unity-pattern-factory-method

name	unity-pattern-factory-method
description	Factory Method
category	unity
source	unity-design-patterns-skills
source_path	patterns/factory-method/SKILL.md

Factory Method

Purpose

Delegate concrete object creation to a factory method so callers depend on a product contract instead of product selection logic.

Use When

• Callers should not know which concrete product variant to instantiate.
• Creation logic includes setup or wiring.
• Product variants are expected to grow.

Avoid When

• There is only one trivial concrete type.
• The main issue is staged assembly, which fits Builder better.

Unity Implementation Recipe

1. Define a product contract.
2. Move selection logic into a factory method or type.
3. Return ready-to-use products or document remaining initialization.
4. Test input-to-product mapping.

Common Pitfalls

• A mega-factory with unrelated domain logic.
• Hidden side effects during creation.

Minimal Unity Sketch

public interface IProjectile { }

public sealed class ProjectileFactory
{
    public IProjectile Create(WeaponType type)
    {
        return type switch
        {
            WeaponType.Arrow => new ArrowProjectile(),
            WeaponType.Fireball => new FireballProjectile(),
            _ => throw new ArgumentOutOfRangeException(nameof(type))
        };
    }
}

Response Shape

• Say whether Factory Method is actually the right fit.
• Map it to concrete Unity types, ownership, and lifecycle.
• Prefer a production-shaped skeleton over a long tutorial dump.
• Call out the simpler alternative if this pattern would be overengineering.