Node.js EventEmitter‘see’事件不能“查看”类属性

Question

这是我的previous SO question的后续。在这个问题中，我定义了一个包含index.js事件处理程序的on文件。从那以后，我更新了index.js文件如下：

index.js

const EventEmitter = require('events');
const Student = require('./student');
const Whiteboard = require('./whiteboard');

class Classroom {
  constructor() {
    this.whiteboard = new Whiteboard();
  }

  async function start() {
    eventEmitter.on('newListener', (event, listener) => {
      console.log(`Added ${event} listener.`);
    });

    eventEmitter.on('handRaised', (question) => {
      console.log(question);
      this.whiteboard.write(question);
    });

    for (let i=0; i<10; i++) {
      let student = new Student(`Student #${i+1}`);
      student.attend();
    }
  }
}

当我运行我的应用程序时，使用this.whiteboard.write(question);的行会生成一个错误。这个错误是：Cannot read property 'write' of undefined。我已经确认this.whiteboard是未定义的。我的问题是，如何访问事件的类属性？

谢谢!

Answer 1

使用this.whiteboard应该没有问题，但是类定义很奇怪，实际上，我希望通过方法定义实现SyntaxError。

尝试像这样修改代码：

const EventEmitter = require('events');
const Student = require('./student');
const Whiteboard = require('./whiteboard');

const eventEmitter = new EventEmitter(); /* create an emitter instance */

class Classroom {
  constructor() {
    this.whiteboard = new Whiteboard();
  }

  async start() { /* no 'function' here */
    eventEmitter.on('newListener', (event, listener) => {
      console.log(`Added ${event} listener.`);
    });

    eventEmitter.on('handRaised', (question) => {
      console.log(question);
      this.whiteboard.write(question);
    });

    for (let i=0; i<10; i++) {
      let student = new Student(`Student #${i+1}`);
      student.attend();
    }
  }
}

编辑：EventEmitter的实例

Answer 2

在任何事件的回调中，您的this都不会指向您的类。尝尝这个。

var self = this;
eventEmitter.on('handRaised', (question) => {
  console.log(question);
  self.whiteboard.write(question);
});