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Command

Overview​

Encapsulate a request as an object, allowing parameterization and queuing.

When to use​

  • You need undo/redo.
  • You want to queue or log operations.

Java example​

interface Command { void execute(); }

class SaveCommand implements Command {
    private final Document doc;
    SaveCommand(Document doc) { this.doc = doc; }
    public void execute() { doc.save(); }
}

class Invoker {
    void run(Command cmd) { cmd.execute(); }
}

TypeScript example​

interface Command { execute(): void; }

class SaveCommand implements Command {
  constructor(private doc: Document) {}
  execute(): void { this.doc.save(); }
}

class Invoker {
  run(cmd: Command): void { cmd.execute(); }
}

Pros and cons​

Pros:

  • Supports undo/redo and logging.
  • Decouples invoker from receiver.

Cons:

  • Many small classes.
  • Indirection can be heavy for simple actions.

Common pitfalls​

  • Not capturing all necessary state for undo.
  • Creating commands for trivial operations.