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skills/quodsoler/unreal-engine-skills/ue-world-level-streaming

ue-world-level-streaming

SKILL.md

UE World & Level Streaming

You are an expert in Unreal Engine's world management and level streaming systems.

Context

Read .agents/ue-project-context.md before advising. Pay attention to:

  • Engine version — World Partition is UE5 only; sub-level streaming works in both UE4 and UE5.
  • Build targets — Dedicated server has no rendering-driven streaming; streaming must be server-safe.
  • World size — Determines whether World Partition or manual sub-level streaming is appropriate.
  • Multiplayer — Seamless travel requirements and per-player streaming radius.

Information to Gather

Before recommending a streaming approach, confirm:

  1. World size and type: Is this an open world (World Partition), a set of discrete levels, or a hub-and-spoke map?
  2. Multiplayer: Are you running a dedicated server? Are per-player streaming radii needed?
  3. Streaming control: Does gameplay code need to control load/unload explicitly, or should proximity drive it?
  4. Level travel: Non-seamless (lobby flows), seamless (multiplayer round transitions), or no travel?
  5. Persistent data: What must survive a level transition — player state, inventory, session state?

World Partition (UE5)

Enabling World Partition

Enable via the Level menu: World -> World Partition -> Convert Level. Once enabled, all actors in the level are managed by World Partition's grid. The level can no longer have traditional sub-levels. Use One File Per Actor (OFPA) for collaborative editing: each actor is saved as its own .uasset under __ExternalActors__.

Runtime Data Layers

Data layers replace the old sub-level toggle pattern. A runtime data layer can be loaded/unloaded at runtime without traveling to a new map.

// MyGameMode.cpp
#include "WorldPartition/DataLayer/DataLayerManager.h"

void AMyGameMode::ActivateDungeonDataLayer()
{
    UDataLayerManager* DLMgr = UDataLayerManager::GetDataLayerManager(GetWorld());
    if (!DLMgr) return;

    // Get by asset reference (set up in editor as a UDataLayerAsset)
    UDataLayerAsset* DungeonLayer = DungeonDataLayerAsset.LoadSynchronous();
    DLMgr->SetDataLayerRuntimeState(DungeonLayer, EDataLayerRuntimeState::Activated);
}

void AMyGameMode::DeactivateDungeonDataLayer()
{
    UDataLayerManager* DLMgr = UDataLayerManager::GetDataLayerManager(GetWorld());
    if (!DLMgr) return;

    UDataLayerAsset* DungeonLayer = DungeonDataLayerAsset.LoadSynchronous();
    DLMgr->SetDataLayerRuntimeState(DungeonLayer, EDataLayerRuntimeState::Unloaded);
}

Data layer states:

  • Unloaded — not loaded, not visible.
  • Loaded — loaded into memory, not visible (pre-warming).
  • Activated — loaded and visible (fully active).

Streaming Sources

Each player controller is a streaming source by default. For custom sources (cinematic cameras, AI directors), implement IWorldPartitionStreamingSourceProvider.

HLOD

HLOD provides distant merged-mesh representations of World Partition cells. Configure HLOD layers in the World Partition editor; build before shipping via Build -> Build World Partition HLODs. Without HLOD, content beyond the streaming radius is simply absent.

Converting Sub-Levels to World Partition

Use Tools -> World Partition -> Convert Level. Actors migrate into the persistent level under WP management. Audit cross-level references beforehand — hard references to converted actors become invalid.

World Partition and Multiplayer

In a multiplayer session, each player controller acts as a streaming source with a configurable radius. The server streams based on server-side sources; clients receive visibility updates via AServerStreamingLevelsVisibility. On dedicated servers, rendering-based streaming does not apply — streaming is driven by server-side sources only.

Streaming radius is configured per-partition in the World Partition editor UI (LoadingRange on URuntimePartition), not via ini.

Level Streaming (Manual Sub-Levels)

ULevelStreaming State Machine

From LevelStreaming.h, the full state sequence is:

Removed -> Unloaded -> Loading -> LoadedNotVisible -> MakingVisible -> LoadedVisible -> MakingInvisible -> LoadedNotVisible
                                      |
                                 FailedToLoad   (check logs; level asset missing or corrupt)

Query state with:

ULevelStreaming* StreamingLevel = /* ... */;
ELevelStreamingState State = StreamingLevel->GetLevelStreamingState();

switch (State)
{
    case ELevelStreamingState::Unloaded:         /* not in memory */ break;
    case ELevelStreamingState::Loading:          /* async load in progress */ break;
    case ELevelStreamingState::LoadedNotVisible: /* in memory, not rendered */ break;
    case ELevelStreamingState::MakingVisible:    /* adding to world */ break;
    case ELevelStreamingState::LoadedVisible:    /* fully active */ break;
    case ELevelStreamingState::MakingInvisible:  /* removing from rendering */ break;
    case ELevelStreamingState::FailedToLoad:     /* check logs */ break;
}

UGameplayStatics: LoadStreamLevel / UnloadStreamLevel

For Blueprint-friendly async streaming with latent actions (from GameplayStatics.h):

// MyActor.cpp — async load using FLatentActionInfo
#include "Kismet/GameplayStatics.h"

void AMyActor::StreamInRoom(FName LevelName)
{
    FLatentActionInfo LatentInfo;
    LatentInfo.CallbackTarget = this;
    LatentInfo.ExecutionFunction = FName("OnRoomLoaded");
    LatentInfo.Linkage = 0;
    LatentInfo.UUID = GetUniqueID();

    UGameplayStatics::LoadStreamLevel(
        this,           // WorldContextObject
        LevelName,      // e.g., FName("Room_01")
        true,           // bMakeVisibleAfterLoad
        false,          // bShouldBlockOnLoad — keep false for async
        LatentInfo
    );
}

UFUNCTION()
void AMyActor::OnRoomLoaded()
{
    // Room is now loaded and visible
}

void AMyActor::StreamOutRoom(FName LevelName)
{
    FLatentActionInfo LatentInfo;
    LatentInfo.CallbackTarget = this;
    LatentInfo.ExecutionFunction = FName("OnRoomUnloaded");
    LatentInfo.Linkage = 0;
    LatentInfo.UUID = GetUniqueID() + 1;

    UGameplayStatics::UnloadStreamLevel(
        this,
        LevelName,
        LatentInfo,
        false // bShouldBlockOnUnload
    );
}

For soft object pointers (preferred for packaging safety), use LoadStreamLevelBySoftObjectPtr with the same arguments.

ULevelStreamingDynamic: Runtime Level Instances

Use ULevelStreamingDynamic::LoadLevelInstance to load the same level package multiple times at different transforms — for procedural dungeons, modular buildings, or instanced rooms (from LevelStreamingDynamic.h):

#include "Engine/LevelStreamingDynamic.h"

void AMyDungeonGenerator::SpawnRoom(FVector Location, FRotator Rotation)
{
    bool bSuccess = false;
    ULevelStreamingDynamic* StreamingLevel = ULevelStreamingDynamic::LoadLevelInstance(
        this,                              // WorldContextObject
        TEXT("/Game/Levels/Room_Corridor"), // LongPackageName — full path
        Location,
        Rotation,
        bSuccess
    );

    if (bSuccess && StreamingLevel)
    {
        // Bind to delegate to know when visible
        StreamingLevel->OnLevelShown.AddDynamic(this, &AMyDungeonGenerator::OnRoomShown);
        StreamingLevel->OnLevelHidden.AddDynamic(this, &AMyDungeonGenerator::OnRoomHidden);

        LoadedRooms.Add(StreamingLevel);
    }
}

void AMyDungeonGenerator::UnloadRoom(ULevelStreamingDynamic* StreamingLevel)
{
    if (StreamingLevel)
    {
        StreamingLevel->SetShouldBeLoaded(false);
        StreamingLevel->SetShouldBeVisible(false);
        StreamingLevel->SetIsRequestingUnloadAndRemoval(true);
    }
}

For networking: use OptionalLevelNameOverride to give all clients and server the same package name for a given instance. Without this, names are auto-generated uniquely per process and will not match across connections.

ULevelStreamingDynamic::FLoadLevelInstanceParams Params(
    GetWorld(),
    TEXT("/Game/Levels/Room_Corridor"),
    FTransform(Rotation, Location)
);
Params.OptionalLevelNameOverride = &InstanceName; // FString, same on server and clients
Params.bInitiallyVisible = true;

bool bSuccess = false;
ULevelStreamingDynamic* Level = ULevelStreamingDynamic::LoadLevelInstance(Params, bSuccess);

OnLevelShown / OnLevelHidden Delegates

From LevelStreaming.h — four BlueprintAssignable delegates: OnLevelLoaded, OnLevelUnloaded, OnLevelShown, OnLevelHidden. Bind with AddDynamic:

StreamingLevel->OnLevelShown.AddDynamic(this, &UMyManager::HandleLevelShown);
StreamingLevel->OnLevelLoaded.AddDynamic(this, &UMyManager::HandleLevelLoaded);

Streaming Volumes

ALevelStreamingVolume automatically controls sub-level loading when the player camera is inside or outside the volume. From LevelStreamingVolume.h:

// EStreamingVolumeUsage — set on the volume in editor
SVB_Loading                 // load but do not make visible
SVB_LoadingAndVisibility    // load and make visible (most common)
SVB_VisibilityBlockingOnLoad // force blocking load when entering
SVB_BlockingOnLoad          // block load of associated levels
SVB_LoadingNotVisible       // load, keep invisible (pre-warm)

Volumes are assigned to a sub-level via its EditorStreamingVolumes array. Disable volume-driven streaming for a level with ULevelStreaming::bDisableDistanceStreaming = true when you want code-only control.

Manual Visibility Control

// Get streaming level reference from world
const TArray<ULevelStreaming*>& Levels = GetWorld()->GetStreamingLevels();
for (ULevelStreaming* Level : Levels)
{
    if (Level->GetWorldAssetPackageFName() == FName("/Game/Levels/MySubLevel"))
    {
        Level->SetShouldBeLoaded(true);
        Level->SetShouldBeVisible(true);
        break;
    }
}

Force flush all streaming (blocks until complete — use sparingly):

UGameplayStatics::FlushLevelStreaming(this);

Level Instances

ALevelInstance places a level as a reusable chunk in the editor. Actors inside are editable as a unit. For runtime instancing, see ULevelStreamingDynamic above.

Packed Level Actors merge instance meshes into a single static mesh for performance. Enable via right-click on Level Instance → Pack Level Actor.

Per-instance property overrides (UE5.1+): Each placed ALevelInstance can override individual actor properties (materials, gameplay values) without modifying the source level. Configure overrides in the Details panel; overridden values bake into packed level data at cook time.

Level Travel

Non-Seamless: UGameplayStatics::OpenLevel

Destroys the current world and loads a new one; all clients disconnect. From GameplayStatics.h:

UGameplayStatics::OpenLevel(this, FName("/Game/Maps/MainMenu"), true);
UGameplayStatics::OpenLevel(this, FName("/Game/Maps/GameLevel"), true, TEXT("?Difficulty=Hard"));
UGameplayStatics::OpenLevelBySoftObjectPtr(this, GameLevelAsset, true); // packaging-safe

Server Travel (Multiplayer, Non-Seamless)

Initiated on the server; all connected clients follow (World.h):

GetWorld()->ServerTravel(TEXT("/Game/Maps/Level02?listen"), /*bAbsolute=*/false);

Seamless Travel

Seamless travel loads the destination map in the background via a transition (midpoint) map. Clients stay connected. From World.h:

void UWorld::SeamlessTravel(const FString& InURL, bool bAbsolute);
bool UWorld::IsInSeamlessTravel() const;
void UWorld::SetSeamlessTravelMidpointPause(bool bNowPaused);

Setup requirements:

  1. Set bUseSeamlessTravel = true on AGameModeBase:
// bUseSeamlessTravel is already declared in AGameModeBase — do NOT redeclare it.
// Just set it in the constructor:

// MyGameMode.cpp constructor
bUseSeamlessTravel = true;
  1. Set a transition map in DefaultEngine.ini:
[/Script/Engine.GameMapsSettings]
TransitionMap=/Game/Maps/Transition
  1. Override GetSeamlessTravelActorList to control which actors persist:
// GameMode — called on server side during transition
void AMyGameMode::GetSeamlessTravelActorList(bool bToTransition, TArray<AActor*>& ActorList)
{
    Super::GetSeamlessTravelActorList(bToTransition, ActorList);

    if (!bToTransition)
    {
        // bToTransition=false means we're moving TO the destination
        // Add actors that should survive (e.g., GameState, custom managers)
        ActorList.Add(MyPersistentManager);
    }
}

// GameMode — called after destination map is loaded
void AMyGameMode::PostSeamlessTravel()
{
    Super::PostSeamlessTravel();
    // Re-initialize any post-travel systems
}

// GameMode — handle re-possessing players after travel
void AMyGameMode::HandleSeamlessTravelPlayer(AController*& C)
{
    Super::HandleSeamlessTravelPlayer(C);
    // Restore player-specific state here
}
  1. Trigger on server:
// From GameMode, server-only
GetWorld()->ServerTravel(TEXT("/Game/Maps/Level02?listen"));
// Seamless travel is automatic because bUseSeamlessTravel is true

Travel sequence: current world -> transition map -> destination world. Use SetSeamlessTravelMidpointPause(true) to pause at midpoint for pre-loading.

Client Travel

For client-initiated travel (join server, change options), call APlayerController::ClientTravel(URL, ETravelType::TRAVEL_Absolute) from the player controller.

World Subsystems

UWorldSubsystem (from Subsystems/WorldSubsystem.h) is auto-instantiated once per UWorld. It is destroyed when the world is destroyed — including on level travel. It is the correct place for per-world singleton logic: streaming managers, zone trackers, world-state caches.

// MyStreamingManager.h
UCLASS()
class MYGAME_API UMyStreamingManager : public UWorldSubsystem
{
    GENERATED_BODY()
public:
    virtual void PostInitialize() override;                          // after all subsystems init
    virtual void OnWorldBeginPlay(UWorld& InWorld) override;         // after all BeginPlay
    virtual void PreDeinitialize() override;                         // cleanup hook
    virtual bool ShouldCreateSubsystem(UObject* Outer) const override; // filter world type

    void RequestLoadZone(FName ZoneName);
    void RequestUnloadZone(FName ZoneName);
private:
    TMap<FName, TWeakObjectPtr<ULevelStreaming>> ActiveZones;
};

Access from anywhere with a world context:

UMyStreamingManager* Manager = GetWorld()->GetSubsystem<UMyStreamingManager>();
if (Manager)
{
    Manager->RequestLoadZone(FName("Zone_A"));
}

UTickableWorldSubsystem

For per-frame updates (distance checks, zone detection). Inherit from UTickableWorldSubsystem. Must call Super::Initialize and Super::Deinitialize to enable/disable ticking. Implement GetStatId returning a RETURN_QUICK_DECLARE_CYCLE_STAT.

Persistent Data Across Level Transitions

Mechanism Lifetime Use Case
UGameInstance Entire application session Cross-level player state, session config
UGameInstanceSubsystem Entire application session Services that outlive any world
Seamless travel actor list Transition only Actors that physically cross (GameState, managers)
USaveGame + SaveGameToSlot Disk-persistent Long-term saves, progression
UWorldSubsystem Per world World-scoped cache; push data to UGameInstance in Deinitialize() before travel clears it

GameInstance Pattern

Store cross-level data in UGameInstance properties (survives all level travel). Access from anywhere with a world context:

UMyGameInstance* GI = GetGameInstance<UMyGameInstance>();
if (GI) GI->PlayerScore += 100;

Common Mistakes and Anti-Patterns

Loading everything at once. Setting bShouldBlockOnLoad = true on many sub-levels causes hitches. Use async loading and the latent action pattern. Only block on load when the game is behind a loading screen.

Streaming volume gaps. Overlapping volumes cause spurious unload/reload cycles. Use MinTimeBetweenVolumeUnloadRequests on the streaming level to add a cooldown and prevent flickering.

Broken seamless travel in multiplayer. If bUseSeamlessTravel is true but no transition map is set, seamless travel silently falls back to non-seamless. Always set TransitionMap in DefaultEngine.ini.

Cross-level hard references. Hard object references (UPROPERTY() UObject*) between actors in different streaming levels cause the entire referenced level to stay loaded. Always use TSoftObjectPtr or TSoftClassPtr across level boundaries.

Dynamic streaming level names not matching server and client. When using ULevelStreamingDynamic::LoadLevelInstance, each process generates a unique name. In multiplayer, supply OptionalLevelNameOverride with the same name on server and all clients.

World Partition on dedicated server. The server does not use rendering-driven streaming. Streaming sources must be explicitly added server-side (e.g., player positions) or World Partition will not stream in actors correctly on the server.

Modifying StreamingLevels directly. Do not add to UWorld::StreamingLevels directly. Use AddStreamingLevels, AddUniqueStreamingLevels, and RemoveStreamingLevels (from World.h) which handle internal bookkeeping and StreamingLevelsToConsider.

Forgetting to call Super::Initialize / Super::Deinitialize in UTickableWorldSubsystem. These calls enable and disable ticking respectively. Skipping them results in a subsystem that never ticks or never stops ticking.

Quick Reference: Key APIs

API Header Notes
UGameplayStatics::LoadStreamLevel Kismet/GameplayStatics.h Async latent load of named sub-level
UGameplayStatics::UnloadStreamLevel Kismet/GameplayStatics.h Async latent unload
UGameplayStatics::FlushLevelStreaming Kismet/GameplayStatics.h Blocking flush — use behind loading screens
UGameplayStatics::OpenLevel Kismet/GameplayStatics.h Non-seamless level travel
ULevelStreamingDynamic::LoadLevelInstance Engine/LevelStreamingDynamic.h Runtime level instancing
ULevelStreaming::GetLevelStreamingState Engine/LevelStreaming.h Query current stream state
ULevelStreaming::SetShouldBeLoaded Engine/LevelStreaming.h Drive load state
ULevelStreaming::SetShouldBeVisible Engine/LevelStreaming.h Drive visibility
ULevelStreaming::SetIsRequestingUnloadAndRemoval Engine/LevelStreaming.h Remove level from world
UWorld::ServerTravel Engine/World.h Multiplayer level transition
UWorld::SeamlessTravel Engine/World.h Background seamless transition
UWorld::GetStreamingLevels Engine/World.h Iterate all streaming levels
UDataLayerManager::SetDataLayerRuntimeState WorldPartition/DataLayer/DataLayerManager.h World Partition data layer control (use UDataLayerManager::GetDataLayerManager(World))
UWorldSubsystem::OnWorldBeginPlay Subsystems/WorldSubsystem.h Post-BeginPlay init hook
AGameModeBase::GetSeamlessTravelActorList GameFramework/GameModeBase.h Control actor persistence

Related Skills

  • ue-gameplay-framework — GameMode travel callbacks, PostSeamlessTravel, actor persistence rules.
  • ue-data-assets-tables — async asset loading patterns that complement level streaming.
  • ue-networking-replication — net visibility transactions, server streaming authority.
  • ue-cpp-foundations — subsystem patterns, UGameInstance lifetime.
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