The entity pipeline is the mechanism which an Unreal worker uses to create and manage its representation of entities.
A SpatialOS deployment communicates with connected workers to instruct them when an entity or its components need to be added or removed from the worker’s view. For example, when an entity moves into or out of a worker’s checkout area. The entity pipeline provides an interface for calling user-defined functions in response to entity-related ops from SpatialOS. As part of the Unreal SDK, we provide a standard implementation that should suit most people’s needs, but we’ll be adding support for customisation of this system in the future.
What the entity pipeline does
Different workers may represent the same entity in different ways. In Unreal, workers represent entities using Actors
which are based on a blueprint or C++ class. Entities have a
Metadata component with an
property that is used to specify which Actor should be used to represent it.
Placing entities in the world
All entities have a
Position component that represents their canonical position in the world. It contains a vector of three
double values, and it’s used to govern which entities a worker checks out, load-balancing, and more.
In most cases, entities need to be positioned on a worker in a way that meaningfully represents their
in the simulated world. However, there are some cases where you’d want to implement custom logic for transforming
positions - for example, if the world is very large or very small. Or you might want a secondary representation for
Handling component-related changes
When entities change, workers get instructions to make the relevant change; eg
AuthorityChange. The entity pipeline handles these instructions, and uses the code
generated from your schemas to respond to these changes.
Using the entity pipeline
The entity pipeline is made up of one or more blocks that together define how incoming operations should be handled. Blocks must be added to the pipeline before a connection to a SpatialOS deployment is created. Any attempts to add blocks after the connection has been established will result in an assertion.
The default pipeline behaviour
The SDK currently comes with a basic implementation of an entity pipeline block that replaces the old
USimpleEntitySpawnerBlock class provides the following behaviour:
Adding entities and components to the world
- Upon receiving an
AddEntityoperation, the block waits to receive the
AddComponentoperation for the
Metadatacomponents that correspond to that particular entity.
- The received
Positioncomponents are used to spawn an Actor in the world by using their respective
- All future
AddComponentoperations are used to set the initial state of the corresponding spawned Actor’s generated
Removing entities and components from the world
- Upon receiving a
RemoveComponentoperation, the Actor referring to this specific entity has
Disable()called on its corresponding generated
- Upon receiving a
RemoveEntityoperation, the Actor associated with this entity has
DestroyActor()called on it.
Adding blocks to your pipeline
Blocks are added to a pipeline like this:
USimpleEntitySpawnerBlock* EntitySpawnerBlock = NewObject<USimpleEntitySpawnerBlock>(); EntitySpawnerBlock->Init(EntityRegistry); SpatialOSInstance->GetEntityPipeline()->AddBlock(EntitySpawnerBlock);
The lifetime of all pipeline block objects are controlled the
USpatialOS object once they’ve been added to the pipeline.
Allowing the pipeline to process operations
In order to process all the operations that are being received, the pipeline must have its
ProcessOps() function called
periodically. We recommend calling this every
Tick(), similarly to the
SpatialOS object. In the RPGDemo project, this
is done in the
SpatialOSInstance->ProcessOps(); SpatialOSInstance->GetEntityPipeline()->ProcessOps(SpatialOSInstance->GetView(), SpatialOSInstance->GetConnection(), GetWorld());
Customizing the entity pipeline
We’re working towards making the entity pipeline fully configurable in order to provide complete control over how and when entities exist in your game. As an example, you could create your own pipeline block that limits the number of actors that are spawned in a particular game frame.
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