Let's talk about event drivers and EventEmitter class in Node.js

This article will take you to understand the events in Node, and talk about the event driver and EventEmitter class. I hope it will be helpful to everyone!

Nodejs is a single-process, single-threaded application, but because of the asynchronous execution callback interfaces provided by the V8 engine, a large amount of concurrency can be processed through these interfaces. So the performance is very high.

Almost every API in Node.js supports callback functions.

Node.js Basically all event mechanisms are implemented using the observer pattern in the design pattern.

Node.js single thread is similar to entering a while(true) event loop until no event observer exits. Each asynchronous event generates an event observer. If an event occurs, the callback function is called.

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Event-driven program

Node.js uses the event-driven model. When the web server receives a request, it closes it, processes it, and then serves it. Next web request.

When the request is completed, it is put back into the processing queue, and when the beginning of the queue is reached, the result is returned to the user.

This model is very efficient and scalable because the webserver always accepts requests without waiting for any read or write operations. (This is also called non-blocking IO or event-driven IO)

In the event-driven model, a main loop is generated to listen for events, and a callback function is triggered when an event is detected.

Node.js has multiple built-in events. We can bind and listen to events by introducing the events module and instantiating the EventEmitter class, as shown in the following example:

// 引入 events 模块
var events = require('events');
// 创建 eventEmitter 对象
var eventEmitter = new events.EventEmitter();

The following program binds Event handler:

// 绑定事件及事件的处理程序
eventEmitter.on('eventName', eventHandler);

We can trigger events through the program:

// 触发事件
eventEmitter.emit('eventName');

Instance

Create index.js file, The code looks like this:

//引入 fs 模块
var fs = require("fs");

// 引入 events 模块
var events = require('events');

// 创建对象
var ee = new events.EventEmitter();

// 绑定事件及事件的处理程序
ee.on('res', function (data) {
    console.log('res-1');
    console.log(data);
});
ee.on('res', function () {
    console.log('res-2');
});

fs.readFile('hello.txt',{flag:'r',encoding:'utf-8'},function(err,data){
    if(err){
        console.log("读取出错:"+err);
    }else{
        console.log("读取成功:"+data);
        // 触发res事件
        ee.emit('res',data);
    }
})

Next let us execute the above code:

##EventEmitter Class

events The module only provides one object: events.EventEmitter. The core of EventEmitter is the encapsulation of event triggering and event listener functions.

You can access this module through require("events");.

// 引入 events 模块
var events = require('events');
// 创建 eventEmitter 对象
var eventEmitter = new events.EventEmitter();

EventEmitter object will trigger the error event if an error occurs during instantiation. The newListener event is fired when a new listener is added, and the removeListener event is fired when a listener is removed.

Below we use a simple example to illustrate the usage of EventEmitter:

//event.js 文件
var EventEmitter = require('events').EventEmitter; 
var event = new EventEmitter(); 
event.on('some_event', function() { 
    console.log('some_event 事件触发'); 
}); 
setTimeout(function() { 
    event.emit('some_event'); 
}, 1000);

The execution results are as follows:

Run this code, and the console outputs

after 1 second 'some_event event triggers'. The principle is that the event object registers a listener for the event some_event, and then we use setTimeout to send the event some_event to the event object after 1000 milliseconds. At this time, the listener for some_event will be called.

$ node event.js 
some_event 事件触发

EventEmitter Each event consists of an event name and several parameters. The event name is a string, which usually expresses certain semantics. For each event, EventEmitter supports several event listeners.

When an event is triggered, the event listeners registered to this event are called in sequence, and the event parameters are passed as callback function parameters.

Let us explain this process with the following example:

//event.js 文件
var events = require('events'); 
var emitter = new events.EventEmitter(); 
emitter.on('someEvent', function(arg1, arg2) { 
    console.log('listener1', arg1, arg2); 
}); 
emitter.on('someEvent', function(arg1, arg2) { 
    console.log('listener2', arg1, arg2); 
}); 
emitter.emit('someEvent', 'arg1 参数', 'arg2 参数');

Execute the above code, the running results are as follows:

$ node event.js 
listener1 arg1 参数 arg2 参数
listener2 arg1 参数 arg2 参数

In the above example,

emitter Two event listeners were registered for the event someEvent, and then the someEvent event was triggered.

You can see in the running results that two event listener callback functions are called successively. This is the simplest usage of

EventEmitter.

EventEmitter provides several properties such as on and emit. The on function is used to bind the event function, and the emit attribute is used to trigger an event.

For more node-related knowledge, please visit:

nodejs tutorial! !

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