EventEmitter 实现

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

on(event, callback) { let callbacks = this.events[event] || []; callbacks.push(callback); this.events[event] = callbacks;

return this;

}

off(event, callback) { let callbacks = this.events[event]; this.events[event] = callbacks && callbacks.filter(fn => fn !== callback);

return this;

}

emit(event, ...args) { let callbacks = this.events[event]; callbacks.forEach(fn => { fn(...args); });

return this;

}

once(event, callback) { let wrapFun = function(...args) { callback(...args);

  this.off(event, wrapFun);
};
this.on(event, wrapFun);

return this;

} }