

Each class exists on specific machines for specific reasons. Getting this wrong is the primary source of multiplayer bugs.

AGameModeBase / AGameMode — Server Only

Exists on: Server and standalone only. Never instantiated on clients.

Why server-only: GameMode is the authoritative referee. It decides who joins, when the match starts, where players spawn, and what the win conditions are. Client execution would allow cheating via local state manipulation.

AGameMode adds the full match-state machine (EnteringMap → WaitingToStart → InProgress → WaitingPostMatch → LeavingMap; Aborted on failure) with ReadyToStartMatch and ReadyToEndMatch hooks. Use AGameModeBase for lobby/simple games, AGameMode for match flow.

Key API from source (GameModeBase.h):

// Class assignments — set in constructor
TSubclassOf<APawn>             DefaultPawnClass;
TSubclassOf<AGameStateBase>    GameStateClass;
TSubclassOf<APlayerController> PlayerControllerClass;
TSubclassOf<APlayerState>      PlayerStateClass;
TSubclassOf<AHUD>              HUDClass;
uint32                         bUseSeamlessTravel : 1;

// Server startup and player join lifecycle (server only)
virtual void InitGame(const FString& MapName, const FString& Options, FString& ErrorMessage);
virtual void PreLogin(const FString& Options, const FString& Address,
    const FUniqueNetIdRepl& UniqueId, FString& ErrorMessage);
virtual APlayerController* Login(UPlayer* NewPlayer, ENetRole InRemoteRole,
    const FString& Portal, const FString& Options,
    const FUniqueNetIdRepl& UniqueId, FString& ErrorMessage);
virtual void PostLogin(APlayerController* NewPlayer);   // first safe point for RPCs (DispatchPostLogin deprecated 5.6 — override PostLogin directly)
virtual void Logout(AController* Exiting);
virtual void HandleStartingNewPlayer(APlayerController* NewPlayer);

// Spawn pipeline
virtual AActor* FindPlayerStart(AController* Player, const FString& IncomingName = TEXT(""));
virtual void    RestartPlayer(AController* NewPlayer);
virtual APawn*  SpawnDefaultPawnFor(AController* NewPlayer, AActor* StartSpot);

// Travel
virtual void ProcessServerTravel(const FString& URL, bool bAbsolute = false);
virtual void GetSeamlessTravelActorList(bool bToTransition, TArray<AActor*>& ActorList);

AGameStateBase / AGameState — Server + All Clients

Exists on: Everywhere. Fully replicated.

Why everywhere: Clients cannot read GameMode (it does not exist on them). Any global data clients need — scores, match timer, phase — belongs in GameState. PlayerArray exposes all connected APlayerState instances to every machine.

Key API from source (GameStateBase.h):

// All PlayerStates, always replicated
UPROPERTY(Transient, BlueprintReadOnly)
TArray<TObjectPtr<APlayerState>> PlayerArray;

// The GameMode class (not instance) replicated to clients
UPROPERTY(Transient, BlueprintReadOnly, ReplicatedUsing=OnRep_GameModeClass)
TSubclassOf<AGameModeBase> GameModeClass;

// Server-authoritative clock, automatically synced
virtual double GetServerWorldTimeSeconds() const;
virtual bool   HasBegunPlay() const;
virtual bool   HasMatchStarted() const;
virtual bool   HasMatchEnded() const;

Custom replicated match data:

UCLASS()
class AMyGameState : public AGameStateBase
{
    GENERATED_BODY()
public:
    UPROPERTY(Replicated, BlueprintReadOnly) int32 TeamAScore;
    UPROPERTY(Replicated, BlueprintReadOnly) int32 TeamBScore;
    UPROPERTY(ReplicatedUsing=OnRep_MatchTimer) float MatchTimeRemaining;

    virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const override;
};

void AMyGameState::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const
{
    Super::GetLifetimeReplicatedProps(OutLifetimeProps);
    DOREPLIFETIME(AMyGameState, TeamAScore);
    DOREPLIFETIME(AMyGameState, TeamBScore);
    DOREPLIFETIME(AMyGameState, MatchTimeRemaining);
}

APlayerController — Server (all) + Owning Client (own only)

Exists on: Server holds one per connected player. Each client holds only its own. Remote clients do not see other players' PlayerControllers.

Why this split: The PlayerController bridges one human to the server. Both ends run it for client-side prediction and server validation. A client has no reason to know another player's input state.

Key API from source (PlayerController.h):

TObjectPtr<APlayerCameraManager> PlayerCameraManager;  // camera, local only
TObjectPtr<APawn>                AcknowledgedPawn;      // server-confirmed possession
TObjectPtr<AHUD>                 MyHUD;                 // local only

uint32 bShowMouseCursor : 1;
uint32 bEnableStreamingSource : 1;  // drives World Partition loading for this viewport

void SetInputMode(const FInputModeDataBase& InData); // FInputModeGameOnly, UIOnly, GameAndUI
virtual void PlayerTick(float DeltaTime);            // only ticked locally
virtual void SetupInputComponent() override;

`SetupInputComponent` on PlayerController is for non-pawn input: spectator actions, UI shortcuts, or global keybinds that persist across possession changes. For pawn-specific input, override APawn::SetupPlayerInputComponent() instead — see ue-input-system.

Enhanced Input setup:

void AMyPlayerController::BeginPlay()
{
    Super::BeginPlay();
    if (IsLocalController())
    {
        if (auto* Sub = ULocalPlayer::GetSubsystem<UEnhancedInputLocalPlayerSubsystem>(GetLocalPlayer()))
            Sub->AddMappingContext(DefaultMappingContext, 0);
        SetInputMode(FInputModeGameOnly());
    }
}

RPC patterns:

UFUNCTION(Server, Reliable, WithValidation) void ServerRequestRespawn();   // client → server
UFUNCTION(Client, Reliable)                void ClientNotifyMatchStart();  // server → client

Possess/UnPossess (server-authority required):

// Take control of a new pawn — must run on server
PlayerController->Possess(NewPawn);
// Release the currently possessed pawn
PlayerController->UnPossess();

Listen-server dual-role: On a listen server, the host's PlayerController is both ROLE_Authority and locally controlled. Guard dual-role logic with IsLocalController() checks. This is a common source of bugs where code assumes authority implies non-local (i.e., code written for dedicated servers runs incorrectly on a listen server host).

ClientTravel — connect this client to a different server:

PlayerController->ClientTravel(TEXT("127.0.0.1:7777"), TRAVEL_Absolute);

ServerTravel — move all players to a new map (called from GameMode, server only):

GetWorld()->ServerTravel(TEXT("/Game/Maps/NewMap?listen"));

AController — Shared Base

AController is the base class for both APlayerController and AAIController. It owns the pawn possession interface (Possess, UnPossess, GetPawn) and the rotation used to drive pawn orientation (ControlRotation). Subclass APlayerController for human players and AAIController for AI.

APlayerState — Server + All Clients (Always Relevant)

Exists on: Server and all clients. Marked always-relevant so it replicates to everyone regardless of distance.

Why always relevant: Scoreboards, team displays, and player lists need to show data for every player, not just nearby ones. PlayerState survives pawn death — when a pawn is destroyed and respawned, the PlayerController keeps its PlayerState, preserving accumulated stats.