injection.md

Injection

Injection allows your system to declare its dependencies, while those dependencies are then automatically injected into the injected variables before OnCreateManager, OnDestroyManager, and OnUpdate.

Component Group Injection

ComponentGroup injection automatically creates a ComponentGroup based on the required component types.

This lets you iterate over all the entities matching those required component types. Each index refers to the same Entity on all arrays.

class MySystem : ComponentSystem
{
    public struct Group
    {
        // ComponentDataArray lets us access IComponentData 
        [ReadOnly]
        public ComponentDataArray<Position> Position;
        
        // ComponentArray lets us access any of the existing class Component                
        public ComponentArray<Rigidbody> Rigidbodies;

        // Sometimes it is necessary to not only access the components
        // but also the Entity ID.
        public EntityArray Entities;

        // The GameObject Array lets us retrieve the game object.
        // It also constrains the group to only contain GameObject based entities.                  
        public GameObjectArray GameObjects;

        // Excludes entities that contain a MeshCollider from the group
        public SubtractiveComponent<MeshCollider> MeshColliders;
        
        // The Length can be injected for convenience as well 
        public int Length;
    }
    [Inject] private Group m_Group;


    protected override void OnUpdate()
    {
        // Iterate over all entities matching the declared ComponentGroup required types
        for (int i = 0; i != m_Group.Length; i++)
        {
            m_Group.Rigidbodies[i].position = m_Group.Position[i].Value;

            Entity entity = m_Group.Entities[i];
            GameObject go = m_Group.GameObjects[i];
        }
    }
}

ComponentDataFromEntity injection

ComponentDataFromEntity<> can also be injected, this lets you get / set the ComponentData by Entity from a job.

class PositionSystem : JobComponentSystem
{
    [Inject] ComponentDataFromEntity<Position> m_Positions;
}

Injecting other systems

class PositionSystem : JobComponentSystem
{
    [Inject] OtherSystem m_SomeOtherSystem;
}

Lastly you can also inject a reference to another system. This will populate the reference to the other system for you.

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