/ue-gameplay-framework

UE Gameplay Framework

Target engine: UE 5.8. APIs below are verified against the 5.8 headers; older forms are listed under "Deprecated — do not use".

The gameplay framework is the set of classes the engine spawns for you — AGameModeBase, AGameStateBase, APlayerController, APlayerState, APawn, ACharacter, AHUD, UGameInstance — plus the join, spawn, match-state, travel and damage flows that wire them together. All of it ships in the Engine module (Classes/GameFramework, Classes/Engine, Classes/Kismet), so Build.cs needs "Core", "CoreUObject", "Engine". Add "EnhancedInput" when binding input, and "OnlineSubsystem" + "OnlineSubsystemUtils" (or "OnlineServicesInterface") only when touching sessions.

Context

Read .agents/ue-project-context.md if it exists (module names, conventions, enabled plugins, GAS/networking setup). Do not stop if it is missing.

Identify the area from the request and the codebase. Ask only when two plausible readings would produce different code.

Presence/authority matrix, the full class declarations these snippets come from, lifecycle order and inherited-property tables: references/framework-class-map.md.

Class Responsibility Map

APlayerState sets bReplicates = true and bAlwaysRelevant = true in its constructor, which is why scoreboards work at any distance.

GameMode (Server Only)

AGameModeBase is the minimal base. AGameMode adds the match state machine — use it when you need MatchState::EnteringMap → WaitingToStart → InProgress → WaitingPostMatch → LeavingMap (plus Aborted on failure).

Join sequence (server only)

// MyGameMode.cpp - include the header of every class named below
#include "MyGameMode.h"

AMyGameMode::AMyGameMode()
{
    DefaultPawnClass      = AMyCharacter::StaticClass();
    PlayerControllerClass = AMyPlayerController::StaticClass();
    GameStateClass        = AMyGameState::StaticClass();
    PlayerStateClass      = AMyPlayerState::StaticClass();
    HUDClass              = AMyHUD::StaticClass();
    bUseSeamlessTravel    = true;
}

bool AMyGameMode::ReadyToStartMatch_Implementation()
{
    return GetNumPlayers() >= MinPlayersToStart;
}

ChoosePlayerStart, FindPlayerStart, SpawnDefaultPawnFor, PlayerCanRestart, MustSpectate, ReadyToStartMatch and ReadyToEndMatch are BlueprintNativeEvent — override the generated _Implementation, never the bare name. Verbatim forms: virtual AActor* ChoosePlayerStart_Implementation(AController* Player), virtual AActor* FindPlayerStart_Implementation(AController* Player, const FString& IncomingName), virtual APawn* SpawnDefaultPawnFor_Implementation(AController* NewPlayer, AActor* StartSpot). Plain virtuals in the same pipeline: virtual void RestartPlayerAtPlayerStart(AController* NewPlayer, AActor* StartSpot), virtual void RestartPlayerAtTransform(AController* NewPlayer, const FTransform& SpawnTransform), virtual bool ShouldSpawnAtStartSpot(AController* Player). Spawn points are APlayerStart actors, selected by their FName PlayerStartTag.

Match state (AGameMode only)

Public: FName GetMatchState() const, virtual bool IsMatchInProgress() const, virtual void StartMatch(), virtual void EndMatch(), virtual void AbortMatch(), virtual void RestartGame(). Protected hooks: virtual void SetMatchState(FName NewState), virtual void OnMatchStateSet(), virtual void HandleMatchIsWaitingToStart(), virtual void HandleMatchHasStarted(), virtual void HandleMatchHasEnded(), virtual void HandleLeavingMap(), virtual void HandleMatchAborted().

SetMatchState is the only sanctioned way to move states; it pushes the new state onto AGameState::MatchState, which replicates to clients. Do not write MatchState directly.

GameState (Everywhere)

Clients cannot see the GameMode, so everything they need goes here.

// MyGameState.h
#pragma once

#include "CoreMinimal.h"
#include "GameFramework/GameStateBase.h"
#include "MyGameState.generated.h"

UCLASS()
class MYGAME_API AMyGameState : public AGameStateBase
{
    GENERATED_BODY()

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

    UPROPERTY(Replicated, BlueprintReadOnly, Category = "Match")
    int32 TeamAScore = 0;

    UPROPERTY(ReplicatedUsing = OnRep_MatchTimeRemaining, BlueprintReadOnly, Category = "Match")
    float MatchTimeRemaining = 0.f;

protected:
    UFUNCTION()
    void OnRep_MatchTimeRemaining();
};

// MyGameState.cpp - also include Net/UnrealNetwork.h

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

void AMyGameState::OnRep_MatchTimeRemaining()
{
    // Client-side reaction: refresh the timer widget.
}

Every ReplicatedUsing = X needs a matching UFUNCTION() void X(); declaration, and every Replicated/ReplicatedUsing property needs a DOREPLIFETIME line inside GetLifetimeReplicatedProps. Conditions, COND_*, push model and RPC rules belong to ue-networking-replication.

Inherited: TArray<TObjectPtr<APlayerState>> PlayerArray (not itself replicated; each APlayerState adds itself on every machine via AddPlayerState, which works because PlayerStates are always relevant). Inherited and replicated: TSubclassOf<AGameModeBase> GameModeClass (ReplicatedUsing = OnRep_GameModeClass), TSubclassOf<ASpectatorPawn> SpectatorClass, bReplicatedHasBegunPlay, ReplicatedWorldTimeSecondsDouble. Accessors: virtual double GetServerWorldTimeSeconds() const, virtual bool HasBegunPlay() const, virtual bool HasMatchStarted() const, virtual bool HasMatchEnded() const, virtual void AddPlayerState(APlayerState* PlayerState), virtual void RemovePlayerState(APlayerState* PlayerState). AGameState adds FName MatchState (ReplicatedUsing = OnRep_MatchState) and int32 ElapsedTime (replicatedUsing = OnRep_ElapsedTime).

PlayerController

// MyPlayerController.cpp
#include "MyPlayerController.h"
#include "EnhancedInputSubsystems.h"
#include "Engine/LocalPlayer.h"
#include "InputMappingContext.h"

void AMyPlayerController::BeginPlay()
{
    Super::BeginPlay();

    if (IsLocalController())
    {
        if (UEnhancedInputLocalPlayerSubsystem* Subsystem =
                ULocalPlayer::GetSubsystem<UEnhancedInputLocalPlayerSubsystem>(GetLocalPlayer()))
        {
            Subsystem->AddMappingContext(DefaultMappingContext, 0);
        }
        SetInputMode(FInputModeGameOnly());
    }
}

void AMyPlayerController::ServerRequestRespawn_Implementation()
{
    // Runs on the server.
}

bool AMyPlayerController::ServerRequestRespawn_Validate()
{
    return true;
}

Declarations for the above (full class body in the reference): virtual void BeginPlay() override;, virtual void SetupInputComponent() override;, virtual void OnPossess(APawn* aPawn) override;, virtual void OnUnPossess() override;, and UFUNCTION(Server, Reliable, WithValidation) void ServerRequestRespawn();.

AController::Possess and AController::UnPossess are virtual final; override OnPossess / OnUnPossess instead. APlayerController::SetupInputComponent() is a protected virtual declared on APlayerController itself — use it for input that outlives possession (menus, spectator keys, global shortcuts). Pawn-specific bindings go in APawn::SetupPlayerInputComponent; mapping contexts, actions, modifiers and triggers belong to ue-input-system.

Notable members: TObjectPtr<APawn> AcknowledgedPawn (server-confirmed possession), TObjectPtr<AHUD> MyHUD, TObjectPtr<APlayerCameraManager> PlayerCameraManager, TObjectPtr<UPlayerInput> PlayerInput (local only), TSubclassOf<UCheatManager> CheatClass, uint32 bShowMouseCursor:1, uint32 bEnableClickEvents:1, uint32 bEnableStreamingSource:1 (drives World Partition loading for this viewport), uint16 SeamlessTravelCount. Input modes FInputModeGameOnly, FInputModeUIOnly and FInputModeGameAndUI are applied with virtual void SetInputMode(const FInputModeDataBase& InData).

Listen-server dual role: the host's PlayerController has both ROLE_Authority and a local player. Guard with IsLocalController() as well as HasAuthority(); code written for a dedicated server often runs twice on a listen server host.

PlayerState

// MyPlayerState.h
#pragma once

#include "CoreMinimal.h"
#include "GameFramework/PlayerState.h"
#include "MyPlayerState.generated.h"

UCLASS()
class MYGAME_API AMyPlayerState : public APlayerState
{
    GENERATED_BODY()

public:
    virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const override;
    virtual void CopyProperties(APlayerState* PlayerState) override;

    UPROPERTY(Replicated, BlueprintReadOnly, Category = "Score")
    int32 Kills = 0;

    UPROPERTY(ReplicatedUsing = OnRep_TeamIndex, BlueprintReadOnly, Category = "Team")
    uint8 TeamIndex = 0;

protected:
    UFUNCTION()
    void OnRep_TeamIndex();
};

// MyPlayerState.cpp - also include Net/UnrealNetwork.h

void AMyPlayerState::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const
{
    Super::GetLifetimeReplicatedProps(OutLifetimeProps);
    DOREPLIFETIME(AMyPlayerState, Kills);
    DOREPLIFETIME(AMyPlayerState, TeamIndex);
}

void AMyPlayerState::OnRep_TeamIndex()
{
    // Client-side reaction: recolour the team badge.
}

void AMyPlayerState::CopyProperties(APlayerState* PlayerState)
{
    Super::CopyProperties(PlayerState);

    if (AMyPlayerState* Target = Cast<AMyPlayerState>(PlayerState))
    {
        Target->Kills     = Kills;
        Target->TeamIndex = TeamIndex;
    }
}

Without both the UFUNCTION() OnRep declaration and the DOREPLIFETIME entries the properties compile, run, and silently never replicate. Replication conditions and push model: ue-networking-replication.

Seamless-travel hooks: virtual void CopyProperties(APlayerState* PlayerState) (protected; called on the old PlayerState with the surviving one as the argument, and when an inactive PlayerState is saved on disconnect), virtual void OverrideWith(APlayerState* PlayerState), virtual void SeamlessTravelTo(class APlayerState* NewPlayerState). Built-ins: FString GetPlayerName() const, float GetScore() const / void SetScore(const float NewScore), int32 GetPlayerId() const, APawn* GetPawn() const, class APlayerController* GetPlayerController() const, virtual void ClientInitialize(class AController* C).

Reach one with MyPawn->GetPlayerState() or MyController->PlayerState; reach all of them through PlayerArray on the GameState:

if (const AGameStateBase* GS = GetWorld()->GetGameState<AGameStateBase>())
{
    for (APlayerState* PS : GS->PlayerArray) { /* every connected player */ }
}

Pawn and Character

APawn is the minimal possessable actor: no mesh, no collision component, no movement component. ACharacter adds a capsule root, a skeletal mesh and UCharacterMovementComponent with networked prediction. ADefaultPawn is the engine's flying placeholder (GetCollisionComponent(), GetMeshComponent(), bAddDefaultMovementBindings); ASpectatorPawn derives from it.

// Pawn-side overrides
virtual void SetupPlayerInputComponent(UInputComponent* PlayerInputComponent) override;
virtual void PossessedBy(AController* NewController) override;
virtual void UnPossessed() override;
virtual void OnRep_Controller() override;
virtual void NotifyControllerChanged() override;

PossessedBy runs on the server only. OnRep_Controller is the client-side counterpart for the replicated Controller pointer; NotifyControllerChanged fires on the server and the owning client (not simulated proxies, Pawn.h:396), which makes it the right place for owner-side logic such as refreshing the input mapping context.

In the Character constructor, size the capsule with GetCapsuleComponent()->SetCapsuleSize(42.f, 96.f) and place the mesh with GetMesh()->SetRelativeLocation(...) / SetRelativeRotation(...); that needs #include "Components/CapsuleComponent.h" and #include "Components/SkeletalMeshComponent.h".

Character API you will reach for directly: virtual void Jump(), virtual void StopJumping(), bool CanJump() const, virtual void LaunchCharacter(FVector LaunchVelocity, bool bXYOverride, bool bZOverride), virtual void Crouch(bool bClientSimulation = false), virtual void UnCrouch(bool bClientSimulation = false), virtual void OnRep_IsCrouched(). Replicated state: float JumpMaxHoldTime, int32 JumpMaxCount, uint8 bIsCrouched:1. Component getters: GetCapsuleComponent(), GetMesh(), GetCharacterMovement(), and GetArrowComponent() (declared only under #if WITH_EDITORONLY_DATA; guard any call).

HUD

AHUD is the immediate-mode canvas overlay for one local player, spawned from AGameModeBase::HUDClass. It never exists on a dedicated server. For retained UI (menus, health bars, layout) use UMG — see ue-ui-umg-slate.

// MyHUD.cpp — declares: virtual void DrawHUD() override; virtual void NotifyHitBoxClick(FName BoxName) override;
#include "Engine/Canvas.h"   // plus MyHUD.h and GameFramework/PlayerController.h

static const FName MenuBoxName(TEXT("MenuButton"));

void AMyHUD::DrawHUD()
{
    Super::DrawHUD();

    if (Canvas == nullptr)
    {
        return;
    }

    const float CentreX = Canvas->SizeX * 0.5f;
    const float CentreY = Canvas->SizeY * 0.5f;

    DrawRect(FLinearColor::White, CentreX - 2.f, CentreY - 2.f, 4.f, 4.f);
    DrawText(TEXT("Menu"), FLinearColor::White, 40.f, 40.f);
    AddHitBox(FVector2D(40.f, 40.f), FVector2D(120.f, 24.f), MenuBoxName, true, 0);
}

void AMyHUD::NotifyHitBoxClick(FName BoxName)
{
    Super::NotifyHitBoxClick(BoxName);

    if (BoxName == MenuBoxName)
    {
        if (APlayerController* PC = GetOwningPlayerController())
        {
            PC->SetInputMode(FInputModeUIOnly());
        }
    }
}

TObjectPtr<UCanvas> Canvas is only valid inside DrawHUD — never cache it. Drawing helpers: DrawText, DrawLine, DrawRect, DrawTexture, DrawTextureSimple. Hit boxes added with void AddHitBox(FVector2D Position, FVector2D Size, FName InName, bool bConsumesInput, int32 Priority = 0) raise the native virtual void NotifyHitBoxClick(FName BoxName), NotifyHitBoxRelease, NotifyHitBoxBeginCursorOver and NotifyHitBoxEndCursorOver, plus the ReceiveHitBoxClick / ReceiveHitBoxRelease / ReceiveHitBoxBeginCursorOver Blueprint events. Owner access: TObjectPtr<APlayerController> PlayerOwner, APlayerController* GetOwningPlayerController() const, APawn* GetOwningPawn() const.

Debug display: uint8 bShowHUD:1 gates all drawing and is toggled by the ShowHUD exec; virtual void ShowDebug(FName DebugType = NAME_None) backs the showdebug console command (engine-supported values include AI, physics, net, camera, collision); override virtual void ShowDebugInfo(float& YL, float& YPos) to add your own lines.

Damage without GAS

Engine damage is one virtual plus four UGameplayStatics entry points. Use it when you do not have GAS; attributes, execution calculations and cues belong to ue-gameplay-abilities.

// Dealing damage - server authority only. Needs Kismet/GameplayStatics.h.
// Target, Hit, ShotDirection, InstigatorController and IgnoreActors are locals of the caller.
UGameplayStatics::ApplyDamage(Target, 25.f, InstigatorController, this, UMyDamageType::StaticClass());
UGameplayStatics::ApplyPointDamage(Target, 40.f, ShotDirection, Hit,
    InstigatorController, this, UMyDamageType::StaticClass());
UGameplayStatics::ApplyRadialDamage(this, 100.f, Hit.ImpactPoint, 500.f,
    UMyDamageType::StaticClass(), IgnoreActors, this, InstigatorController, false, ECC_Visibility);
UGameplayStatics::ApplyRadialDamageWithFalloff(this, 100.f, 10.f, Hit.ImpactPoint, 200.f, 600.f, 1.f,
    UMyDamageType::StaticClass(), IgnoreActors, this, InstigatorController, ECC_Visibility);

// Receiving damage. AMyCharacter declares UPROPERTY(Replicated) float Health = 100.f; and FName LastHitBone;
// Declaration:
// virtual float TakeDamage(float DamageAmount, struct FDamageEvent const& DamageEvent, class AController* EventInstigator, AActor* DamageCauser) override;
#include "Engine/DamageEvents.h"

float AMyCharacter::TakeDamage(float DamageAmount, FDamageEvent const& DamageEvent, AController* EventInstigator, AActor* DamageCauser)
{
    const float Applied = Super::TakeDamage(DamageAmount, DamageEvent, EventInstigator, DamageCauser);

    if (!HasAuthority() || Applied <= 0.f)
    {
        return 0.f;
    }

    if (DamageEvent.IsOfType(FPointDamageEvent::ClassID))
    {
        const FPointDamageEvent& Point = static_cast<const FPointDamageEvent&>(DamageEvent);
        LastHitBone = Point.HitInfo.BoneName;
    }

    Health = FMath::Max(0.f, Health - Applied);
    return Applied;
}

FDamageEvent carries TSubclassOf<UDamageType> DamageTypeClass and dispatches through virtual bool IsOfType(int32 InID) const. FPointDamageEvent adds float Damage, FVector_NetQuantizeNormal ShotDirection and FHitResult HitInfo; FRadialDamageEvent adds FRadialDamageParams Params, FVector Origin and TArray<FHitResult> ComponentHits. UDamageType is a UObject used as a class, never instanced per hit — subclass it to carry float DamageImpulse, float DestructibleImpulse, float DamageFalloff, uint32 bCausedByWorld:1, uint32 bScaleMomentumByMass:1.

Dynamic multicast alternatives to overriding TakeDamage, all on AActor:

Bind with OnTakeAnyDamage.AddDynamic(this, &AMyCharacter::HandleAnyDamage); and mark the handler UFUNCTION(). Gate damage with void SetCanBeDamaged(bool bInCanBeDamaged) / bool CanBeDamaged() const. Falling below AWorldSettings::KillZ calls virtual void FellOutOfWorld(const class UDamageType& dmgType) with AWorldSettings::KillZDamageType.

GameInstance

One per process; survives every level load, including seamless travel. Override virtual void Init() (once at startup; Super::Init() initializes the game instance subsystems last, so use them only after that call), the protected virtual void OnStart() (once the instance is ready), and virtual void Shutdown() (on exit). UGameInstance::GetWorld() is final — do not override it. Game instance subsystems (UGameInstanceSubsystem) are the right home for cross-level services; the full subsystem table lives in ue-cpp-foundations.

Sessions

Two stacks ship in 5.8. Legacy OnlineSubsystem (modules "OnlineSubsystem", "OnlineSubsystemUtils"; backends Null, Steam, EOS) is what most existing projects use. Online Services v2 (Plugins/Online/OnlineServices, module "OnlineServicesInterface") is the modern path for new code, with OnlineServicesOSSAdapter bridging the two. Session code belongs on UGameInstance or a game instance subsystem, because it must outlive the level.

// Legacy OnlineSubsystem - host. Needs OnlineSubsystem.h, OnlineSessionSettings.h and
// Interfaces/OnlineSessionInterface.h; CreateSessionHandle is an FDelegateHandle member.
IOnlineSubsystem* OSS = IOnlineSubsystem::Get();
IOnlineSessionPtr Sessions = OSS ? OSS->GetSessionInterface() : nullptr;
if (Sessions.IsValid())
{
    FOnlineSessionSettings Settings;
    Settings.NumPublicConnections = 4;
    Settings.bShouldAdvertise = true;
    Settings.bIsLANMatch = false;
    Settings.bUsesPresence = true;

    CreateSessionHandle = Sessions->AddOnCreateSessionCompleteDelegate_Handle(
        FOnCreateSessionCompleteDelegate::CreateUObject(this, &UMyGameInstance::HandleCreateSessionComplete));
    Sessions->CreateSession(0, NAME_GameSession, Settings);
}

Search with FOnlineSessionSearch and Sessions->FindSessions(0, SessionSearch.ToSharedRef()); join with Sessions->JoinSession(0, NAME_GameSession, SessionSearch->SearchResults[Index]); then resolve the address with Sessions->GetResolvedConnectString(NAME_GameSession, ConnectInfo) and call PlayerController->ClientTravel(ConnectInfo, TRAVEL_Absolute). Handlers are void HandleCreateSessionComplete(FName SessionName, bool bWasSuccessful), void HandleFindSessionsComplete(bool bWasSuccessful) and void HandleJoinSessionComplete(FName SessionName, EOnJoinSessionCompleteResult::Type Result). Release every stored FDelegateHandle with the matching Clear...Delegate_Handle. Full search/join code: references/framework-class-map.md.

// Online Services (v2). Needs Online/OnlineServices.h and Online/Sessions.h.
using namespace UE::Online;
TSharedPtr<IOnlineServices> Services = GetServices(EOnlineServices::Default);
ISessionsPtr SessionsV2 = Services.IsValid() ? Services->GetSessionsInterface() : nullptr;

v2 calls return TOnlineAsyncOpHandle / TOnlineResult instead of delegate handles; the sibling interfaces alongside ISessions are IAuth, ILobbies, IAchievements, IPresence and ILeaderboards.

Travel

From the GameMode, a listen-server map change is GetWorld()->ServerTravel(TEXT("/Game/Maps/NewMap?listen")).

Seamless travel runs in two legs (current map → transition map → destination), and virtual void GetSeamlessTravelActorList(bool bToTransition, TArray<AActor*>& ActorList) is called for both. UGameInstance, APlayerController and APlayerState are carried across; AGameModeBase, AGameStateBase, pawns and level actors are not. AGameModeBase::ProcessServerTravel(const FString& URL, bool bAbsolute = false) is the override point for custom URL handling. Level streaming, World Partition and Data Layers belong to ue-world-level-streaming.

Deprecated — do not use

Common Mistakes

GameMode dereferenced on a client: GetAuthGameMode() returns nullptr everywhere except the server, so GetWorld()->GetAuthGameMode<AMyGameMode>()->EndMatch() crashes there. Write if (AMyGameMode* GM = GetWorld()->GetAuthGameMode<AMyGameMode>()) { GM->EndMatch(); } instead.

ReplicatedUsing with no OnRep declaration: UHT fails the build. Every ReplicatedUsing = OnRep_X needs UFUNCTION() void OnRep_X(); in the same class body.

Replicated property with no DOREPLIFETIME: it compiles, runs, and silently never replicates. Add GetLifetimeReplicatedProps with Super:: first, then one DOREPLIFETIME per property.

Overriding Possess / UnPossess: both are virtual final on AController. Override OnPossess(APawn* aPawn) and OnUnPossess().

Overriding a BlueprintNativeEvent by its bare name: ChoosePlayerStart, FindPlayerStart, SpawnDefaultPawnFor, HandleStartingNewPlayer, ReadyToStartMatch, ReadyToEndMatch, PlayerCanRestart and MustSpectate all need the _Implementation suffix, and the matching Super:: call carries that suffix too when you chain.

HUD, camera or audio work on a dedicated server: those objects do not exist there. Guard presentation code with if (GetNetMode() != NM_DedicatedServer).

Wrong class for the data: a score every client can see belongs on APlayerState, not APlayerController; a match timer clients read belongs on a replicated AGameStateBase property, not AGameModeBase; data that outlives a level belongs on UGameInstance, not AGameStateBase; pawn input binding belongs in APawn::SetupPlayerInputComponent, not the Character constructor.

GetPawn() before the server acknowledges possession: use AcknowledgedPawn when you need the pawn the server has confirmed.

PIE with multiple players: each player gets its own PlayerController but they share one GameMode instance. Set PIE "Number of Players" to 2 or more before trusting any multiplayer path.

Related Skills

• ue-gameplay-abilities — GAS attributes, gameplay effects and ability-driven damage, the alternative to
• ue-networking-replication — DOREPLIFETIME conditions, COND_*, RPC rules, push model, Iris (Beta in 5.8)
• ue-character-movement — UCharacterMovementComponent, movement modes, PhysCustom, prediction, Mover
• ue-gameplay-cameras — APlayerCameraManager, view targets and blends, camera modifiers, Gameplay
• ue-input-system — Enhanced Input mapping contexts, input actions, modifiers and triggers
• ue-actor-component-architecture — actor lifecycle, components, attachment, tick groups
• ue-cpp-foundations — UCLASS/UPROPERTY/UFUNCTION specifiers, TObjectPtr, the subsystem table
• ue-ui-umg-slate — UMG widgets and retained UI, the usual alternative to canvas HUD drawing
• ue-serialization-savegames — USaveGame, slots, what to persist off UGameInstance
• ue-world-level-streaming — level streaming, World Partition, Data Layers, streaming sources
• ue-ai-navigation — AI controllers, behavior trees, perception, EQS and navmesh queries
• ue-audio-system — UAudioComponent, MetaSounds, submixes, attenuation and concurrency
• Also relevant: ue-blueprint-cpp-interop, ue-game-features, ue-mover, ue-physics-collision, ue-state-trees, ue-testing-debugging