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Design Pattern / Behavioral

[Design Pattern] Observer Pattern

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

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  • Design Pattern
  • Behavioral
Contents

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.

flowchart LR

  subject["HealthModel"]
  notify["Notify()"]

  subgraph observers["Observers"]
    hpbar["HealthBar"]
    sfx["DamageFX"]
    achievement["AchievementTracker"]
  end

  subject -->|health changed| notify
  notify --> hpbar
  notify --> sfx
  notify --> achievement

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