component_group.md

ComponentGroup

The ComponentGroup is a foundation class on top of which all iteration methods are built (Injection, foreach, IJobProcessComponentData etc)

Essentially a ComponentGroup is constructed with a set of required components, and/or subtractive components.

The ComponentGroup lets you extract individual arrays. All these arrays are guaranteed to be in sync (same length and the index of each array refers to the same Entity).

Generally speaking GetComponentGroup is used rarely, since ComponentGroup Injection and IJobProcessComponetnData is simpler and more expressive.

However the ComponentGroup API can be used for more advanced use cases like filtering a Component Group based on specific SharedComponent values.

struct SharedGrouping : ISharedComponentData
{
    public int Group;
}

class PositionToRigidbodySystem : ComponentSystem
{
    ComponentGroup m_Group;

    protected override void OnCreateManager(int capacity)
    {
        // GetComponentGroup should always be cached from OnCreateManager, never from OnUpdate
        // - ComponentGroup allocates GC memory
        // - Relatively expensive to create
        // - Component type dependencies of systems need to be declared during OnCreateManager,
        //   in order to allow automatic ordering of systems
        m_Group = GetComponentGroup(typeof(Position), typeof(Rigidbody), typeof(SharedGrouping));
    }

    protected override void OnUpdate()
    {
        // Only iterate over entities that have the SharedGrouping data set to 1
        // (This could for example be used as a form of gamecode LOD)
        m_Group.SetFilter(new SharedGrouping { Group = 1 });
        
        var positions = m_Group.GetComponentDataArray<Position>();
        var rigidbodies = m_Group.GetComponentArray<Rigidbody>();

        for (int i = 0; i != positions.Length; i++)
            rigidbodies[i].position = positions[i].Value;
            
        // NOTE: GetAllUniqueSharedComponentDatas can be used to find all unique shared components 
        //       that are added to entities. 
        // EntityManager.GetAllUniqueSharedComponentDatas(List<T> shared);
    }
}

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