Kuidas oma sündmusemitterit kodeerida Node.js-is: samm-sammuline juhend

Mõistke sõlme sisemisi külgi, pakkides väikepakette / mooduleid

Kui olete Node.js-i jaoks uus, on siin Mediumis ja mujal palju õpetusi. Võite vaadata näiteks minu artiklit All About Core Node.JS.

Kuid pikemalt mõtlemata jõuame arutlusel oleva teema juurde: “Sündmuse kiirgajad”. Sündmuse kiirgajad mängivad Node.js ökosüsteemis väga olulist rolli.

EventEmitter on moodul, mis hõlbustab sõlmes olevate objektide vahelist suhtlemist / suhtlemist. EventEmitter on sõlme asünkroonse sündmuspõhise arhitektuuri keskmes. Paljud Node'i sisseehitatud moodulid on päritud EventEmitterilt, sealhulgas silmapaistvad raamistikud nagu Express.js.

Kontseptsioon on üsna lihtne: kiirgavad objektid kiirgavad nimega sündmusi, mille tõttu kutsutakse varem registreeritud kuulajaid. Seega on emitter-objektil põhimõtteliselt kaks peamist omadust:

  • Nimesündmuste emiteerimine.
  • Kuulaja funktsioonide registreerimine ja registreerimise tühistamine.

See on nagu pubi / ala või vaatleja kujundus (kuigi mitte täpselt).

Mida me selles õpetuses üles ehitame

  • Klass EventEmitter
  • on / addEventListener meetod
  • meetod off / removeEventListener
  • kord meetod
  • emiteerida meetodit
  • rawListeners meetod
  • meetod kuulajaCount

Ülaltoodud põhifunktsioonidest piisab sündmuste mudeli abil täieliku süsteemi juurutamiseks.

Enne kodeerimisega alustamist heidame pilgu sellele, kuidas me EventEmitter klassi kasutame. Pange tähele, et meie kood jäljendab mooduli Node.js täpse API-d.

Tegelikult, kui asendate meie EventEmitteri Node.js sisseehitatud 'sündmuste' mooduliga, saate sama tulemuse.

Näide 1 - looge sündmuse kiirgaja eksemplar ja registreerige paar tagasihelistamist

const myEmitter = new EventEmitter(); function c1() { console.log('an event occurred!'); } function c2() { console.log('yet another event occurred!'); } myEmitter.on('eventOne', c1); // Register for eventOne myEmitter.on('eventOne', c2); // Register for eventOne

Kui sündmus 'eventOne' on välja antud, tuleks tugineda mõlemale ülaltoodud tagasihelistamisele.

myEmitter.emit('eventOne');

Konsooli väljund on järgmine:

an event occurred! yet another event occurred!

Näide 2 - registreerumine sündmuse käivitamiseks ainult üks kord, kasutades üks kord.

myEmitter.once('eventOnce', () => console.log('eventOnce once fired')); 

Ürituse 'eventOnce' emiteerimine:

myEmitter.emit('eventOne');

Konsoolis peaks ilmuma järgmine väljund:

eventOnce once fired

Taas registreeritud sündmuste avaldamine ei avalda mingit mõju.

myEmitter.emit('eventOne');

Kuna sündmus väljastati ainult üks kord, ei avalda ülaltoodud väide mingit mõju.

Näide 3 - Sündmusele registreerimine tagasihelistamise parameetritega

myEmitter.on('status', (code, msg)=> console.log(`Got ${code} and ${msg}`));

Sündmuse edastamine parameetritega:

myEmitter.emit('status', 200, 'ok');

Konsooli väljund on järgmine:

Got 200 and ok

MÄRKUS. Saate sündmusi emiteerida mitu korda (välja arvatud need, mis on registreeritud ühe korra meetodil).

Example 4— Unregistering events

myEmitter.off('eventOne', c1);

Now if you emit the event as follows, nothing will happen and it will be a noop:

myEmitter.emit('eventOne'); // noop

Example 5— Getting Listener count

console.log(myEmitter.listenerCount('eventOne'));

NOTE: If the event has been unregistered using off or removeListener method, then the count will be 0.

Example 6— Getting Raw Listeners

console.log(myEmitter.rawListeners('eventOne'));

Example 7— Async Example demo

// Example 2->Adapted and thanks to Sameer Buna class WithTime extends EventEmitter { execute(asyncFunc, ...args) { this.emit('begin'); console.time('execute'); this.on('data', (data)=> console.log('got data ', data)); asyncFunc(...args, (err, data) => { if (err) { return this.emit('error', err); } this.emit('data', data); console.timeEnd('execute'); this.emit('end'); }); } }

Using the withTime event emitter:

const withTime = new WithTime(); withTime.on('begin', () => console.log('About to execute')); withTime.on('end', () => console.log('Done with execute')); const readFile = (url, cb) => { fetch(url) .then((resp) => resp.json()) // Transform the data into json .then(function(data) { cb(null, data); }); } withTime.execute(readFile, '//jsonplaceholder.typicode.com/posts/1');

Check the output in the console. The list of posts will be displayed along with other logs.

The Observer Pattern for Our Event Emitter

Visual Diagram 1 (Methods in our EventEmitter)

Since we now understand the usage API, let’s get to coding the module.

The complete boilerplate code for the EventEmitter class

We will be filling in the details incrementally in the next couple sections.

class EventEmitter { listeners = {}; // key-value pair addListener(eventName, fn) {} on(eventName, fn) {} removeListener(eventName, fn) {} off(eventName, fn) {} once(eventName, fn) {} emit(eventName, ...args) { } listenerCount(eventName) {} rawListeners(eventName) {} }

We begin by creating the template for the EventEmitter class along with a hash to store the listeners. The listeners will be stored as a key-value pair. The value could be an array (since for the same event we allow multiple listeners to be registered).

1. The addListener() method

Let us now implement the addListener method. It takes in an event name and a callback function to be executed.

 addListener(event, fn)  []; this.listeners[event].push(fn); return this; 

A little explanation:

The addListener event checks if the event is already registered. If yes, returns the array, otherwise empty array.

this.listeners[event] // will return array of events or undefined (first time registration)

For example…

Let’s understand this with a usage example. Let’s create a new eventEmitter and register a ‘test-event’. This is the first time the ‘test-event’ is being registered.

const eventEmitter = new EventEmitter(); eventEmitter.addListener('test-event', ()=> { console.log ("test one") } );

Inside addListener () method:

this.listeners[event] => this.listeners['test-event'] => undefined || [] => []

The result will be:

this.listeners['test-event'] = []; // empty array

and then the ‘fn’ will be pushed to this array as shown below:

this.listeners['test-event'].push(fn);

I hope this makes the ‘addListener’ method very clear to decipher and understand.

A note: Multiple callbacks can be registered against that same event.

2. The on method

This is just an alias to the ‘addListener’ method. We will be using the ‘on’ method more than the ‘addListener’ method for the sake of convenience.

on(event, fn) { return this.addListener(event, fn); }

3. The removeListener(event, fn) method

The removeListener method takes an eventName and the callback as the parameters. It removes said listener from the event array.

NOTE: If the event has multiple listeners then other listeners will not be impacted.

First, let’s take a look at the full code for removeListener.

removeListener (event, fn) { let lis = this.listeners[event]; if (!lis) return this; for(let i = lis.length; i > 0; i--) { if (lis[i] === fn) { lis.splice(i,1); break; } } return this; }

Here’s the removeListener method explained step-by-step:

  • Grab the array of listeners by ‘event’
  • If none found return ‘this’ for chaining.
  • If found, loop through all listeners. If the current listener matches with the ‘fn’ parameter use the splice method of the array to remove it. Break from the loop.
  • Return ‘this’ to continue chaining.

4. The off(event, fn) method

This is just an alias to the ‘removeListener’ method. We will be using the ‘on’ method more than the ‘addListener’ method for sake of convenience.

 off(event, fn) { return this.removeListener(event, fn); }

5. The once(eventName, fn) method

Adds a one-timelistener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

Use for setup/init kind of events.

Let’s take a peek at the code.

once(eventName, fn) { this.listeners[event] = this.listeners[eventName] || []; const onceWrapper = () => { fn(); this.off(eventName, onceWrapper); } this.listeners[eventName].push(onceWrapper); return this; }

Here’s the once method explained step-by-step:

  • Get the event array object. Empty array if the first time.
  • Create a wrapper function called onceWrapper which will invoke the fn when the event is emitted and also removes the listener.
  • Add the wrapped function to the array.
  • Return ‘this’ for chaining.

6. The emit (eventName, ..args) method

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

emit(eventName, ...args) { let fns = this.listeners[eventName]; if (!fns) return false; fns.forEach((f) => { f(...args); }); return true; }

Here’s the emit method explained step-by-step:

  • Get the functions for said eventName parameter
  • If no listeners, return false
  • For all function listeners, invoke the function with the arguments
  • Return true when done

7. The listenerCount (eventName) method

Returns the number of listeners listening to the event named eventName.

Here’s the source code:

listenerCount(eventName) 

Here’s the listenerCount method explained step-by-step:

  • Get the functions/listeners under consideration or an empty array if none.
  • Return the length.

8. The rawListeners(eventName) method

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()). The once wrappers in this implementation will not be available if the event has been emitted once.

rawListeners(event) { return this.listeners[event]; }

The full source code for reference:

class EventEmitter { listeners = {} addListener(eventName, fn)  on(eventName, fn) { return this.addListener(eventName, fn); } once(eventName, fn) { this.listeners[eventName] = this.listeners[eventName] || []; const onceWrapper = () => { fn(); this.off(eventName, onceWrapper); } this.listeners[eventName].push(onceWrapper); return this; } off(eventName, fn) { return this.removeListener(eventName, fn); } removeListener (eventName, fn) { let lis = this.listeners[eventName]; if (!lis) return this; for(let i = lis.length; i > 0; i--) { if (lis[i] === fn) { lis.splice(i,1); break; } } return this; } emit(eventName, ...args) { let fns = this.listeners[eventName]; if (!fns) return false; fns.forEach((f) => { f(...args); }); return true; } listenerCount(eventName)  rawListeners(eventName) { return this.listeners[eventName]; } }

The complete code is available here:

//jsbin.com/gibofab/edit?js,console,output

As an exercise feel free to implement other events’ APIs from the documentation //nodejs.org/api/events.html.

If you liked this article and want to see more of similar articles, feel free to give a couple of claps :)

MÄRKUS . Kood on optimeeritud loetavuse ja mitte jõudluse jaoks. Võib-olla saate harjutusena koodi optimeerida ja kommentaaride jaotises jagada. Ei ole servajuhtumeid täielikult testinud ja mõned valideerimised võivad olla välja lülitatud, kuna see oli kiire kirjutamine.

See artikkel on osa eelseisvast videokursusest “Node.JS Master Class - Build Your Own ExpressJS-like MVC Framework nullist”.

Kursuse pealkiri pole veel lõplikult vormistatud.