Behavioural patterns: Strategy, Observer, Command, State

The goal

Behavioural patterns distribute responsibility between objects and define how they communicate.

Strategy

Define a family of interchangeable algorithms behind an interface and pick one at runtime.

type Pricing = (subtotalCents: number) => number;

const regular: Pricing = (s) => s;
const member:  Pricing = (s) => Math.round(s * 0.9);
const blackFriday: Pricing = (s) => Math.round(s * 0.7);

function checkout(subtotal: number, pricing: Pricing) { return pricing(subtotal); }
checkout(10_000, member);   // the strategy is passed in

Replaces long if/else or switch chains and supports the open/closed principle. In languages with first-class functions the "strategy" is simply a function you pass in.

Observer (publish and subscribe)

Objects subscribe to a subject and are notified when it changes, without the subject knowing who listens.

class Emitter<T> {
  private handlers = new Set<(v: T) => void>();
  on(h: (v: T) => void) { this.handlers.add(h); return () => this.handlers.delete(h); } // returns "unsubscribe"
  emit(v: T) { this.handlers.forEach((h) => h(v)); }
}
const cart = new Emitter<number>();
const stop = cart.on((count) => updateBadge(count));
cart.emit(3);
stop();                                              // always clean up

Used for UI events, domain events and message buses. Watch for leaks (forgotten unsubscribes), ordering surprises and cascades of updates that are hard to follow.

Observer as a picture

100%

Figure SDA-SDE-05-1 · One subject, many observers · © forcsguy.com

Command

Turn a request into an object containing everything needed to run it. That lets you queue, log, retry, schedule and undo it.

interface Command { run(): void; undo(): void }

class InsertText implements Command {
  constructor(private doc: Doc, private at: number, private text: string) {}
  run()  { this.doc.insert(this.at, this.text); }
  undo() { this.doc.remove(this.at, this.text.length); }
}

const history: Command[] = [];
function perform(c: Command) { c.run(); history.push(c); }
function undo() { history.pop()?.undo(); }

Editors, job queues, macros and transaction logs all use it.

State

Let an object change behaviour when its internal state changes, by delegating to a state object (or a transition table).

type State = "new" | "paid" | "shipped" | "cancelled";
const next: Record<State, Partial<Record<"pay" | "ship" | "cancel", State>>> = {
  new:       { pay: "paid", cancel: "cancelled" },
  paid:      { ship: "shipped", cancel: "cancelled" },
  shipped:   {},
  cancelled: {},
};
function apply(state: State, action: "pay" | "ship" | "cancel"): State {
  const to = next[state][action];
  if (!to) throw new Error(`Cannot ${action} an order that is ${state}`);
  return to;
}

Makes illegal transitions explicit and impossible, replacing scattered flags and nested conditions.

Template Method

Define the skeleton of an algorithm in one place and let steps vary.

abstract class Importer {
  run() { const raw = this.read(); const rows = this.parse(raw); this.save(rows); } // fixed order
  protected abstract read(): string;
  protected abstract parse(raw: string): Row[];
  protected save(rows: Row[]) { /* default */ }
}

Frameworks use it widely ("override these hooks"). With functions, you can pass the varying steps in instead of subclassing.

Chain of Responsibility

Pass a request along a chain of handlers until one handles it (or each adds something). Middleware pipelines in web servers are this pattern.

type Handler = (req: Req, next: () => Promise<Res>) => Promise<Res>;
// authenticate -> rateLimit -> validate -> handle

Iterator and others

• Iterator: walk a collection without exposing its structure. In most languages this is built in (for...of, generators).
• Mediator: one object coordinates many (a chat room, a form controller) so participants do not reference each other.
• Memento: capture and restore an object's state (undo snapshots).
• Visitor: add new operations over a structure of object types without modifying them; common in compilers.
• Interpreter: evaluate expressions of a small language.

Choosing

Problem	Pattern
Many alternative algorithms or rules	Strategy
React to changes without coupling	Observer
Undo, queue, log or retry actions	Command
Behaviour depends on lifecycle state	State
Same steps, different details	Template Method
A pipeline of optional processing steps	Chain of Responsibility

Remember: with first-class functions many of these become callbacks, closures and tables. Use the lightest form that keeps the code clear.

Real-world reading