JavaScript观察者模式：EventEmitter实现详解

class EventEmitter {
  constructor() {
    this.events = {};
  }

  on(event, callback) {
    if (!this.events[event]) {
      this.events[event] = [];
    }
    this.events[event].push(callback);
  }

  emit(event, ...args) {
    if (this.events[event]) {
      this.events[event].forEach(callback => callback(...args));
    }
  }

  off(event, callback) {
    if (this.events[event]) {
      const index = this.events[event].indexOf(callback);
      if (index > -1) {
        this.events[event].splice(index, 1);
      }
    }
  }

  once(event, callback) {
    const wrapped = (...args) => {
      callback.apply(this, args);
      this.off(event, wrapped);
    };
    this.on(event, wrapped);
  }
}

const emitter = new EventEmitter();

emitter.on('greet', (name) => {
  console.log(`Hello, ${name}!`);
});

emitter.once('init', () => {
  console.log('This will run only once.');
});

emitter.emit('greet', 'Alice'); // 输出: Hello, Alice!
emitter.emit('init');           // 输出: This will run only once.
emitter.emit('init');           // 无输出