JS中自定义事件与观察者模式详解

var Event = (function() {
  let list = {};
  let listen = function(key, fn) {
    (list[key] || (list[key] = [])).push(fn);
  };
  let remove = function(key, fn) {
    let fns = list[key];
    if (!fns) return;
    if (!fn) {
      fns && (fns.length = 0);
    } else {
      for (let i = fns.length - 1; i >= 0; i--) {
        fns[i] === fn && (fns.splice(i, 1));
      }
    }
  };
  let trigger = function() {
    let keys = [].shift.call(arguments);
    let fns = list[keys];
    if (!fns || fns.length === 0) return;
    for (let i = 0, fn; fn = fns[i++];) {
      fn.apply(this, arguments);
    }
  };
  return {
    listen,
    remove,
    trigger
  };
})();
// 使用
function log(name) {
  console.log(name)
}
Event.listen('to-do', log)    // 订阅事件
Event.trigger('to-do', '111') // 111
Event.trigger('to-do', '222') // 222
Event.remove('to-do', log)    // 移除事件
Event.trigger('to-do', '333') // 不执行

function EventEmitter() {
  this._maxListeners = 10;
  this._events = Object.create(null);
}
// 向事件队列添加事件
// prepend为true表示向事件队列头部添加事件
EventEmitter.prototype.addListener = function (type, listener, prepend) {
  if (!this._events) {
    this._events = Object.create(null);
  }
  if (this._events[type]) {
    if (prepend) {
      this._events[type].unshift(listener);
    } else {
      this._events[type].push(listener);
    }
  } else {
    this._events[type] = [listener];
  }
};
// 移除某个事件
EventEmitter.prototype.removeListener = function (type, listener) {
  if (Array.isArray(this._events[type])) {
    if (!listener) {
      delete this._events[type]
    } else {
      this._events[type] = this._events[type].filter(e => e !== listener && e.origin !== listener)
    }
  }
};
// 向事件队列添加事件，只执行一次
EventEmitter.prototype.once = function (type, listener) {
  const only = (...args) => {
    listener.apply(this, args);
    this.removeListener(type, listener);
  }
  only.origin = listener;
  this.addListener(type, only);
};
// 执行某类事件
EventEmitter.prototype.emit = function (type, ...args) {
  if (Array.isArray(this._events[type])) {
    this._events[type].forEach(fn => {
      fn.apply(this, args);
    });
  }
};
// 设置最大事件监听个数
EventEmitter.prototype.setMaxListeners = function (count) {
  this.maxListeners = count;
};
// 使用
var emitter = new EventEmitter();
var onceListener = function (args) {
  console.log('我只能被执行一次', args, this);
}
var listener = function (args) {
  console.log('我是一个listener', args, this);
}
emitter.once('click', onceListener);
emitter.addListener('click', listener);
emitter.emit('click', '参数');
emitter.emit('click');
emitter.removeListener('click', listener);
emitter.emit('click');

// 创建
var customEvent = new Event('custom-event');
// 监听
window.addEventListener('custom-event', function(e) {
  // do something ...
  console.log(`事件对象：`, e)
})
// 触发
window.dispatchEvent(customEvent)

// 创建
var customEvent = new CustomEvent('custom-event', {
  data: {
    name: 'Tom',
    age: 18,
    other: 'otherMessage',
  }
});
// 监听
var eBox = document.getElementById('box')
eBox.addEventListener('custom-event', function(e) {
  // do something ...
  console.log(`传入的数据：`, e.data)
})
// 触发 兼容写法
if(window.dispatchEvent) {
  eBox.dispatchEvent(customEvent);
} else {
  // IE8
  eBox.fireEvent(customEvent);
}