What is the difference between Component, Behaviour and MonoBehaviour? And why these are separated?

Question

MonoBehaviour extends Behaviour and Behaviour extends Component. I want to know why these classes are separated and semantic meanings of these classes. Are there any purpose to separate these classes? And are there any classes extending Behaviour or Component directly?

I know we must use MonoBehaviour to create C# codes in Unity. However, I'm interesting in architecture of Unity as a game engine.

Answer 1

Component:

Component is a base class for both Behaviour and MonoBehaviour. It is a base class of any script that can be attached to a GameObject.

This is what almost every Unity build-in script derives from. This includes Collider and Rigidbody.

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Behaviour:

The most important thing to know about Behaviour is that anything that inherits from it can be enabled or disabled. Do not inherit your script from this if you don't want it to be enabled/disabled.

For example, Rigidbody cannot be enabled/disabled. This is why it inherits from the Component class instead of Behaviour.

Behaviour inherits from Component.

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MonoBehaviour:

The most important thing to note about MonoBehaviour is that you need it when you have to use coroutines, Invoking, or any Unity callback functions such as physics OnCollisionEnter function, Start, OnEnable, OnDisable, etc.

MonoBehaviour inherits from Behaviour so that your scripts can be enabled/disabled.

Are there any purpose to separate these classes?

Yes, like the answer above, they are separated so that you can use different features by inheriting from the appropriate component.

You want coroutine, Invoke, InvokeRepeating and be able to enable/disable your script? Use inherits from MonoBehaviour.

You want to be able to enable or disable your script but don't need coroutine, Invoke, InvokeRepeating? Use Behaviour.

When you have a scripts that should never be enabled/disabled, that should inherit from Component.

Note that Behaviour and Component are used by Unity for internal stuff. You should not try to inherit your script from these.

The main reason for separating these is to conserve memory and also make the code in each component easy to maintain. This is better than writing thousands of lines of codes for a feature that one rarely use. By separating them and when they are loaded into the memory they are actually being used and not wasting space in the memory.

And are there any classes extending Behaviour or Component directly?

Yes.

• MonoBehaviour inherits directly from Behaviour which then inherits from Component.

• Collider inherits from Component directly. Then non base colliders such as BoxCollider and SphereCollider inherits from that Collider.