How to use C# Events in Unity

Objective

The main objective of this blog post is to give you an idea about how to use C# Events in Unity.

What is problem with Delegates?

What is an Event?

Why Events are used?

How to declare an Event?

You may have all these questions in your mind.

Don’t worry, you are at the right place.

Let’s start, Do you remember the problem of Delegates?

If we use a single delegate after the use of multicast delegates then it will reset the existing invocation list.

This situation is very difficult to track if multiple classes subscribe to the same delegate.

This is the reason why we have Events.

Let’s see how they work.

Events are a type of special delegates which are used when you want to notify other classes when something happens. Like, on GameStart, on Gameover.

Let’s take a look on declaration and use of an Events.

1. Declare a Delegate

public delegate void OnGameStart();

2.Create an Event

public static event OnGameStart onGameStart;

3. Point to method

onGameStart += Move;

4. Invoke an Event

onGameStart ();

Let's, create a simple game using Event Delegate.

Step 1

Step 2

Write following code in EventManager.cs

public class EventManager : MonoBehaviour {
          public delegate void OnButtonClick();
          public static event OnButtonClick onButtonClick;
          public void RaiseOnButtonClick() {
            if (onButtonClick != null) {
                onButtonClick();
            }
        }
    }

Step 3

Step 4

Step 5

Write following code in PlayerTransformAndColor.cs

public class PlayerTransformAndColor : MonoBehaviour {
       
        public SpriteRenderer spriteRenderer;
        public Color color1, color2;
        private void OnEnable()
        {
            EventManager.onButtonClick += ChangePosition;
            EventManager.onButtonClick += ChangeColor;
         }
         public void ChangePosition() {
          transform.position += Vector3.one * 2;
        }
        public void ChangeColor() {
            spriteRenderer.color = color2;
        }
    }

Step 6

Now let’s do some changes in PlayerTransformAndColor.cs

public class PlayerTransformAndColor : MonoBehaviour {
        public SpriteRenderer spriteRenderer;
        public Color color1, color2;
        private void OnEnable()
        {
            EventManager.onButtonClick += ChangePosition;
            EventManager.onButtonClick += ChangeColor;
            EventManager.onButtonClick = ChangeRotation;
        }
        public void ChangePosition() {
            transform.position += Vector3.one * 2;
        }
        public void ChangeColor() {
            spriteRenderer.color = color2;
        }
        public void ChangeRotation() {
            transform.Rotate(0, 90, 0);
        }
       
    }

You may find the error. Like shown in below,

Got the  idea, How Events overcome the problem of Delegates.??

Events adds a layer of abstraction and protection on delegate, this protection prevents client of the delegate from resetting the delegate and invocation list.  

Now let’s take one more practical example. That gives you more clear idea about how event handle multiple event listeners by only one event Invocation.

For this, we’ll follow these steps.

Step 1

Step 2

Write following code in EventManager.cs

public class EventManager : MonoBehaviour {
    public delegate void OnRest();              //Declare a Delegate
    public static event OnRest onReset;         //Create an Event
    public delegate void OnReStart();           //Declare a Delegate
    public static event OnReStart onReStart;    //Create an Event
    public static void RaiseOnReset()
    {
        if (onReset != null)
        {
            onReset();                          //Invoke an Event
        }
    }
    public static void RaiseOnReStart()
    {
        if (onReStart != null)
        {
            onReStart();                       //Invoke an Event
        }
    }
}

Step 3

Step 4

Step 5

Write following code in EventHandler.cs

public class EventHandler : MonoBehaviour
{
      void Update()
    {
        if (Input.GetKeyDown(KeyCode.P))
        {
            EventManager.RaiseOnReset();                   // Function call to invoke an Event
        }
        else if (Input.GetKeyDown(KeyCode.S))
        {
            EventManager.RaiseOnReStart();                // Function call to invoke an Event
        }
    }
}

Step 6

Step 7

Create a 3D game object(Cube).

Step 8

Write following code in ObjectMotion.cs

  public class ObjectMotion : MonoBehaviour
    {
        private float minRange, maxRange;
        private float speed, restartSpeed;
        private Vector3 newPosition;
        private void OnEnable()
        {
            EventManager.onReset += Reset;
            EventManager.onReStart += Restart;
        }
          void Start()
        {
            minRange = 1;
            maxRange = 3;
            speed = Random.Range(minRange, maxRange);
            restartSpeed = speed;
        }
         void Update()
        {
            newPosition = transform.position;
            newPosition.x = Mathf.Sin(Time.time) * speed;
            transform.position = newPosition;
        }
        private void OnDisable()
        {
            EventManager.onReset -= Reset;
            EventManager.onReStart -= Restart;
        }
        public void Reset()
        {
            speed = 0;
        }
        public void Restart()
        {
            speed = restartSpeed;
        }
    }

Step 9

Now create multiple copies of 3D GameObject.

Run Unity, and press "P" to reset the position and "S" to restart the motion.

From the above examples we may come to the  following conclusion,

Conclusion

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