笔试题：手写一个观察者模式？ ​

var events = (function () {
  var topics = {};

  return {
    // 注册监听函数
    subscribe: function (topic, handler) {
      if (!topics.hasOwnProperty(topic)) {
        topics[topic] = [];
      }
      topics[topic].push(handler);
    },

    // 发布事件，触发观察者回调事件
    publish: function (topic, info) {
      if (topics.hasOwnProperty(topic)) {
        topics[topic].forEach(function (handler) {
          handler(info);
        });
      }
    },

    // 移除主题的一个观察者的回调事件
    remove: function (topic, handler) {
      if (!topics.hasOwnProperty(topic)) return;

      var handlerIndex = -1;
      topics[topic].forEach(function (item, index) {
        if (item === handler) {
          handlerIndex = index;
        }
      });

      if (handlerIndex >= 0) {
        topics[topic].splice(handlerIndex, 1);
      }
    },

    // 移除主题的所有观察者的回调事件
    removeAll: function (topic) {
      if (topics.hasOwnProperty(topic)) {
        topics[topic] = [];
      }
    },
  };
})();