# Composite Pattern

> A design pattern that gives leaves and groups one interface, so a tree is traversed without checking which kind each node is.

Record: `architecture:composite-pattern`
Kind: pattern
Layer: [Design Patterns Core](https://banes-lab.com/records/layer/design-patterns-core.md)
Severity: recommended
Scope: structure, tree, hierarchy
Canonical: https://banes-lab.com/ontology#architecture-composite-pattern

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Decorator Pattern](https://banes-lab.com/records/architecture/decorator-pattern.md) and before [Flyweight Pattern](https://banes-lab.com/records/architecture/flyweight-pattern.md).

## Repair

- Refactored by: Unify Leaf and Composite behind one interface
- Detected by: isContainer/isLeaf conditionals during traversal
- Violated by: callers branching on leaf-vs-container at every node
- Measured by: node-kind conditional count
- Enforced by: [design review](https://banes-lab.com/records/architecture/design-review.md)

## Requires

- [Uniform Component Interface](https://banes-lab.com/records/lexicon/uniform-component-interface.md)

## Reinforces

- [Uniform Interface](https://banes-lab.com/records/architecture/uniform-interface.md)
- [Open/Closed Principle (OCP)](https://banes-lab.com/records/architecture/open-closed.md)

## Enables

- [Recursive Composition](https://banes-lab.com/records/lexicon/recursive-composition.md)
- [Leaf/Composite Transparency](https://banes-lab.com/records/lexicon/leaf-composite-transparency.md)

## Conflicts with

- [Leaf-vs-Container Special-Casing](https://banes-lab.com/records/lexicon/leaf-vs-container-special-casing.md)

## In tension with

- [Type Safety](https://banes-lab.com/records/architecture/type-safety.md)

## Tensions

- [Composite Pattern / Type Safety](https://banes-lab.com/records/tension/composite-pattern-type-safety.md)

## Severity

- [recommended](https://banes-lab.com/records/vocabulary/severity-recommended.md)

## Category

- [Structural Patterns](https://banes-lab.com/records/architecture-category/structural-patterns.md)

## Linked from

- [Contracts / Interfaces / Compatibility](https://banes-lab.com/ontology/principles/architecture-category-contracts-interfaces-compatibility.md)
- [SOLID / Object-Oriented Design](https://banes-lab.com/ontology/principles/architecture-category-solid-object-oriented-design.md)
- [Schema / Canonical Data / Semantics](https://banes-lab.com/ontology/principles/architecture-category-schema-canonical-data-semantics.md)
- [Structural Patterns](https://banes-lab.com/ontology/lexicon/lexicon-category-structural-patterns.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)