Observer

Observer

One-line pattern summary

A reactive pattern where subscribers are automatically notified when the publisher’s state changes.

Typical Unity use cases

• When UI, sound, and achievements should all react to health changes.
• When a loosely coupled event connection is needed.

Parts (roles)

• Subject
• Observer
• Subscribe / Unsubscribe

Unity example (C#)

The code below is a simplified Unity example based on the scenario described above.

using System;

public sealed class HealthModel
{
    public event Action<int, int> HealthChanged;

    public void SetHealth(int currentHealth, int maxHealth)
    {
        HealthChanged?.Invoke(currentHealth, maxHealth);
    }
}

public sealed class HealthBarPresenter
{
    public void Bind(HealthModel healthModel)
    {
        healthModel.HealthChanged += OnHealthChanged;
    }

    private void OnHealthChanged(int currentHealth, int maxHealth) { }
}

Advantages

• Behavior is separated into smaller units, which reduces the impact of changes.
• Adding or swapping rules is relatively safe.

Things to watch out for

• As the number of objects and indirect calls increases, the flow can become harder to follow.
• Ordering bugs should be pinned down with tests.

Interaction diagram

This shows the flow where a subject state change is automatically propagated to subscribers.