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Composition Over Inheritance

At Gnarly Game Studio, we strongly favor the composition pattern over traditional object-oriented inheritance when building our game systems. This decision stems from the need for flexibility, scalability, and maintainability—all critical elements of game development. While object-oriented programming (OOP) has its place, its limitations can be a significant hindrance in a dynamic environment like game development, where systems need to evolve quickly and remain decoupled.

An added benefit of using the composition pattern in Unity is that Unity itself already follows a component-based architecture. This makes composition a more natural and seamless approach in the context of Unity development, where objects are already designed to be composed of independent components.

Why Composition Over Inheritance?​

1. Flexibility and Modularity​

The primary advantage of the composition pattern is the flexibility it offers. Instead of creating rigid hierarchies through inheritance, objects are built by composing behaviors from smaller, modular components. This allows us to mix and match behaviors as needed, resulting in systems that are more adaptable and easier to maintain.

For example:

  • If you want to create multiple types of enemies (e.g., flying, walking, swimming enemies), a traditional OOP approach might involve creating a class hierarchy where each enemy type inherits from a common base class. However, as behaviors (such as flying or swimming) become more complex, you end up with deep inheritance trees that are difficult to extend or refactor.
  • With composition, you can build each enemy by assembling components (like "FlyingBehavior", "SwimmingBehavior", or "AttackingBehavior") that define specific functionality. This avoids the inflexibility of class inheritance and allows behaviors to be combined and reused without being locked into a rigid class hierarchy.

2. Unity’s Component Architecture​

Unity already uses a component-based architecture, which naturally aligns with the composition pattern. In Unity, every game object is made up of a collection of components, where each component handles a specific piece of functionality—such as movement, physics, rendering, or input handling.

This architecture emphasizes composition by design, where instead of inheriting from a monolithic base class, game objects are built by attaching multiple components to a single object. This makes the composition pattern a natural fit for Unity development, where breaking down behaviors into reusable, independent components is already the standard way of working.

Example in Unity:​

  • A Player game object might have separate components like PlayerMovement, PlayerHealth, PlayerInventory, and PlayerWeapon. Each of these components handles a specific aspect of the player’s behavior, and they can be developed, tested, and modified independently.
  • By following this architecture, new functionality can be added or existing behaviors modified without changing the underlying game object structure. If we want to add a new ability to the player (like a DashComponent), it’s simply a matter of attaching the new component, without changing the core player class or worrying about inheritance issues.

3. Avoiding Deep Inheritance Hierarchies​

In OOP, inheritance can lead to deep class hierarchies where functionality is shared among parent and child classes. While this is often useful, it has its downsides:

  • Fragility: Changes in base classes can unintentionally break functionality in subclasses.
  • Tight Coupling: Subclasses are tightly coupled to their parent classes, making it harder to modify or refactor the code without affecting the entire hierarchy.
  • Code Duplication: When behavior doesn’t fit neatly into the existing hierarchy, developers often end up duplicating code across different classes.

With composition, we avoid these pitfalls by creating systems that are based on the assembly of independent components. Each component is responsible for a specific piece of functionality and can be reused across different objects. This leads to looser coupling between systems and allows for easier code reuse without creating deep dependencies.

Example in Game Development:​

In a traditional OOP approach, if you have an RPG game with characters like Warrior and Mage, they might both inherit from a Character base class, with unique combat behavior implemented in each subclass. Over time, as you add more specialized character types (e.g., Paladin, Sorcerer), the inheritance tree grows and becomes increasingly difficult to manage.

By using composition, you can instead create components like MeleeAttack, MagicAttack, and DefenseBuff, which can be combined dynamically to create any character type without relying on deep inheritance.

4. Reusability of Components​

A major benefit of the composition pattern is reusability. Once you’ve created a component, it can be used across different game objects or systems without modification. This contrasts with inheritance, where behaviors are often locked into specific class hierarchies and are difficult to reuse in other contexts.

For example:

  • A HealthComponent could be added to any game object that requires health tracking, whether it's a player, an NPC, or even destructible environment objects like crates.
  • A Damageable component could be added to anything that can take damage, independent of whether it’s an enemy or a player character. This modularity allows behaviors to be reused across many different object types without rewriting code.

5. Ease of Extensibility​

As games evolve, developers often need to add new behaviors or modify existing ones. With inheritance, this can lead to extensive refactoring or creating multiple layers of subclasses to accommodate new behaviors. This leads to what is known as the fragile base class problem—modifying the base class to introduce new behavior can break subclasses that rely on the previous functionality.

Composition offers a much more extensible architecture. New components can be added or swapped in without disrupting the existing system. Want to give a character new abilities or modify their movement? Simply create a new component and attach it to the character without altering the core logic.