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Common issues and their solutions when working with UMI.

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Replication Issues

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Items Not Appearing on Clients

Symptoms:
  • Items added on server show in server player’s inventory
  • Items don’t appear for clients
  • Client inventory UI is empty or outdated
Causes:
  1. Component not set to replicate
  2. Items array not replicated
  3. Events not firing on clients
Solutions: Check Component Replication: 
// In your setup
InventoryComponent->SetIsReplicated(true);
Verify Array Replication: 
// InventoryComponent.h should have:
UPROPERTY(ReplicatedUsing=OnRep_Items)
TArray<FInventorySlot> Items;
Implement OnRep Function: 
UFUNCTION()
void OnRep_Items()
{
    OnInventoryChanged.Broadcast();
}
Check Network Role: 
if (GetOwnerRole() == ROLE_Authority)
{
    // Server only
    InventoryComponent->AddItem(ItemData, Quantity);
}

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Desynced Inventory Between Server and Client

Symptoms:
  • Server shows different items than client
  • Quantity mismatches
  • Items disappear randomly
Causes:
  1. Client prediction without server validation
  2. Client modifying inventory directly
  3. Missed replication updates
Solutions: Always Use Server Authority: 
// Wrong - don't do this
void AddItemClientSide()
{
    InventoryComponent->AddItem(ItemData, 1);  // Client calls directly
}

// Correct
UFUNCTION(Server, Reliable)
void Server_AddItem(UItemDataAsset* ItemData, int32 Quantity)
{
    if (GetLocalRole() == ROLE_Authority)
    {
        InventoryComponent->AddItem(ItemData, Quantity);
    }
}
Validate on Server: 
bool AddItemWithValidation(UItemDataAsset* ItemData, int32 Quantity)
{
    if (GetLocalRole() != ROLE_Authority)
    {
        Server_AddItem(ItemData, Quantity);
        return false;  // Don't know result yet
    }

    // Server validates
    if (!InventoryComponent->CanAddItemWeight(ItemData, Quantity))
    {
        return false;
    }

    return InventoryComponent->AddItem(ItemData, Quantity);
}

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Items Not Appearing

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Items Added But Not Visible in UI

Symptoms:
  • AddItem returns true
  • GetItemCount shows correct count
  • UI doesn’t display items
Causes:
  1. UI not bound to inventory events
  2. UI not refreshing
  3. Slots initialized incorrectly
Solutions: Bind to Events: 
// In your widget
void UInventoryWidget::NativeConstruct()
{
    Super::NativeConstruct();

    if (InventoryComponent)
    {
        InventoryComponent->OnInventoryChanged.AddDynamic(this, &UInventoryWidget::RefreshInventory);
        InventoryComponent->OnSlotUpdated.AddDynamic(this, &UInventoryWidget::UpdateSlot);

        RefreshInventory();  // Initial population
    }
}
Force Refresh: 
void RefreshInventory()
{
    for (int32 i = 0; i < InventoryComponent->MaxSlots; i++)
    {
        FItemInstance Item = InventoryComponent->GetItemAtIndex(i);
        UpdateSlotVisual(i, Item);
    }
}

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WorldItemActor Not Pickable

Symptoms:
  • World item visible
  • Interaction prompt doesn’t appear
  • Can’t pick up item
Causes:
  1. InteractionComponent not tracing correctly
  2. Item outside trace range
  3. Item not implementing interaction interface
  4. Collision not set up
Solutions: Check Trace Settings: 
InteractionComponent->TraceDistance = 500.0f;  // Increase range
InteractionComponent->bDrawDebugTrace = true;  // Visualize
Verify Collision: 
// On WorldItemActor mesh
MeshComponent->SetCollisionEnabled(ECollisionEnabled::QueryOnly);
MeshComponent->SetCollisionResponseToChannel(ECC_Visibility, ECR_Block);
MeshComponent->SetCollisionResponseToChannel(ECC_Camera, ECR_Ignore);
Check Interaction Setup: 
// WorldItemActor should have interaction option
FInteractionOption PickupOption;
PickupOption.Tag = FGameplayTag::RequestGameplayTag("Interaction.Pickup");
PickupOption.PromptText = FText::FromString("Press E to Pick Up");
PickupOption.Range = 300.0f;

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Stacking Problems

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Items Not Stacking When They Should

Symptoms:
  • Same items occupy multiple slots
  • Items with same ID don’t merge
Causes:
  1. Different stacking policies
  2. Rarity included in stack key
  3. Different durability buckets
  4. Different crafters (ProvenanceMatters)
Solutions: Check Stacking Policy: 
ItemDataAsset:
   Stacking Policy: CommodityDecayAware  (not FullInstance)
   Include Rarity In Stack Key: false
Debug Stack Keys: 
FString Key1 = UStackingUtils::CalculateStackKey(Item1);
FString Key2 = UStackingUtils::CalculateStackKey(Item2);

UE_LOG(LogTemp, Log, TEXT("Item1 Key: %s"), *Key1);
UE_LOG(LogTemp, Log, TEXT("Item2 Key: %s"), *Key2);

if (Key1 == Key2)
{
    UE_LOG(LogTemp, Log, TEXT("Should stack"));
}
Force Stacking: 
// Merge two stacks manually
FItemInstance Item1 = InventoryComponent->GetItemAtIndex(Slot1);
FItemInstance Item2 = InventoryComponent->GetItemAtIndex(Slot2);

if (Item1.ItemID == Item2.ItemID)
{
    int32 TotalQuantity = Item1.Quantity + Item2.Quantity;
    int32 MaxStack = Item1.ItemData->MaxStackSize;

    if (TotalQuantity <= MaxStack)
    {
        Item1.Quantity = TotalQuantity;
        InventoryComponent->UpdateSlot(Slot1, Item1);
        InventoryComponent->ClearSlot(Slot2);
    }
}

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Items Stacking When They Shouldn’t

Symptoms:
  • Items with different properties merge
  • Unique items stack together
  • Durability values merge incorrectly
Causes:
  1. Wrong stacking policy
  2. Custom properties not included in stack key
Solutions: Use Correct Policy: 
Unique Items:
   Stacking Policy: FullInstance
   Max Stack Size: 1

Equipment:
   Stacking Policy: GearDurabilityBucket
   Bucket Width Percentage: 25

Crafted Items:
   Stacking Policy: ProvenanceMatters
Custom Stack Key: Extend stacking logic to include your custom properties (see Extending the System).

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Loot Not Spawning

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LootComponent Generates Empty Results

Symptoms:
  • GenerateLoot returns empty array
  • No items drop from enemies/containers
Causes:
  1. Empty loot table
  2. All drop weights = 0
  3. MaxItemsToSpawn = 0
  4. Level filtering too strict
Solutions: Verify Loot Table: 
Loot Table:
   Loot Entries: [Must have at least one entry]
   Max Items To Spawn: 1 or more
Check Weights: 
Loot Entry:
   Drop Weight: 50.0  (not 0)
Debug Loot Generation: 
FLootResult Result = LootComponent->GenerateLoot();

UE_LOG(LogTemp, Log, TEXT("Loot generated: %d items"), Result.SpawnedItems.Num());

for (const FLootedItem& Item : Result.SpawnedItems)
{
    UE_LOG(LogTemp, Log, TEXT("  - %s x%d"), *Item.ItemData->ItemID.ToString(), Item.Quantity);
}

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Loot Spawns But Items Invisible

Symptoms:
  • Loot generates correctly (logs show items)
  • WorldItemActors not visible
Causes:
  1. Mesh not set
  2. Spawned underground
  3. Culled by visibility
Solutions: Initialize Properly: 
AWorldItemActor* DroppedItem = GetWorld()->SpawnActor<AWorldItemActor>(
    WorldItemActorClass,
    SpawnLocation + FVector(0, 0, 50),  // Spawn slightly above ground
    FRotator::ZeroRotator
);

DroppedItem->InitializeFromInstance(LootedItem);

// Verify mesh
if (DroppedItem->MeshComponent)
{
    UE_LOG(LogTemp, Log, TEXT("Mesh set: %s"), *DroppedItem->MeshComponent->GetStaticMesh()->GetName());
}

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UI Not Updating

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Inventory Changes Don’t Reflect in UI

Symptoms:
  • Items added/removed
  • UI shows old data
  • Need to close/reopen UI to see changes
Causes:
  1. Events not bound
  2. Events not firing
  3. UI caching old data
Solutions: Rebind Events: 
void UInventoryWidget::NativeConstruct()
{
    Super::NativeConstruct();

    // Clear old bindings
    if (InventoryComponent)
    {
        InventoryComponent->OnInventoryChanged.RemoveAll(this);
        InventoryComponent->OnSlotUpdated.RemoveAll(this);
    }

    // Bind events
    InventoryComponent->OnInventoryChanged.AddDynamic(this, &UInventoryWidget::RefreshInventory);
    InventoryComponent->OnSlotUpdated.AddDynamic(this, &UInventoryWidget::UpdateSlot);

    RefreshInventory();
}
Manual Refresh: 
// After any inventory change
InventoryComponent->OnInventoryChanged.Broadcast();

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Equipment Visuals Not Updating

Symptoms:
  • Item equipped successfully
  • Character mesh doesn’t change
  • Old equipment still visible
Causes:
  1. Mesh components not set up
  2. Master Pose not configured
  3. Meshes not assigned in ItemDataAsset
Solutions: Verify Mesh Setup: 
// Equipment meshes should use Master Pose
HeadMeshComponent->SetMasterPoseComponent(GetMesh());  // Main character mesh
ChestMeshComponent->SetMasterPoseComponent(GetMesh());
Force Visual Update: 
EquipmentComponent->UpdateAllEquipmentVisuals();
Check ItemDataAsset: 
Equipment Mesh Variants:
   Default: SK_Helmet  (must be set)

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Crafting Issues

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Can’t Start Crafting

Symptoms:
  • StartCrafting returns false
  • No error message
  • Recipe appears valid
Causes:
  1. Missing ingredients
  2. Missing tools
  3. No fuel (if required)
  4. All slots busy (MultiSlot mode)
Solutions: Debug Validation: 
if (!CraftingComponent->CanStartCrafting(Recipe, Quantity))
{
    if (!CraftingComponent->HasRequiredIngredients(Recipe))
    {
        UE_LOG(LogTemp, Warning, TEXT("Missing ingredients"));
    }

    if (!CraftingComponent->HasRequiredTools(Recipe))
    {
        UE_LOG(LogTemp, Warning, TEXT("Missing tools"));
    }

    if (CraftingComponent->bRequiresFuel && !CraftingComponent->HasSufficientFuel())
    {
        UE_LOG(LogTemp, Warning, TEXT("Missing fuel"));
    }

    int32 EmptySlot = CraftingComponent->FindEmptySlot();
    if (EmptySlot == -1)
    {
        UE_LOG(LogTemp, Warning, TEXT("All slots busy"));
    }
}

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Crafting Progress Stuck

Symptoms:
  • Crafting starts
  • Progress doesn’t increase
  • Never completes
Causes:
  1. Tick not running
  2. Crafting paused
  3. Time dilation = 0
Solutions: Check Tick: 
// CraftingComponent should tick
PrimaryComponentTick.bCanEverTick = true;
PrimaryComponentTick.bStartWithTickEnabled = true;
Resume if Paused: 
if (CraftingSlot.bIsPaused)
{
    CraftingComponent->ResumeCrafting(SlotIndex);
}

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Save/Load Issues

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Data Not Persisting

Symptoms:
  • Items lost on reload
  • Inventory resets to default
  • Equipment not saved
Causes:
  1. Component not registered as saveable
  2. GUID not persistent
  3. Save not being called
  4. Wrong bucket
Solutions: Register Component: 
void BeginPlay()
{
    Super::BeginPlay();

    USaveSubsystem* SaveSubsystem = GetWorld()->GetSubsystem<USaveSubsystem>();
    if (SaveSubsystem)
    {
        SaveSubsystem->RegisterSaveable(this);
    }
}
Use Persistent GUID: 
UPROPERTY(SaveGame)
FGuid ComponentGuid;

void BeginPlay()
{
    if (!ComponentGuid.IsValid())
    {
        ComponentGuid = FGuid::NewGuid();
    }
}

FGuid GetSaveGuid() const override
{
    return ComponentGuid;  // Same GUID every time
}
Trigger Save: 
// Mark dirty when data changes
InventoryComponent->OnInventoryChanged.AddLambda([this]()
{
    USaveSubsystem* SaveSubsystem = GetWorld()->GetSubsystem<USaveSubsystem>();
    SaveSubsystem->MarkDirty(this);
});

// Or manual save
SaveSubsystem->FlushSaves();

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Corrupted Save Data

Symptoms:
  • Load fails
  • Crash on load
  • Partial data loaded
Causes:
  1. Serialization mismatch
  2. Version change
  3. Corrupted file
Solutions: Version Your Saves: 
const int32 SAVE_VERSION = 2;

FSaveRecord BuildSaveRecord() override
{
    FSaveRecord Record;
    FMemoryWriter Writer(Record.Bytes);

    // Write version first
    int32 Version = SAVE_VERSION;
    Writer << Version;

    // Write data
    Writer << MyData;

    return Record;
}

void ApplySaveRecord(const FSaveRecord& Record) override
{
    FMemoryReader Reader(Record.Bytes);

    // Read version
    int32 Version;
    Reader << Version;

    if (Version != SAVE_VERSION)
    {
        UE_LOG(LogTemp, Warning, TEXT("Save version mismatch. Expected %d, got %d"), SAVE_VERSION, Version);
        return;
    }

    // Read data
    Reader << MyData;
}

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Performance Issues

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Frame Rate Drops When Opening Inventory

Causes:
  1. Too many UI updates
  2. Inefficient refresh logic
  3. Large inventory size
Solutions: Batch Updates: 
void RefreshInventory()
{
    // Disable tick during refresh
    SetComponentTickEnabled(false);

    for (int32 i = 0; i < MaxSlots; i++)
    {
        UpdateSlotVisual(i);
    }

    SetComponentTickEnabled(true);
}
Virtualization: 
// Only create widgets for visible slots
void ScrollToSlot(int32 SlotIndex)
{
    int32 StartSlot = SlotIndex - VisibleSlots / 2;
    int32 EndSlot = SlotIndex + VisibleSlots / 2;

    for (int32 i = StartSlot; i <= EndSlot; i++)
    {
        if (!SlotWidgets.Contains(i))
        {
            CreateSlotWidget(i);
        }
    }
}

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Multiplayer Lag

Causes:
  1. Too much replication
  2. Frequent updates
  3. Large inventory sizes
Solutions: Reduce Replication Frequency: 
InventoryComponent->SetNetUpdateFrequency(10.0f);  // 10 updates per second max
Replicate Only Changes: 
// Use RepNotify to update only changed slots
UFUNCTION()
void OnRep_Inventory(const TArray<FInventorySlot>& OldInventory)
{
    // Compare and update only changed slots
    for (int32 i = 0; i < Items.Num(); i++)
    {
        if (i >= OldInventory.Num() || Items[i] != OldInventory[i])
        {
            OnSlotUpdated.Broadcast(i, Items[i].Item);
        }
    }
}

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Common Questions

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Q: Can I use UMI in multiplayer?

A: Yes! UMI is designed for multiplayer with full replication support.

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Q: Does UMI support dedicated servers?

A: Yes, all components work on dedicated servers with server authority.

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Q: Can I modify UMI source code?

A: Yes, but extending via inheritance is recommended for easier updates.

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Q: Does UMI work with the Gameplay Ability System (GAS)?

A: Yes. UMI fully supports GAS and includes optional hooks so items can trigger Gameplay Abilities, apply Gameplay Effects, or integrate with GAS-driven stats if your project uses it.

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Q: Does UMI require GAS?

A: No. GAS integration is optional. UMI works entirely without GAS, and all inventory, equipment, crafting, and loot functionality operates normally even if GAS is disabled.