unity-pattern-composite

name	unity-pattern-composite
description	Composite
category	unity
source	unity-design-patterns-skills
source_path	patterns/composite/SKILL.md

Composite

Purpose

Treat individual objects and groups through one interface so callers can ignore tree shape.

Use When

• A feature naturally forms a tree.
• The same operation should work on a leaf or a group.
• Callers should not branch on single-versus-many.

Avoid When

• The structure is shallow and explicit fields are clearer.
• Leaf and container behavior are too different for one contract.

Unity Implementation Recipe

1. Define a small shared interface.
2. Implement leaves directly.
3. Let composites manage children and delegate recursion.
4. Document ownership and traversal cost.

Common Pitfalls

• Forcing child-management methods onto leaves unnecessarily.
• Hiding expensive recursive work behind a cheap-looking method.

Minimal Unity Sketch

public interface IBuffNode
{
    void Apply(CharacterStats stats);
}

public sealed class BuffGroup : IBuffNode
{
    private readonly List<IBuffNode> _children = new();
    public void Add(IBuffNode node) => _children.Add(node);
    public void Apply(CharacterStats stats)
    {
        foreach (var child in _children) child.Apply(stats);
    }
}

Response Shape

• Say whether Composite 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.