Dmytro Gladkyi
Dmytro Gladkyi

Dmytro Gladkyi

The Magic of Start/Update Events in Unity's MonoBehaviour

The Magic of Start/Update Events in Unity's MonoBehaviour

Dmytro Gladkyi's photo
Dmytro Gladkyi
·May 25, 2022·

2 min read

The Magic of Start/Update Events in MonoBehaviour

If you programmed at least something in the Unity then you definitely extended the MonoBehaviour class and defined your custom Start, Update, Awake, etc methods.

But the method is private and it is not possible to override private methods in C# . They have to be either public or protected...

And did you know that...these methods were not derived from the parent MonoBehaviour class?

 // Start is called before the first frame update
void Start()
{
    audioSource = GetComponent<AudioSource>();
    player = FindObjectOfType<PlayerController>();
    if (objectPool.Count == 0)
    {
        prepareContainerObjects();
    }

    hideRequirementInfo();
}


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

Go to MonoBehaviour.cs and check that it has no methods. Also its parent - Behaviour...

mono_behaviour.png

Then how does it work? How does Unity know there is Start method that should be called on MonoBehaviour subclasses?

Event System

Easy! Unity uses its own messaging system that's using C# reflection feature

The methods Start/Update/etc are magic methods that can be defined in your MonoBehaviour subclasses. They do not override the implementation from base class. The Unity engine uses Component.SendMessage method in order to dispatch commands to your MonoBehaviour subclass.

/// <summary>
///   <para>Calls the method named methodName on every MonoBehaviour in this game object.</para>
/// </summary>
/// <param name="methodName">Name of the method to call.</param>
/// <param name="value">Optional parameter for the method.</param>
/// <param name="options">Should an error be raised if the target object doesn't implement the method for the message?</param>
public void SendMessage(string methodName, object value) => this.SendMessage(methodName, value, SendMessageOptions.RequireReceiver);

The class hierarchy looks like this for your script:

YourScript -> MonoBehaviour -> Behaviour -> Component

So, whenever the Unity engine decides to call Update method on all objects it calls Component.SendMessage method on the object . It checks whether Start/Update/Whatever from the magic list of methods is defined and will call them.

So what?

So, if you have these events with empty bodies - delete them from your code! Before that I thought that defining empty method had 0 impact as it would be called anyway from parent class. But as you can see from the investigation: the call will be ignored! So you can save another 1-2 nanoseconds on each object as the Engine will just skip it.

 
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