event-systems

Event Systems

Events decouple systems. The publisher doesn't know who's listening. The subscriber doesn't know who's publishing.

When to Use Each Type

C# Events (Preferred for Code)

public sealed class HealthSystem : MonoBehaviour
{
    // Declare event
    public event System.Action<float, float> OnHealthChanged; // current, max
    public event System.Action OnDied;

    [SerializeField] private float _maxHealth = 100f;
    private float _currentHealth;

    public void TakeDamage(float amount)
    {
        _currentHealth = Mathf.Max(0f, _currentHealth - amount);
        OnHealthChanged?.Invoke(_currentHealth, _maxHealth);

        if (_currentHealth <= 0f)
        {
            OnDied?.Invoke();
        }
    }
}

// Subscriber
public sealed class HealthBar : MonoBehaviour
{
    [SerializeField] private HealthSystem _health;

    private void OnEnable()
    {
        _health.OnHealthChanged += UpdateBar;
        _health.OnDied += HandleDeath;
    }

    private void OnDisable()
    {
        _health.OnHealthChanged -= UpdateBar;
        _health.OnDied -= HandleDeath;
    }

    private void UpdateBar(float current, float max)
    {
        // Update UI
    }

    private void HandleDeath()
    {
        // Show death screen
    }
}

CRITICAL: Always Unsubscribe

Subscribe in OnEnable, unsubscribe in OnDisable. This handles:

• Object deactivated (SetActive(false) → OnDisable)
• Object destroyed (OnDisable → OnDestroy)
• Scene unloaded

// GOOD — symmetric subscribe/unsubscribe
private void OnEnable() => _source.OnEvent += HandleEvent;
private void OnDisable() => _source.OnEvent -= HandleEvent;

// BAD — memory leak if object is destroyed or deactivated
private void Start() => _source.OnEvent += HandleEvent;
// No unsubscribe → delegate holds reference → object can't be GC'd