How to use EventEmitter to handle events in Node.js?

In this tutorial we learn about Node.js’ native EvenEmitter class. After finishing the course, you will understand events, how to use EvenEmitter, and how to utilize events in your programs. In addition, you will also learn how the EventEmitter class extends from other local modules, and understand the principles behind it through some examples.

Recommended tutorial: node js tutorial

In short, this article covers everything about the EventEmitter class.

What is an event?

Event-driven architecture is very common today. Event-driven programs can generate, detect and respond to various events.

The core part of Node.js is event-driven, and many modules such as the file system (fs) and stream themselves use EventEmitter Written by.

In event-driven programming, Event (event) is the result of one or more actions, which may be the user's operation or the timing output of the sensor, etc.

We can think of event-driven programs as a publish-subscribe model, where the publisher triggers events and the subscribers listen to the events and take appropriate actions.

For example, suppose there is a server to which users can upload images. In event-driven programming, an action such as uploading an image will emit an event, which will also have 1 to n subscribers in order to take advantage of it.

After the upload event is triggered, a subscriber can let them know and react to the site's administrator by emailing the site's administrator; another subscriber may collect information about the action , and save it in the database.

These events are usually independent of each other, although they may also be dependent on each other.

What is EventEmitter? The

EventEmitter class is a built-in class of Node.js, located in the events module. According to the description in the documentation:

Most of the Node.js core API is implemented based on an idiomatic asynchronous event-driven architecture, in which certain types of objects (called " Emitter") emits named events that cause calls to Function objects ("listeners")"

This class can be described to some extent as a publish-subscribe model The implementation of auxiliary tools, because it can help event emitters (publishers) publish events (messages) to listeners (subscribers) in a simple way.

Create EventEmitters

Having said that , but it is more practical to create an EventEmitter first. This can be done by creating an instance of the class itself or implementing it through a custom class, and then creating an instance of the class.

Create EventEmitter Object

Start with a simple example: Create an EventEmitter that emits an event containing program running time information every second.

First start with events Import the EventEmitter class in the module:

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

Then create an EventEmitter:

const timerEventEmitter = new EventEmitter();

Use this object to publish events. Easy:

timerEventEmitter.emit("update");

The event name has been specified previously and it was published as an event. But the program did not respond because no listener has reacted to this event yet.

Let first This event repeats once every second. Use the setInterval() method to create a timer and publish the update event once every second:

let currentTime = 0;

// 每秒触发一次 update 事件
setInterval(() => {
    currentTime++;
    timerEventEmitter.emit('update', currentTime);
}, 1000);

EventEmitter The instance is used to accept event names and parameters. Pass update as the event name and currentTime as the time since the program started.

Pass emit( ) method triggers the emitter, which pushes the event with the information we provide. Once the event emitter is ready, subscribe an event listener to it:

timerEventEmitter.on('update', (time) => {
    console.log('从发布者收到的消息：');
    console.log(`程序已经运行了 ${time} 秒`);
});

via on() Method creates a listener, passing the event name to specify which event you want the listener to be attached to. On the update event, runs a method that records the time.

on() The second parameter of the function is a callback that can accept additional data emitted by the event.

Running the code will output:

从发布者收到的消息：
程序已经运行了 1 秒
从发布者收到的消息：
程序已经运行了 2 秒
从发布者收到的消息：
程序已经运行了 3 秒
...

If only when the event is triggered for the first time Only when you need to perform certain operations, you can also subscribe using the once() method:

timerEventEmitter.once('update', (time) => {
    console.log('从发布者收到的消息：');
    console.log(`程序已经运行了 ${time} 秒`);
});

Running this code will output:

从发布者收到的消息：
程序已经运行了 1 秒

EventEmitter With multiple listeners

Create another event sender below. This is a timer program with three listeners. The first listener updates the time every second, the second listener fires when the timer is about to end, and the last one fires when the timer ends:

• update: every second Trigger once
• end: Trigger when the countdown ends
• end-soon: Trigger 2 seconds before the end of the countdown

First write a function to create this event emitter:

const countDown = (countdownTime) => {
    const eventEmitter = new EventEmitter();

    let currentTime = 0;

    // 每秒触发一次 update 事件
    const timer = setInterval(() => {
        currentTime++;
        eventEmitter.emit('update', currentTime);

        // 检查计时是否已经结束
        if (currentTime === countdownTime) {
            clearInterval(timer);
            eventEmitter.emit('end');
        }

        // 检查计时是否会在 2 秒后结束
        if (currentTime === countdownTime - 2) {
            eventEmitter.emit('end-soon');
        }
    }, 1000);
    return eventEmitter;
};

这个函数启动了一个每秒钟发出一次 update 事件的事件。

第一个 if 用来检查计时是否已经结束并停止基于间隔的事件。如果已结束将会发布 end 事件。

如果计时没有结束，那么就检查计时是不是离结束还有 2 秒，如果是则发布 end-soon 事件。

向该事件发射器添加一些订阅者：

const myCountDown = countDown(5);

myCountDown.on('update', (t) => {
    console.log(`程序已经运行了 ${t} 秒`);
});

myCountDown.on('end', () => {
    console.log('计时结束');
});

myCountDown.on('end-soon', () => {
    console.log('计时将在2秒后结束');
});

这段代码将会输出：

程序已经运行了 1 秒
程序已经运行了 2 秒
程序已经运行了 3 秒
计时将在2秒后结束
程序已经运行了 4 秒
程序已经运行了 5 秒
计时结束

扩展 EventEmitter

接下来通过扩展 EventEmitter 类来实现相同的功能。首先创建一个处理事件的 CountDown 类：

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

class CountDown extends EventEmitter {
    constructor(countdownTime) {
        super();
        this.countdownTime = countdownTime;
        this.currentTime = 0;
    }

    startTimer() {
        const timer = setInterval(() => {
            this.currentTime++;
            this.emit('update', this.currentTime);
    
            // 检查计时是否已经结束
            if (this.currentTime === this.countdownTime) {
                clearInterval(timer);
                this.emit('end');
            }
    
            // 检查计时是否会在 2 秒后结束
            if (this.currentTime === this.countdownTime - 2) {
                this.emit('end-soon');
            }
        }, 1000);
    }
}

可以在类的内部直接使用 this.emit()。另外 startTimer() 函数用于控制计时开始的时间。否则它将在创建对象后立即开始计时。

创建一个 CountDown 的新对象并订阅它：

const myCountDown = new CountDown(5);

myCountDown.on('update', (t) => {
    console.log(`计时开始了 ${t} 秒`);
});

myCountDown.on('end', () => {
    console.log('计时结束');
});

myCountDown.on('end-soon', () => {
    console.log('计时将在2秒后结束');
});

myCountDown.startTimer();

运行程序会输出：

程序已经运行了 1 秒
程序已经运行了 2 秒
程序已经运行了 3 秒
计时将在2秒后结束
程序已经运行了 4 秒
程序已经运行了 5 秒
计时结束

on() 函数的别名是 addListener()。看一下 end-soon 事件监听器：

myCountDown.on('end-soon', () => {
    console.log('计时将在2秒后结束');
});

也可以用 addListener() 来完成相同的操作，例如：

myCountDown.addListener('end-soon', () => {
    console.log('计时将在2秒后结束');
});

EventEmitter 的主要函数

eventNames()

此函数将以数组形式返回所有活动的侦听器名称：

const myCountDown = new CountDown(5);

myCountDown.on('update', (t) => {
    console.log(`程序已经运行了 ${t} 秒`);
});

myCountDown.on('end', () => {
    console.log('计时结束');
});

myCountDown.on('end-soon', () => {
    console.log('计时将在2秒后结束');
});

console.log(myCountDown.eventNames());

运行这段代码会输出：

[ 'update', 'end', 'end-soon' ]

如果要订阅另一个事件，例如 myCount.on('some-event', ...)，则新事件也会添加到数组中。

这个方法不会返回已发布的事件，而是返回订阅的事件的列表。

removeListener()

这个函数可以从 EventEmitter 中删除已订阅的监听器：

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

const emitter = new EventEmitter();

const f1 = () => {
    console.log('f1 被触发');
}

const f2 = () => {
    console.log('f2 被触发');
}

emitter.on('some-event', f1);
emitter.on('some-event', f2);

emitter.emit('some-event');

emitter.removeListener('some-event', f1);

emitter.emit('some-event');

在第一个事件触发后，由于 f1 和 f2 都处于活动状态，这两个函数都将被执行。之后从 EventEmitter 中删除了 f1。当再次发出事件时，将会只执行 f2：

f1 被触发
f2 被触发
f2 被触发

An alias for removeListener() is off(). For example, we could have written:

removeListener() 的别名是 off()。例如可以这样写：

emitter.off('some-event', f1);

removeAllListeners()

该函数用于从 EventEmitter 的所有事件中删除所有侦听器：

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

const emitter = new EventEmitter();

const f1 = () => {
    console.log('f1 被触发');
}

const f2 = () => {
    console.log('f2 被触发');
}

emitter.on('some-event', f1);
emitter.on('some-event', f2);

emitter.emit('some-event');

emitter.removeAllListeners();

emitter.emit('some-event');

第一个 emit() 会同时触发 f1 和 f2，因为它们当时正处于活动状态。删除它们后，emit() 函数将发出事件，但没有侦听器对此作出响应：

f1 被触发
f2 被触发

错误处理

如果要在 EventEmitter 发出错误，必须用 error 事件名来完成。这是 Node.js 中所有 EventEmitter 对象的标准配置。这个事件必须还要有一个 Error 对象。例如可以像这样发出错误事件：

myEventEmitter.emit('error', new Error('出现了一些错误'));

error 事件的侦听器都应该有一个带有一个参数的回调，用来捕获 Error 对象并处理。如果 EventEmitter 发出了 error 事件，但是没有订阅者订阅 error 事件，那么 Node.js 程序将会抛出这个 Error。这会导致 Node.js 进程停止运行并退出程序，同时在控制台中显示这个错误的跟踪栈。

例如在 CountDown 类中，countdownTime参数的值不能小于 2，否则会无法触发 end-soon 事件。在这种情况下应该发出一个 error 事件：

class CountDown extends EventEmitter {
    constructor(countdownTime) {
        super();

        if (countdownTimer < 2) {
            this.emit(&#39;error&#39;, new Error(&#39;countdownTimer 的值不能小于2&#39;));
        }

        this.countdownTime = countdownTime;
        this.currentTime = 0;
    }

    // ...........
}

处理这个错误的方式与其他事件相同：

myCountDown.on('error', (err) => {
    console.error('发生错误:', err);
});

始终对 error 事件进行监听是一种很专业的做法。

使用 EventEmitter 的原生模块

Node.js 中许多原生模块扩展了EventEmitter 类，因此它们本身就是事件发射器。

一个典型的例子是 Stream 类。官方文档指出：

流可以是可读的、可写的，或两者均可。所有流都是 EventEmitter 的实例。

先看一下经典的 Stream 用法：

const fs = require('fs');
const writer = fs.createWriteStream('example.txt');

for (let i = 0; i < 100; i++) {
  writer.write(`hello, #${i}!\n`);
}

writer.on(&#39;finish&#39;, () => {
  console.log('All writes are now complete.');
});

writer.end('This is the end\n');

但是，在写操作和 writer.end() 调用之间，我们添加了一个侦听器。 Stream 在完成后会发出一个 finished 事件。在发生错误时会发出 error 事件，把读取流通过管道传输到写入流时会发出 pipe 事件，从写入流中取消管道传输时，会发出 unpipe 事件。

另一个类是 child_process 类及其 spawn() 方法：

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

当  child_process 写入标准输出管道时，将会触发  stdout 的 data 事件。当输出流遇到错误时，将从 stderr 管道发送 data 事件。

Finally, after the process exits, the close event will be triggered.

Summary

Event-driven architecture enables us to create systems with high cohesion and low coupling. An event represents the result of an action, and one or more listeners can be defined and react to it.

This article takes an in-depth look at the EventEmitter class and its capabilities. Instantiate it and use it directly, and extend its behavior into a custom object.

Finally, some important functions of this class are introduced.

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