Iris maintains backward compatibility with Unreal Engine’s (UE) generic replication system as much as possible. However, you might need to make changes to your gameplay code to accommodate key differences between the two systems.
One of Iris's key design principles is to minimize the number of interactions between the replication system and gameplay code. To accomplish this, Iris reduces the number of virtual function calls. These virtual function calls from the generic system are replaced with explicit calls to API functions through the Iris replication system.
The following table lists:
- Features of the current replication system.
- Whether they have changed in Iris.
- What the corresponding Iris feature is if the system has changed.
- Links to the documentation page for more information.
| Existing Replication Feature | Changed in Iris | Iris Replication Feature |
|---|---|---|
| Dormancy | ||
| Priority | ✓ | Iris Prioritization |
| Property Replication | ||
| Relevancy | ✓ | Iris Filtering |
| Remote Procedure Calls | ||
| Subobject Replication | ✓ | Iris Subobject Replication |
Anything in the above table that is not checked as Changed in Iris works as it does in the generic replication system.
Push Model
Iris aims to be fully push-based. It is possible to use Iris without push model replication. If push model replication is not enabled for a particular object, Iris automatically falls back on polling the object based on its NetUpdateFrequency. By default, Iris honors the standard push model settings.
Replicated Properties
Replicated properties work as they do in the generic replication system. For more information about how replicated properties work in UE, see the Replicate Actor Properties documentation.
Remote Procedure Calls
Remote Procedure Call (RPC) declaration and execution in Iris works as it does in the generic replication system and replication graph. For more information about how RPC and replicated property updates are executed on receiving machines, see the Replication Execution Order documentation.
Subobject Replication
Iris requires you enable the registered subobject list. To use the registered subobject list for your actor class, add the following to your replicated actor’s constructor:
bReplicateUsingRegisteredSubObjectList = true;
If you are using Iris, replicated subobjects not derived from AActor or UActorComponent must also implement the virtual function RegisterReplicationFragments to register their replicated properties and functions. To implement RegisterReplicationFragments for your UObject-derived class UMyDerivedObject, add the following code to your MyDerivedObject.h and MyDerivedObject.cpp files, respectively:
MyDerivedObject.h
#if UE_WITH_IRIS
// Register replication fragments
virtual void RegisterReplicationFragments(UE::Net::FFragmentRegistrationContext& Context, UE::Net::EFragmentRegistrationFlags RegistrationFlags) override;
#endif // UE_WITH_IRIS
MyDerivedObject.cpp
#if UE_WITH_IRIS
#include "Iris/ReplicationSystem/ReplicationFragmentUtil.h"
#endif // UE_WITH_IRIS
#if UE_WITH_IRIS
void UMyDerivedObject::RegisterReplicationFragments(UE::Net::FFragmentRegistrationContext& Context, UE::Net::EFragmentRegistrationFlags RegistrationFlags)
{
// Build descriptors and allocate PropertyReplicaitonFragments for this object
UE::Net::FReplicationFragmentUtil::CreateAndRegisterFragmentsForObject(this, Context, RegistrationFlags);
}
#endif // UE_WITH_IRIS
For more information about replicated subobjects, see the Replicated Subobjects documentation.
Custom Network Serializers
Iris supports all Unreal Engine primitive types that can be used as replicated properties for network serialization. If a struct uses a custom NetSerialize method and is missing an Iris-specific implementation, the following warning is logged:
Warning: Generating descriptor for struct STRUCT_NAME that has custom serialization.
If the data in the NetSerialize method can replicate using only property network serialize methods, you can silence the warning by adding an entry to your project’s DefaultEngine.ini file:
DefaultEngine.ini
[/Script/IrisCore.ReplicationStateDescriptorConfig]
; Declarate structs that are vetted to work using reflection based struct serialization even though there exists a custom NetSerialize function for the struct
+SupportsStructNetSerializerList=(StructName=STRUCT_NAME)
Fast Array Replication
Iris supports existing fast array definitions. Iris also provides a specialized fast array serializer, FIrisFastArraySerializer, located in IrisFastArraySerializer.h.
Peer-to-Peer
Iris supports Unreal Engine listen servers. Using a listen server, a game instance can act as the host of a multiplayer game session while also supporting its own, local players.
Differences Specific to Replication Graph
The Replication Graph plugin is a network replication system built upon nodes containing lists of persistent objects to replicate. Iris does not support replication graph. Iris and Replication Graph are separate systems, and the network driver can only use one or the other. Iris does not have a concept of a node in the same sense of replication graph to control when and where actors are replicated. Instead, the new network object filters and prioritizers are intended as a replacement for replication graph’s functionality. For more information, see the Filtering and Prioritization documentation.