Attach a non Monobehavior class to a GameObject

Question

T$$anonymous$$s is my issue:

I want to assign a script that inherits from an abstract class to a GameObject. I used to create MonoBehaviour scripts like "BirdScript" or "CarScript" but I want use proper classes and be able to control the gameobject the class is attached to.

Let me show you an example:

I have to following classes

 public abstract class Shape : MonoBehaviour {

    private int weight;
    private string color;

    protected int Weight
    {
        get { return weight; }
        set { weight = value; }
    }

    protected string Color
    {
        get { return color; }
        set { color = value; }
    }

    //Example abstract method
    public abstract void calculateVolume();
 }

 public class Cube : Shape {

    public void Move()
    {
        Vector3 pos = transform.position;
        pos.x = 2;
        transform.position = pos;
    }

    public override void calculateVolume()
    {
       //Just print a string
       MonoBehaviour.print("Cube volume");
    }
 }

I also have a prefab GameObject called MyCube w$$anonymous$$ch has the Cube class attached

Now I want to create an instance of MyCube from another GameObject w$$anonymous$$ch has the following script attached

 public class ShapeManager : MonoBehaviour
 {
    [SerializeField]
    public GameObject cube;

    // Use t$$anonymous$$s for initialization
    void Start()
    {
        GameObject c = Instantiate(cube);
        c.GetComponent<Cube>().calculateVolume();
        c.GetComponent<Cube>().Move();
    }

    // Update is called once per frame
    void Update() {}
 }

The GameObject cube has the MyCube prefab attached using the Inspector.

My questions are:

What if Cube wouldn't inherit a Monobehavior class ?

Is t$$anonymous$$s approach correct ?

Does it violate OOP ?

The way ShapeManager controls the cube is right ?

 GameObject c = Instantiate(cube);
 c.GetComponent<Cube>().calculateVolume();
 c.GetComponent<Cube>().Move();

My apologies for t$$anonymous$$s long post :)

Answer 1

If neither shape nor cube actually use any of the features of a MonoBehavior, like say... defining Update(), serializing data to be saved/loaded, or just accessing the gameObject member, then I would say you can certainly choose to NOT inherit from monobehavior.

However, the cube's move function, DOES access "transform", a property of the monobehavior.

Still, if not actually overriding Monobehavior functions, it's always possible to store a reference to a game object in the class itself (specified as a parameter to the constructor of the class), rather than derive from Monobehvior.

Is it correct? Well, as long as it works, it's just a mater of opinion. Using a reference to an object, rather than deriving from an unnecessary class- does not violate OOP.

I'm not quite sure what shape manager is supposed to do, but it looks like it will create a new object, and if that object has a cube compoent (that is derived from monobehavior), it will call move on it. If the object does not have a cube component it will generate a null exception. If cube is not derived from monobehavior, use of GetComponent will fail, or possibly not even compile.

Update/Edit: Here is an example of how I would do it using both classes derived from monobehavior, and a class NOT derived from monobehavior.

 abstract public class Candy : MonoBehaviour
 {
     public int pointValue;
     public float speed;
 
     abstract public void Move();
     abstract public bool MoveCondition();
 
     void Start() { }
     void Update() { if (MoveCondition()) Move(); }
 }
 public class LemonDrop : Candy
 {
     override public void Move() { /* manipulate member variable "transform" to drop down candy*/ }
     override public bool MoveCondition() { /* use gameObject to check for certain collisions */ }
 }
 public class CottonCandy : Candy
 {
     override public void Move() { /* manipulate member variable "transform" to float around*/ }
     override public bool MoveCondition() { return true; }
 }
 
 //usage
 
 static class CandyManager
 {
     public static List<Candy> allCandy=null; //note that t$$anonymous$$s is "effectively" a list of gameobjects, that all have a candy component
 
     //dont let the <Tparam> throw you off, just t$$anonymous$$nk of it as a parameter that takes a "type"
     static void CreateCandy<Tparam>(Vector3 position) where Tparam : Candy
     {
         //create gameobject & add component (or load prefab with component)
         GameObject g = new GameObject("candy");
         g.transform.position = position;
         Candy c=null;
         c=g.AddComponent<Tparam>();
 
         //add it to allCandy List
         if (allCandy == null)
             allCandy = new List<Candy>();
         allCandy.Add(c);
     }
 
     static void InitLevel()
     {
         CreateCandy<LemonDrop>(Vector3.up);
         CreateCandy<LemonDrop>(Vector3.up+ Vector3.right);
         CreateCandy<CottonCandy>(Vector3.right);
         CreateCandy<CottonCandy>(Vector3.forward);
     }
 }

Attach a non Monobehavior class to a GameObject

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