Components, subobjects, and Fast Arrays

Replicated ActorComponents

Both must be true:

• owning Actor replicates;
• ActorComponent replicates (SetIsReplicatedByDefault(true) in the component constructor for a default behavior, or the appropriate instance setup).

Components can replicate properties, RPCs, and their own subobjects. Component RPCs add more overhead than Actor RPCs; route high-volume calls through the Actor when appropriate.

Replicated UObjects/subobjects

Use a UObject subobject when state belongs to an Actor/component but needs its own identity and replicated properties without becoming a world Actor.

Preferred forward-compatible procedure:

1. Make the outer/owner a replicated Actor or replicated ActorComponent.
2. Enable bReplicateUsingRegisteredSubObjectList on that owner.
3. Create the subobject on the server with a stable ownership relationship.
4. Register it with AddReplicatedSubObject and an optional condition.
5. Remove/unregister it before deletion when required by the lifetime path.
6. Wake/flush the owning Actor before changing a dormant subobject.

The registered subobject list works with Generic Replication and RepGraph and is the only supported subobject method for Iris. Do not enable Iris while relying only on legacy ReplicateSubobjects overrides.

Fast Arrays

Use FFastArraySerializer for replicated collections with item-level additions, removals, and changes where a normal whole-array representation is too costly or lacks item callbacks.

Implementation contract:

• item struct derives from FFastArraySerializerItem;
• container struct derives from FFastArraySerializer and owns the item array;
• provide the required delta serialization trait/function for the chosen pattern;
• call MarkItemDirty after an item changes;
• call MarkArrayDirty for structural changes when required by the operation;
• handle add/change/remove callbacks as received-state reactions;
• use stable item identity and do not retain pointers to entries that may move.

Verify exact signatures against FastArraySerializer.h for the engine build; do not invent them from memory. Iris has a native Fast Array path, but Iris itself remains Experimental.

Selection

Data	Representation
A few scalar states	actor/component replicated properties
Cohesive values needing ordered handling	one replicated struct
Large changing item collection	Fast Array
Modular owned state object	registered replicated UObject subobject
Independent world relevance/transform	replicated Actor