什么时候应该使用 EventEmitter？

Question

我读了很多关于EventEmitter 的东西。但我不知道在哪种情况下我需要在我的 Node.js 应用程序中使用它。

Answer 1

只要代码订阅某事而不是从某事获得回调是有意义的。典型的用例是您的应用程序中有多个代码块，当事件发生时可能需要执行一些操作。

例如，假设您正在创建一个票务系统。处理事情的常见方式可能是这样的：

function addTicket(ticket, callback) {
    insertTicketIntoDatabase(ticket, function(err) {
        if (err)
            return handleError(err);

        callback();
    });
}

但是现在，有人决定当一张票被插入数据库时​​，你应该给用户发电子邮件让他们知道。没关系，你可以将它添加到回调中：

function addTicket(ticket, callback) {
    insertTicketIntoDatabase(ticket, function(err) {
        if (err)
            return handleError(err);

        emailUser(ticket, callback);
    });
}

但是现在，有人还想通知另一个系统票已插入。随着时间的推移，插入票证时可能会发生许多事情。所以让我们稍微改变一下：

function addTicket(ticket, callback) {
    insertTicketIntoDatabase(ticket, function(err) {
        if (err)
            return handleError(err);

        TicketEvent.emit('inserted', ticket);
        callback();
    });
}

在通知用户界面之前，我们不再需要等待所有这些功能完成。在代码的其他地方，您可以轻松地添加这些函数：

TicketEvent.on('inserted', function(ticket) {
    emailUser(ticket);
});

TicketEvent.on('inserted', function(ticket) {
    notifySlack(ticket);
});

Answer 2

Node.js 事件发射器用于将代码库分解为 使用类似 pub-sub 的异步模式调用的组件或服务。 然而，通常当我们谈论 pub-sub 模式时，我们指的是分布式分解系统。此处并非如此，因为所有组件都存在于同一个代码存储库中并在同一个 Node.js 运行时中运行。

请记住，使用 Node.js 事件发射器不会使我们的代码自动成为非阻塞、异步的。需要特别注意的是，事件侦听器（订阅者）不会相互阻塞，即事件侦听器应该异步执行代码。

此外，当使用这种模式时，事件发射器（发布者）并不关心事件监听器所采取的行动的结果。没有回调或返回值。如果这些操作很关键，则需要处理故障。

代码示例：

/**
 * When event listeners execute synchronous blocking code as seen in this example,
 * the next listener is not notified until the first listener completes execution
 * of the synchronous blocking code.
 *
 * Here is an output from running this code:
 *
 * 11:16:40 Listener 1 - processing event
 * 11:16:45 Listener 1 - processed: Test Event
 * 11:16:45 Listener 2 - processing event
 * 11:16:45 Listener 2 - processed: Test Event
 */

const { EventEmitter } = require('events');

const time = () => {
  const currentDate = new Date();
  return `${currentDate.getHours()}:${currentDate.getMinutes()}:${currentDate.getSeconds()}`;
};

const EventBus = new EventEmitter();

// Listener 1
EventBus.on('event', (message) => {
  console.log(`${time()} Listener 1 - processing event`);
  for (let i = 0; i < 6e9; i += 1) {
    // Intentionally empty
  }
  console.log(`${time()} Listener 1 - processed: ${message}`);
});

// Listener 2
EventBus.on('event', (message) => {
  console.log(`${time()} Listener 2 - processing event`);
  console.log(`${time()} Listener 2 - processed: ${message}`);
});

// Emitting event
EventBus.emit('event', 'Test Event');

/**
 *
 * To take full advantage of EventListener the listeners should execute
 * asynchronous non-blocking code. However, wrapping a synchronous code
 * into an async function is not enough. The 2nd listener is still
 * blocked and waiting for the async function to complete
 *
 * Here is an output from running this code:
 * 11:13:52 Listener 1 - processing event
 * 11:13:52 Listener 1 - about to await
 * 11:13:57 Listener 2 - processing event
 * 11:13:57 Listener 2 - processed: Test Event
 * 11:13:57 Listener 1 - await completed
 * 11:13:57 Listener 1 - processed: Test Event
 */

const { EventEmitter } = require('events');

const time = () => {
  const currentDate = new Date();
  return `${currentDate.getHours()}:${currentDate.getMinutes()}:${currentDate.getSeconds()}`;
};

const EventBus = new EventEmitter();

// Listener 1
EventBus.on('event', async (message) => {
  console.log(`${time()} Listener 1 - processing event`);

  async function extracted() {
    for (let i = 0; i < 6e9; i += 1) {
      // Intentionally empty
    }
  }

  console.log(`${time()} Listener 1 - about to await`);
  await extracted();
  console.log(`${time()} Listener 1 - await completed`);
  console.log(`${time()} Listener 1 - processed: ${message}`);
});

// Listener 2
EventBus.on('event', (message) => {
  console.log(`${time()} Listener 2 - processing event`);
  console.log(`${time()} Listener 2 - processed: ${message}`);
});

// Emitting event
EventBus.emit('event', 'Test Event');

/**
 *
 * To take full advantage of EventListener the listeners should execute
 * asynchronous non-blocking code. Here we are using setTimeout() in order
 * to execute code asynchronously.
 *
 * Here is an output from running this code:
 *
 * 11:45:54 Listener 1 - processing event
 * 11:45:54 Listener 1 - about to execute setTimeout
 * 11:45:54 Listener 1 - setTimeout completed
 * 11:45:54 Listener 1 - processed: Test Event
 * 11:45:54 Listener 2 - processing event
 * 11:45:54 Listener 2 - processed: Test Event
 * 11:45:59 Listener 1 - finished the long loop
 */

const { EventEmitter } = require('events');

const time = () => {
  const currentDate = new Date();
  return `${currentDate.getHours()}:${currentDate.getMinutes()}:${currentDate.getSeconds()}`;
};

const EventBus = new EventEmitter();

// Listener 1
EventBus.on('event', async (message) => {
  console.log(`${time()} Listener 1 - processing event`);

  function extracted() {
    for (let i = 0; i < 6e9; i += 1) {
      // Intentionally empty
    }
    console.log(`${time()} Listener 1 - finished the long loop`);
  }

  console.log(`${time()} Listener 1 - about to execute setTimeout`);
  setTimeout(extracted, 0);
  console.log(`${time()} Listener 1 - setTimeout completed`);
  console.log(`${time()} Listener 1 - processed: ${message}`);
});

// Listener 2
EventBus.on('event', (message) => {
  console.log(`${time()} Listener 2 - processing event`);
  console.log(`${time()} Listener 2 - processed: ${message}`);
});

// Emitting event
EventBus.emit('event', 'Test Event');

/**
 *
 * To take full advantage of EventListener the listeners should execute
 * asynchronous non-blocking code. Here we are using setImmediate() in order
 * to execute code asynchronously.
 *
 * Here is an output from running this code:
 *
 * 12:1:3 Listener 1 - processing event
 * 12:1:3 Listener 1 - about to execute setImmediate
 * 12:1:3 Listener 1 - setImmediate completed
 * 12:1:3 Listener 1 - processed: Test Event
 * 12:1:3 Listener 2 - processing event
 * 12:1:3 Listener 2 - processed: Test Event
 * 12:1:9 Listener 1 - finished the long loop
 */

const { EventEmitter } = require('events');

const time = () => {
  const currentDate = new Date();
  return `${currentDate.getHours()}:${currentDate.getMinutes()}:${currentDate.getSeconds()}`;
};

const EventBus = new EventEmitter();

// Listener 1
EventBus.on('event', async (message) => {
  console.log(`${time()} Listener 1 - processing event`);

  function extracted() {
    for (let i = 0; i < 6e9; i += 1) {
      // Intentionally empty
    }
    console.log(`${time()} Listener 1 - finished the long loop`);
  }

  console.log(`${time()} Listener 1 - about to execute setImmediate`);
  setImmediate(extracted);
  console.log(`${time()} Listener 1 - setImmediate completed`);
  console.log(`${time()} Listener 1 - processed: ${message}`);
});

// Listener 2
EventBus.on('event', (message) => {
  console.log(`${time()} Listener 2 - processing event`);
  console.log(`${time()} Listener 2 - processed: ${message}`);
});

// Emitting event
EventBus.emit('event', 'Test Event');