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09 — Multiplayer

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The plugin's main promise: you write the code once, and it works in single-player as well as multiplayer. This chapter explains how that's built, and where the one exception is.

One method instead of two

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

Internally there's just one check:

bool UISDemoInventoryComponent::TryAddItem(...)
{
    if (!HasContainerAuthority())
    {
        Server_AddItem(Definition, Count);   // client -> server
        return true;                          // "request sent"
    }
    return AddItem_Internal(...);             // we have authority - work in place
}
Situation HasAuthority() What happens
Single-player true runs in place, no RPC is created
Listen server (host) true the same, in place
Client false forwarded to the server, result via replication

Single-player serialises not a single byte. The multiplayer capability costs it nothing.

The return value on a client

A client can't learn the result synchronously — the server hasn't answered yet. So:

On a client Try* returns true meaning "request sent", not "succeeded".

// WRONG - on a client this is always true
if (Inventory->TryAddItem(Def, 1, Rem))
{
    ShowPickupAnimation();   // fires even when the server refuses
}

// RIGHT - react to the event that arrives with replication
Inventory->OnItemAdded.AddDynamic(this, &UMyWidget::HandleItemAdded);

The interface is always built on events: OnInventoryChanged, OnItemAdded, OnItemRemoved, OnItemChanged, OnAddRejected. They fire on both the server and the clients, so the same handler serves both modes.

The exception: ownerless containers

This is the only place where the networked nature leaks into the API. Better to understand it right away than to fight the consequences.

The problem. A Server RPC only works when the actor has a network connection through which the request can reach the server. The player's character has one. A chest in the middle of a level doesn't. The engine silently drops such a call.

// DOESN'T WORK on a client: the chest has no connection
ChestInventory->TryRemoveItemFromSlot(0, 1);

The solution. The request always goes through the player's inventory, which does have a connection. The server, on receiving it, authoritatively moves items in both inventories:

// WORKS everywhere
MyInventory->TryTransferFrom(ChestInventory, SlotIndex);   // take
MyInventory->TryTransferTo(ChestInventory, SlotIndex);     // put

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

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

A rule worth remembering: any interaction with a world container is initiated by the player's inventory.

Items on the ground — the same principle, but it's already done for you. A pickup has no connection either, so instead of Pickup->TryCollect(Player) call:

// WORKS everywhere: the route goes through the player's inventory
MyInventory->TryCollect(Pickup);

This is an ordinary Try* — it forwards itself from a client. AISDemoItemPickup::TryCollect also exists, but it's a low-level server call that the previous one ends up in.

What stays server-side

Three things don't route themselves, because they have nothing to route through:

Call What to do instead
AISDemoLootContainer::Open / Close call from the server side of your interaction
AISDemoItemPickup::SpawnForItem dropping an item is a Server RPC on the dropping player
UISDemoInventoryComponent::TryAddItemInstance a client can't pass the server an object pointer; use TryTransferFrom or TryCollect

All three refuse on a client and log — nothing breaks silently.

What stops a client asking for too much

The scheme above lets a client say "move slot 3 of that inventory into mine". Nothing in the shape of that sentence would stop it from naming another player's backpack — so the server takes care of that.

Every Server_* RPC, before doing any work, passes through UISDemoItemContainerComponent::CanAcceptRequestFrom. Who sent the request is taken from the connection the RPC arrived on, not from an argument — the argument is exactly what a modified client controls entirely.

When the request names a second container, it's checked only if items move out of it. The asymmetry is deliberate: taking from someone else's container is theft, putting into one is at worst a nuisance, and you can only give away what's yours. Requiring ownership of the destination too would break handing an item to another player, which the plugin itself cites as an example. A game that doesn't want unsolicited gifts overrides CanAcceptRequestFrom on the receiving side.

The default implementation accepts the request if one of two things holds:

  • the container belongs to the same player who sent the request (UNetConnections are compared, so a controller, pawn and PlayerState are all equally valid);
  • the container has no connection at all — it's a world container (a chest, a corpse, a vending machine), and it's open to anyone who reached it;

and additionally — if the project settings define a MaxInteractionDistance, the requester is within that distance of the container's owner.

[/Script/InventorySystemDemo.ISDemoInventorySettings]
; A server-side sanity limit, not an interaction range: set it several times
; larger than your real range so ordinary latency never trips it.
; 0 (the default) disables the check.
MaxInteractionDistance=1000.0

Override CanAcceptRequestFrom for anything your game adds on top: a locked chest, a shop with opening hours, a corpse only its killer may search.

The check runs only on requests that crossed the network. Calls on the server and every call in single-player go straight to the work.

What replicates and how

Inventory contents

FISDemoInventoryList is built on FFastArraySerializer: one slot changes — one slot is sent, not the whole inventory. The difference between 40 bytes and a few kilobytes when a player picks up a pebble.

Items (UISDemoItemInstance) are replicated UObject subobjects. The plugin supports both of UE's subobject replication mechanisms and follows the one your project chose:

  • the classic ReplicateSubobjects (default in UE 5.7);
  • the subobject registry, if the project turned on net.SubObjects.DefaultUseSubObjectReplicationList.

The plugin deliberately does not force the registry on: it only works when the owning actor turned it on too, and forcing it on a component dropped onto an "ordinary" actor would silently break item replication.

Inventory capacity

MaxSlots and MaxWeight replicate along with the contents. This matters exactly when capacity changes at runtime, and here's why.

A value set in Blueprint reaches clients on its own — it lives in the class defaults, and a client creates the component already carrying it. But a runtime assignment doesn't change the defaults: the server does Inventory->MaxSlots = 40, and without replication a client would stay with the number the component was born with.

The consequences of such a mismatch are purely client-side, but visible: IsFull(), GetFreeSlotCount(), IsValidSlot() and CanAcceptItemCount() on a client would answer for the old size, the grid would draw the wrong number of cells, and dragging into slot 35 would be highlighted as forbidden. The items themselves are never lost — every mutation is server-side, and the server always counts correctly.

This applies to two perfectly ordinary scenarios:

  • a backpack upgrade — a perk, a quest reward, a new bag;
  • auto-creation of components — the subsystem applies DefaultInventorySlots on the server, i.e. also at runtime.

Equipment

Also via FFastArraySerializer. The visual actor is not replicated — each machine creates its own from the Equippable fragment. So a sword in the hand costs exactly as much as the slot entry.

Who sees what

Contents replicate to all observers of the owning actor, not just the owner. This is a deliberate compromise: the same component class serves both a player's backpack and a world chest, whose contents everyone should see.

If you need strict privacy for a player's inventory, wrap access in your own UI — the plugin doesn't hide data at the replication level.

Single-player and multiplayer on one map

Nothing to switch. Components are created where the world has authority: in single-player that's the one world, on a network it's the server, and clients get replicas.

No branch of your code has to ask which mode it's in.

Automatic component attachment

UISDemoInventoryWorldSubsystem can create components for every player itself. Off by default — see 02 — Setup.

One mechanism covers login, late join, respawn, seamless travel and PIE. Manually placed components always take precedence.

Where exactly to create them:

Auto Inventory Host When appropriate
PlayerState the inventory survives death and respawn — a typical backpack
Pawn the inventory is lost with the body — extraction shooters, roguelikes

Testing in multiplayer

In the editor: Play → Net Mode → Play As Client, number of players ≥ 2.

Console commands work on a client too — they forward themselves to the server, and the report comes back:

DemoInvGive Potion 5
DemoInvList
DemoInvOverlay

The inspector shows the net mode with a coloured badge: green — the authoritative side, yellow — a client. Open it on both sides to see whether the state matches.

If an item didn't appear:

Log LogDemoInventorySystem Verbose

Prints every add, remove and move with slot indices.

Where to next