class Logger {
  constructor() {
    if (Logger.instance) {
      return Logger.instance;
    }
    this.logs = [];
    Logger.instance = this;
  }

  log(message) {
    this.logs.push(message);
    console.log(`[LOG] ${message}`);
  }
}

const logger1 = new Logger();
const logger2 = new Logger();
console.log(logger1 === logger2); // true

class Subject {
  constructor() {
    this.observers = [];
  }

  subscribe(observer) {
    this.observers.push(observer);
  }

  notify(data) {
    this.observers.forEach(observer => observer.update(data));
  }
}

class Observer {
  update(data) {
    console.log("Received:", data);
  }
}

const subject = new Subject();
const obs1 = new Observer();
const obs2 = new Observer();

subject.subscribe(obs1);
subject.subscribe(obs2);
subject.notify("State changed!"); // 两个观察者都会收到消息

class AdminService {
  performAction() { return "Admin action executed"; }
}

class UserService {
  performAction() { return "User action executed"; }
}

function ServiceFactory(role) {
  switch(role) {
    case 'admin':
      return new AdminService();
    case 'user':
      return new UserService();
    default:
      throw new Error("Invalid role");
  }
}

const service = ServiceFactory('admin');
console.log(service.performAction());

function log(target, name, descriptor) {
  const original = descriptor.value;
  descriptor.value = function(...args) {
    console.log(`Calling "${name}" with`, args);
    return original.apply(this, args);
  };
  return descriptor;
}

class Calculator {
  @log
  add(a, b) {
    return a + b;
  }
}

const calc = new Calculator();
calc.add(2, 3); // 输出调用信息后再执行