05 — Inventory Operations¶

Everything you can do with an inventory: grant, take, move, use, tidy. Plus filters, weight and events for the UI.

One rule for every operation¶

Every method that changes state is named Try* and is called from anywhere — client or server, Blueprint or C++:

Inventory->TryAddItem(Definition, 3, Remainder);

You never write HasAuthority() and never choose between a "server" and a "client" version of the call. Why this works — 09 — Multiplayer.

What Try* returns on a client. true means "request sent", not "succeeded" — the final decision is always the server's. So build the UI on events, not on the return value. See events below.

Grant an item¶

int32 Remainder = 0;
const bool bAdded = Inventory->TryAddItem(PotionDef, 5, Remainder);

if (Remainder > 0)
{
    // Not everything fit - show "Inventory full" or drop the rest on the ground.
}

Existing partial stacks are topped up first, then new slots are opened. Partial success is a normal result, not an error: Remainder says how many didn't fit.

Find out in advance whether it fits¶

const int32 WillFit = Inventory->CanAcceptItemCount(PotionDef, 5);

An honest answer taking into account filters, room in partial stacks, free slots and the weight limit. This is what to show as a pickup preview or a shop purchase size.

Take an item¶

From a specific slot — when the player acted on a slot they can see:

Inventory->TryRemoveItemFromSlot(SlotIndex, 1);

By type, not caring about slots — when consuming materials:

int32 Remainder = 0;
Inventory->TryRemoveItem(IronOreDef, 10, Remainder);

The second form spans any number of stacks and empties the smallest first — so leftovers consolidate instead of scattering across the inventory.

Check first, then spend. A partial removal that then "fails" leaves the player with no materials and no result:
if (Inventory->HasItem(IronOreDef, 10))  // all or nothing
{
    Inventory->TryRemoveItem(IronOreDef, 10, Remainder);
    CraftSword();
}
Or use TakeItemFrom from the library — it does this check itself.

Move between inventories¶

This is the most important place in the whole chapter, because the direction of the call matters.

// Take from a chest (looting)
PlayerInventory->TryTransferFrom(ChestInventory, SlotIndex);

// Put into a chest
PlayerInventory->TryTransferTo(ChestInventory, SlotIndex);

Both calls are made on the player's inventory, not the chest's. The reason is purely network: a chest in the world has no network connection, so a client's request through it wouldn't reach the server (09 — Multiplayer).

In Blueprint there's a ready node that picks the direction itself:

Move Item Between Actors (From Actor, To Actor, From Slot)

Pick an item up off the floor¶

The same principle, one more example: AISDemoItemPickup (an item lying in the world — 07 — Loot and world items) also has no connection of its own, so pickup is called the same way — on the player's inventory:

PlayerInventory->TryCollect(Pickup);

Not Pickup->TryCollect(...). The difference isn't cosmetic: the pickup variant is a server-only call that does nothing from a client. The inventory variant is an ordinary Try*, like the rest of this page: called from anywhere, and it decides whether to run the request locally or forward it to the server.

Quick move¶

The shift-click behaviour, when the player doesn't pick a destination slot:

PlayerInventory->TryQuickMoveTo(ChestInventory, SlotIndex);

It merges into matching partial stacks first, then takes the first free slot. What doesn't fit stays put — a full destination just moves less rather than destroying items.

Drag-and-drop inside an inventory¶

Inventory->TrySwapSlots(SlotA, SlotB);   // the basis of drag-and-drop
Inventory->TrySplitStack(SlotIndex, 4);  // split 4 copies into a new slot

TrySwapSlots merges stacks if the items are compatible, and works when one of the slots is empty — so it doubles as a "move here" operation.

A split preserves the state of both halves: half of a chipped stack stays chipped.

Use an item¶

const EISDemoItemUseResult Result = Inventory->TryUseItem(SlotIndex, GetPawn());

What happens is decided by the fragments: a potion is spent, a torch loses durability, a rock answers NotUsable.

Result	Meaning
`Success`	used
`NoItem`	the slot is empty
`NotUsable`	no fragment makes the item usable
`Blocked`	a fragment refused (broken, no charges, cooldown)

The last three are normal answers. Show them to the player: Get Use Result Text (Result) gives ready text.

Instigator (the second argument) is worth passing — it's who the fragments apply effects to.

Tidying¶

Inventory->CompactStacks();                        // merge partial stacks
Inventory->SortInventory(EISDemoSortMode::ByName);     // sort and compact

CompactStacks is gentler: it merges scattered stacks of the same item without touching the rest. A backpack with 3+7+12 arrows will have 22 arrows in the earliest of those slots, and everything else stays where it was.

SortInventory rearranges everything and packs from slot 0. Modes: ByName, ByStackCount, ByType, ByWeight.

Sorting visibly moves items under an open UI. Call it from an explicit player action (a "Sort" button), not automatically.

Filters: what an inventory accepts at all¶

Field	Effect
`AllowedItemTags`	if non-empty — the item must carry at least one of these tags
`BlockedItemTags`	an item with any of these tags is refused

BlockedItemTags is checked first and always wins. This lets you allow a broad category and carve exceptions out of it:

Shop:
  AllowedItemTags: Item.Type.Weapon      ← we trade weapons
  BlockedItemTags: Item.Property.QuestItem  ← but not quest ones

Check in advance: CanAcceptItem(Definition).

Slot restrictions¶

A single inventory can have specialised areas — with no second component:

SlotRestrictions:
  └── [0]: FirstSlot 0, LastSlot 3, RequiredTags: Item.Type.Ammo

Slots 0–3 become an ammo belt at the front of an ordinary backpack. Slots not covered by any rule accept whatever the inventory-wide filters allow.

Check a specific slot: CanSlotAcceptItem(SlotIndex, Definition) — this is the call a drag-and-drop UI should make on hover to show the right cursor.

Weight¶

Field	Effect
`MaxWeight`	limit; 0 = weight is ignored

The unit weight comes from the Weight fragment. Items without one weigh nothing and are never refused by weight.

Slots and weight are independent limits: either one can be the binding constraint.

const float Current = Inventory->GetCurrentWeight();  // for an encumbrance bar

Weight is calculated and shown even when MaxWeight = 0 — it just doesn't constrain. That's why the inspector says 5.50 weight carried (no limit set): the number is honest, there's no limit.

How to set capacity correctly¶

The normal case — in the component's Details panel. Open the character or chest Blueprint, select the Inventory component and set MaxSlots and MaxWeight there. That value goes into the class defaults, so both the server and the clients get it the same way, with no replication.

Changing at runtime is just an assignment, but on the server.

// Backpack upgrade. Do it where you have authority.
if (Inventory->HasContainerAuthority())
{
    Inventory->MaxSlots  = 40;
    Inventory->MaxWeight = 120.f;
}

Both fields replicate, so clients see the new capacity and their UI redraws. Assigning on a client does nothing useful: the next update from the server overwrites it — exactly like any replicated field.

What doesn't exist: a global weight default in Project Settings. There's only DefaultInventorySlots for auto-created inventories; their weight is always 0 until you set it yourself.

Watch out. The weight limit doesn't apply to worn items: equipping takes the item out of the inventory, and its weight disappears from the count. If armour should weigh on the player in your game, count that part yourself — via GetTotalStatValue on the equipment component, for example.

Starting items¶

StartingItems (a map of "item type → count") fills the inventory at the start of the game, on the server. Handy for a chest's fixed contents, a trader's stock, or a test character's starting kit.

For random contents use a loot table — 07 — Loot and world items.

Events for the UI¶

This is what you build the interface on — not the Try* return value.

Event	When it fires
`OnInventoryChanged`	any change; carries the change type
`OnItemAdded`	a slot that was empty received an item
`OnItemRemoved`	a slot emptied (the item is still readable during the call)
`OnItemChanged`	the item is the same, but the data changed (stack, durability)
`OnAddRejected`	an add was refused; carries the reason text

All fire on both the server and the clients — the same handler serves both modes.

Inventory->OnInventoryChanged.AddDynamic(this, &UMyWidget::HandleInventoryChanged);
Inventory->OnAddRejected.AddDynamic(this, &UMyWidget::ShowRejectionToast);

OnAddRejected is worth binding right away: without it the player won't understand why a pickup did nothing.

Drawing the grid¶

for (const FISDemoInventoryEntry& Entry : Inventory->GetAllEntries())
{
    DrawSlot(Entry.SlotIndex, Entry.Instance);   // take the index from the entry
}

GetAllEntries returns only occupied slots. The list is sparse — don't assume entry #N is slot #N (01 — Core Concepts).

Where to next¶

Equip an item: 06 — Equipment
Make a chest with loot: 07 — Loot and world items
The full list of methods: 12 — API Reference