Kako kodirati vlastiti odašiljač događaja u Node.js: detaljni vodič

Razumijevanje internih čvorova kodiranjem malih paketa / modula

Ako ste novi u Node.js, ovdje na Mediumu i drugdje postoje mnogi vodiči. Možete pogledati moj članak, na primjer All About Core Node.JS.

No, bez daljnjeg odugovlačenja, krenimo na temu o kojoj se raspravlja: "Emiteri događaja". Emiteri događaja igraju vrlo važnu ulogu u ekosustavu Node.js.

EventEmitter je modul koji olakšava komunikaciju / interakciju između objekata u Nodeu. EventEmitter je u osnovi Node asinkrone arhitekture vođene događajima. Mnogi ugrađeni moduli Node-a nasljeđuju se od EventEmittera, uključujući istaknute okvire poput Express.js.

Koncept je vrlo jednostavan: predmeti emitora emitiraju imenovane događaje zbog kojih se pozivaju prethodno registrirani slušatelji. Dakle, objekt emitera u osnovi ima dvije glavne značajke:

  • Emitiranje događaja imena.
  • Registriranje i poništavanje registracije funkcija slušatelja.

To je nekako poput obrasca dizajna pub / sub ili promatrača (iako ne baš točno).

Što ćemo graditi u ovom vodiču

  • Klasa EventEmitter
  • na / addEventListener metoda
  • off / removeEventListener metoda
  • jednom metoda
  • metoda emitiranja
  • metoda rawListeners
  • method listenerCount

Gore navedene osnovne značajke dovoljne su za implementaciju cjelovitog sustava koji koristi model događaja.

Prije nego što krenemo u kodiranje, pogledajmo kako ćemo koristiti klasu EventEmitter. Imajte na umu da će naš kod oponašati točan API modula Node.js 'događaji'.

U stvari, ako naš EventEmitter zamijenite ugrađenim modulom za događaje Node.js, dobit ćete isti rezultat.

Primjer 1 - Stvorite instancu odašiljača događaja i registrirajte nekoliko povratnih poziva

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

Kada se emitira događaj 'eventOne', treba pozivati ​​oba gore navedena povratna poziva.

myEmitter.emit('eventOne');

Izlaz u konzoli bit će sljedeći:

an event occurred! yet another event occurred!

Primjer 2 - Registriranje za događaj koji se aktivira samo jednom korištenjem jednom.

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

Emitiranje događaja 'eventOnce':

myEmitter.emit('eventOne');

Sljedeći izlaz trebao bi se pojaviti u konzoli:

eventOnce once fired

Emitiranje događaja registriranih s još jednom neće imati utjecaja.

myEmitter.emit('eventOne');

Budući da je događaj emitiran samo jednom, gornja izjava neće imati utjecaja.

Primjer 3 - Registracija događaja s parametrima povratnog poziva

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

Emitiranje događaja s parametrima:

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

Izlaz u konzoli bit će sljedeći:

Got 200 and ok

NAPOMENA: Događaje možete emitirati više puta (osim onih registriranih metodom Once).

Primjer 4 - Poništavanje registracije događaja

myEmitter.off('eventOne', c1);

Sada ako emitirate događaj na sljedeći način, ništa se neće dogoditi i bit će to petlja:

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

Primjer 5 - Dobivanje broja slušatelja

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

NAPOMENA: Ako je događaj neregistriran metodom off ili removeListener, tada će broj biti 0.

Primjer 6 - Dobivanje sirovih slušatelja

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

Primjer 7 - Demonstracija primjera Async

// 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'); }); } }

Korištenje emitera događaja withTime:

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');

Provjerite izlaz u konzoli. Popis postova prikazat će se zajedno s ostalim zapisnicima.

Uzorak promatrača našeg odašiljača događaja

Vizualni dijagram 1 (Metode u našem EventEmitteru)

Budući da sada razumijemo API upotrebe, idemo na kodiranje modula.

Kompletni kôd uzorka za klasu EventEmitter

Pojedinosti ćemo popunjavati u sljedećih nekoliko odjeljaka.

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 :)

NOTE: The code is optimized for readability and not for performance. Maybe as an exercise, you can optimize the code and share it in the comment section. Haven’t tested fully for edge cases and some validations may be off as this was a quick writeup.

This article is part of the upcoming video course “Node.JS Master Class — Build Your Own ExpressJS-Like MVC Framework from scratch”.

The title of the course is not yet finalized.