[unity] Action vs UnityEvent

When developing in Unity, there are two commonly used methods for implementing event systems: C#’s Action delegate and UnityEvent. Each of these methods has different uses and characteristics, with advantages and disadvantages depending on the context. In this post, we will compare the differences between Action and UnityEvent, along with their pros and cons.

What is Action?

Action is a delegate type in C# that can reference a method without a return value. It allows you to easily reference and invoke methods, making it useful for handling callbacks or directly managing events within your code. C#’s Action serves as a pointer to methods or lambda expressions, allowing you to register and invoke multiple methods efficiently. It is particularly helpful for managing events or handling callbacks in a clean and concise way.

Example

public class ActionExample
{
  public static void Main()
  {
    Action<string> printMessage = message => Console.WriteLine(message);
    printMessage("Hello, Action!");
  }
}

Pros

1. Performance : Action is lighter and faster compared to UnityEvent. UnityEvent uses reflection to handle events, whereas Action is resolved at compile time, reducing runtime overhead.
2. Simplicity : Action can be used directly in code, making it straightforward to implement and easy to refactor. Using lambda expressions makes the code even more intuitive and concise.
3. Type Safety : Action undergoes type checking at compile time, which prevents errors related to incorrect type assignments or method calls.

Cons

1. Direct Reference Required : Action needs to be assigned and referenced directly in the code, which makes it difficult for non-programmers to modify configurations.
2. No Design-Time Configuration : Since Action cannot be configured in Unity’s Inspector, it may be inconvenient for visually connecting events.

What is UnityEvent?

UnityEvent is a unique event system provided by Unity, designed to be easily configured and visually connected in the Unity Editor’s Inspector. It is particularly useful for setting up UI events or interactions with game objects, allowing non-programmers to handle event connections intuitively.

UnityEvent allows you to set up and link events easily in the Unity Inspector, which is useful for handling scenarios such as UI button clicks in an intuitive way.

Example

using UnityEngine;
using UnityEngine.Events;
public class UnityEventExample : MonoBehaviour
{
  public UnityEvent onButtonClick;

  void Start()
  {
    if (onButtonClick == null)  onButtonClick = new UnityEvent();
    onButtonClick.AddListener(TaskOnClick);
  }

  void TaskOnClick()
  {
    Debug.Log("Button clicked!");
  }
}

Pros

1. Inspector Integration : UnityEvent allows direct linking of event listeners in the Unity Editor, making it accessible for non-programmers to set up and manage events visually.
2. Visual Approach : Methods responding to events can be linked using drag-and-drop in the Inspector, allowing designers or level designers to set up event-driven behaviors without needing programming knowledge.

Cons

1. Performance Issues : UnityEvent uses reflection internally, which results in lower performance compared to Action. For frequently called events where performance is crucial, Action is a better choice.
2. Difficulty in Code Tracing : When methods are linked through UnityEvent, it can be difficult to trace them in the code. As projects grow, this can make maintenance more challenging.
3. Lack of Type Safety : UnityEvent is only type-checked at runtime, making it less type-safe compared to Action.

When to Use What?

- Code-Centric and Performance-Critical Scenarios : It is better to use Action, especially when events occur frequently or are best handled directly within the code.
- Collaboration with Non-Programmers or Inspector-Linked Events : UnityEvent is more suitable when events need to be configured visually in the Inspector, especially for UI elements or triggers where straightforward and intuitive connections are beneficial.

In conclusion, Action and UnityEvent are used in different contexts, and leveraging their strengths is key. Action offers better performance and code management efficiency, while UnityEvent enables visual and intuitive event configuration. Therefore, it is crucial to choose appropriately based on the requirements of your project.