PolymorphicStructs

See the forum thread for more information https://forum.unity.com/threads/sources-included-polymorphism-in-dots-now-with-source-generators.1264616/

Installing into your project

1. Import the PolymorphicStructs.unitypackage into your project (find it in the Releases section)
2. Make sure all of the code that's going to use PolymorphicStructs is under an .asmdef and references the PolymorphicStructs.asmdef

How to use

1. Create an interface that will represent the various functions your polymorphic structs will share. This interface must have the [PolymorphicStruct] attribute on it

[PolymorphicStruct]
public interface IMyState
{
    public void OnStateEnter(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData);
    public void OnStateExit(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData);
    public void OnStateUpdate(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData); 
}

2. Create the structs that implement that interface, representing all the different forms that the polymorphic stuct can assume. Make sure these structs are partial

[Serializable]
public partial struct StateA : IMyState
{
    public float Duration;
    public float3 MovementSpeed;

    private float _durationCounter;

    public void OnStateEnter(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData)
    {
        _durationCounter = Duration;
    }

    public void OnStateExit(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData)
    {
        refData.Translation = refData.StateMachine.StartTranslation;
    }

    public void OnStateUpdate(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData)
    {
        refData.Translation += MovementSpeed * inData.DeltaTime;

        _durationCounter -= inData.DeltaTime;
        if (_durationCounter <= 0f)
        {
            refData.StateMachine.CurrentStateIndex = refData.StateMachine.StateBIndex;
        }
    }
}


[Serializable]
public partial struct StateB : IMyState
{
    public float Duration;
    public float3 RotationSpeed;

    private float _durationCounter;

    public void OnStateEnter(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData)
    {
        _durationCounter = Duration;
    }

    public void OnStateExit(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData)
    {
        refData.Rotation = quaternion.identity;
    }

    public void OnStateUpdate(ref StateUpdateData_ReadWrite refData, in StateUpdateData_ReadOnly inData)
    {
        refData.Rotation = math.mul(quaternion.Euler(RotationSpeed * inData.DeltaTime), refData.Rotation);

        _durationCounter -= inData.DeltaTime;
        if (_durationCounter <= 0f)
        {
            refData.StateMachine.CurrentStateIndex = refData.StateMachine.StateCIndex;
        }
    }
}

3. This will generate a MyState struct (takes the name of the interface, minus the first character "I") that will have all the functions of the IMyState interface. You can now call polymorphic functions on MyState like this and it will call the implementation of whichever version of the struct was assigned to MyState:

myState.OnStateUpdate(...)

4. You can also create a MyState from the various specific structs and vice-versa like this:

// Creating a MyState from a StateA
StateA stateA = new StateA();
MyState state = stateA.ToMyState();

// Creating a StateA from a MyState
StateA stateA = new StateA(state);

How the sample works

1. TestScene is the sample scene
2. CubeSpawnerSystem initializes the scene by spawning thousands of cube prefabs (Cube)
3. The Cube prefab has a StateMachineAuthoring component
4. The StateMachineAuthoring component adds a MyStateMachine as well as a DynamicBuffer<MyState>. This buffer is the place where we store all of our polymorphic states. You can see how we create a polymorphic MyState from individual state structs in the StateMachineAuthoring.AddStateToStatesBuffer() method
5. MyStateMachineSystem iterates over all state machine entities, and for each one, handles calling OnStateUpdate() on whichever state is set as the CurrentState. It also handles transitions