shared_component_data.md

Shared ComponentData

IComponentData is appropriate for data that varies between entities, such as storing a World position. ISharedComponentData is useful when many entities have something in common, for example in the Boid demo we instantiate many entities from the same Prefab and thus the MeshInstanceRenderer between many Boid entities is exactly the same.

[System.Serializable]
public struct MeshInstanceRenderer : ISharedComponentData
{
    public Mesh                 mesh;
    public Material             material;

    public ShadowCastingMode    castShadows;
    public bool                 receiveShadows;
}

In the Boid demo we never change the MeshInstanceRenderer component, but we do move all the entities' Transform every frame.

The great thing about ISharedComponentData is that there is literally zero memory cost on a per Entity basis.

We use ISharedComponentData to group all entities using the same InstanceRenderer data together and then efficiently extract all matrices for rendering. The resulting code is simple & efficient because the data is laid out exactly as it is accessed.

• MeshInstanceRendererSystem.cs

Some important notes about SharedComponentData:

• Entities with the same SharedComponentData are grouped together in the same Chunks. The index to the SharedComponentData is stored once per Chunk, not per Entity. As a result SharedComponentData have zero memory overhead on a per Entity basis.
• Using ComponentGroup we can iterate over all entities with the same type.
• Additionally we can use ComponentGroup.SetFilter() to iterate specifically over entities that have a specific SharedComponentData value. Due to the data layout this iteration has low overhead.
• Using EntityManager.GetAllUniqueSharedComponents we can retrieve all unique SharedComponentData that is added to any alive entities.
• SharedComponentData are automatically reference counted.
• SharedComponentData should change rarely. Changing a SharedComponentData involves using memcpy to copy all ComponentData for that Entity into a different Chunk.

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