Agent skill
patterns-behavioral
Implement behavioral design patterns (Strategy, Observer, State, Command, Chain of Responsibility, Mediator, Template Method, Iterator, Memento, Visitor). Use when you need pluggable algorithms, event/listener systems, state machines, middleware pipelines, undo/redo, or request chains. NOT for object creation (use patterns-creational); NOT for wrapping/composing objects (use patterns-structural).
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npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/other/other/patterns-behavioral
Metadata
Additional technical details for this skill
- tags
-
behavioral-patterns strategy state-machine eventing pipelines middleware pub-sub observer command undo-redo
- stage
- Mechanics
- aliases
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[ "strategy", "observer", "state-machine", "command", "middleware", "pub-sub", "event-system", "pipeline", "undo-redo" ]
SKILL.md
Patterns (Behavioral)
Overview
Manage algorithms and collaboration without turning your code into nested conditionals and tight coupling. Behavioral patterns help you route requests, encapsulate actions, and swap behavior safely.
A note on scope: these guidelines assume systemic TypeScript (long‑lived apps/services). For short‑lived scripts, you can often simplify (fewer abstractions, more throw) as long as the blast radius stays small.
Workflow
- Decide “scriptic vs systemic” and set policies (error semantics, boundary validation, ownership/lifetimes).
- Identify the interaction pressure: pipelines, events, undo, state machines, or algorithm selection.
- Draw a quick responsibility map: who triggers actions, who owns state, who receives outcomes?
- Pick a pattern that makes responsibilities explicit (interfaces + concrete behaviors).
- Implement with clear contracts and tests for ordering, error semantics (Result vs throw), and lifetimes (unsubscribe/shutdown).
Chooser
- Chain of Responsibility: configurable pipeline; each handler may handle or pass along.
- Command: represent actions as objects; queue/schedule/undo/retry.
- Iterator: traverse collections/graphs without exposing representation.
- Mediator: central coordinator to reduce many-to-many coupling.
- Memento: snapshot/restore object state; undo/redo without leaking internals.
- Observer: pub/sub updates; multiple listeners react to events.
- State: state machine; behavior varies by state; transitions are explicit.
- Strategy: swap algorithms behind a stable interface (runtime/config selection).
- Template Method: stable algorithm skeleton with overridable steps (often via hooks; use inheritance only when it already fits).
- Visitor: add new operations across a stable object structure without changing element classes.
Implementation Checklist
- Make ordering explicit for pipelines and observers; define error semantics (what’s expected vs unknown).
- For expected failures, prefer typed unions/
Result; reservethrowfor unknown/unrecoverable and catch/convert at boundaries. - Treat boundary inputs as
unknown(events/requests) and validate/decode once near the edge. - For async observers/pipelines, make ownership explicit: unsubscribe/shutdown, backpressure/queueing, and cancellation (
AbortSignal). - Keep strategies/states small and pure when possible; inject dependencies via context.
- Prefer composition for Strategy/State; reserve Template Method for cases where inheritance is already a fit.
- For Command/Memento: define serialization and persistence needs early (in-memory vs durable; versioned formats).
Snippets (optional)
- TypeScript:
references/snippets/typescript.md - React:
references/snippets/react.md
References
Read the relevant reference file before implementing or refactoring toward the pattern:
references/chain-of-responsibility.mdreferences/command.mdreferences/iterator.mdreferences/mediator.mdreferences/memento.mdreferences/observer.mdreferences/state.mdreferences/strategy.mdreferences/template-method.mdreferences/visitor.md
Each reference includes: selection cues, minimal structure, pitfalls, and test ideas.
Output Template
When applying a behavioral pattern, return:
- The pressure you’re addressing (pipeline/eventing/undo/state/algorithm selection) and why this pattern fits.
- The proposed seam (interfaces/contracts) and who owns state and lifetimes (subscribe/unsubscribe, start/stop).
- Verification steps (tests for ordering, expected failures, and shutdown/cancellation).
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