JavaScript - fra tilbagekald til asynkronisering / afventning

JavaScript er synkron. Dette betyder, at den udfører din kodeblok efter ordre efter hejsning. Inden koden udføres, varog functionerklæringer "hejses" til toppen af ​​deres anvendelsesområde.

Dette er et eksempel på en synkron kode:

console.log('1') console.log('2') console.log('3')

Denne kode logger pålideligt “1 2 3”.

Asynkrone anmodninger venter på, at en timer er færdig, eller en anmodning om at svare, mens resten af ​​koden fortsætter med at udføre. Så når tiden er inde, springer disse tilbagekald tilbage til disse asynkrone anmodninger.

Dette er et eksempel på en asynkron kode:

console.log('1') setTimeout(function afterTwoSeconds() { console.log('2') }, 2000) console.log('3')

Dette logger faktisk “1 3 2”, da “2” er på en, setTimeoutder kun udføres ved dette eksempel efter to sekunder. Din applikation hænger ikke og venter på, at de to sekunder er færdige. I stedet fortsætter den med at udføre resten af ​​koden, og når timeoutet er afsluttet, vender det tilbage til afterTwoSeconds.

Du kan spørge "Hvorfor er det nyttigt?" eller “Hvordan får jeg min asynkroniseringskode til at blive synkroniseret?”. Forhåbentlig kan jeg vise dig svarene.

"Problemet"

Lad os sige, at vores mål er at søge efter en GitHub-bruger og få alle brugernes opbevaringssteder. Sagen er, at vi ikke ved det nøjagtige navn på brugeren. Så vi er nødt til at liste alle brugere med lignende navn og deres respektive arkiver.

Behøver ikke at være super fancy, noget som dette

I disse eksempler bruger anmodningskoden XHR (XMLHttpRequest). Du kan erstatte det med jQuery $.ajaxeller den nyere native-tilgang kaldet fetch. Begge giver dig løfterne nærmer sig porten.

Det ændres lidt afhængigt af din tilgang, men som en starter:

// url argument can be something like '//api.github.com/users/daspinola/repos' function request(url) { const xhr = new XMLHttpRequest(); xhr.timeout = 2000; xhr.onreadystatechange = function(e) { if (xhr.readyState === 4) { if (xhr.status === 200) { // Code here for the server answer when successful } else { // Code here for the server answer when not successful } } } xhr.ontimeout = function () { // Well, it took to long do some code here to handle that } xhr.open('get', url, true) xhr.send(); }

Husk, at i disse eksempler er den vigtige del ikke, hvad slutresultatet af koden er. I stedet skal dit mål være at forstå forskellene i fremgangsmåderne, og hvordan du kan udnytte dem til din udvikling.

Ring tilbage

Du kan gemme en reference til en funktion i en variabel, når du bruger JavaScript. Derefter kan du bruge dem som argumenter for en anden funktion til at udføre senere. Dette er vores "tilbagekaldelse".

Et eksempel ville være:

// Execute the function "doThis" with another function as parameter, in this case "andThenThis". doThis will execute whatever code it has and when it finishes it should have "andThenThis" being executed. doThis(andThenThis) // Inside of "doThis" it's referenced as "callback" which is just a variable that is holding the reference to this function function andThenThis() { console.log('and then this') } // You can name it whatever you want, "callback" is common approach function doThis(callback) { console.log('this first') // the '()' is when you are telling your code to execute the function reference else it will just log the reference callback() }

Brug af callbacktil at løse vores problem giver os mulighed for at gøre noget lignende til den requestfunktion, vi definerede tidligere:

function request(url, callback) { const xhr = new XMLHttpRequest(); xhr.timeout = 2000; xhr.onreadystatechange = function(e) { if (xhr.readyState === 4) { if (xhr.status === 200) { callback(null, xhr.response) } else { callback(xhr.status, null) } } } xhr.ontimeout = function () { console.log('Timeout') } xhr.open('get', url, true) xhr.send(); }

Vores funktion til anmodningen accepterer nu a, callbackså når en requester lavet, kaldes den i tilfælde af fejl og i tilfælde af succes.

const userGet = `//api.github.com/search/users?page=1&q=daspinola&type=Users` request(userGet, function handleUsersList(error, users) { if (error) throw error const list = JSON.parse(users).items list.forEach(function(user) { request(user.repos_url, function handleReposList(err, repos) { if (err) throw err // Handle the repositories list here }) }) })

Opdel dette:

• Vi anmoder om at få en brugers opbevaringssteder
• Når anmodningen er afsluttet, bruger vi tilbagekald handleUsersList
• Hvis der ikke er nogen fejl, analyserer vi vores serverrespons i et objekt ved hjælp af JSON.parse
• Derefter gentager vi vores brugerliste, da den kan have mere end en

  For hver bruger beder vi om deres opbevaringsliste.

  Vi bruger den url, der returneres pr. Bruger i vores første svar

  Vi kalder repos_urlsom url til vores næste anmodninger eller fra det første svar

• Når anmodningen har gennemført tilbagekaldet, ringer vi op

  Dette håndterer enten dens fejl eller svaret med listen over opbevaringssteder for den bruger

Bemærk : At sende fejlen først som parameter er en almindelig praksis, især når du bruger Node.js.

En mere "komplet" og læsbar tilgang ville være at have en vis fejlhåndtering. Vi vil holde tilbagekaldet adskilt fra anmodningen.

Noget som dette:

try { request(userGet, handleUsersList) } catch (e) { console.error('Request boom! ', e) } function handleUsersList(error, users) { if (error) throw error const list = JSON.parse(users).items list.forEach(function(user) { request(user.repos_url, handleReposList) }) } function handleReposList(err, repos) { if (err) throw err // Handle the repositories list here console.log('My very few repos', repos) }

Dette ender med at have problemer som racing og problemer med fejlhåndtering. Racing sker, når du ikke styrer, hvilken bruger du får først. Vi anmoder om oplysningerne for dem alle, hvis der er mere end en. Vi tager ikke en ordre i betragtning. For eksempel kan bruger 10 komme først og bruger 2 sidst. Vi har en mulig løsning senere i artiklen.

Det største problem med tilbagekald er, at vedligeholdelse og læsbarhed kan blive en smerte. Det er allerede noget, og koden gør næppe noget. Dette er kendt som callback helvede, som kan undgås med vores næste tilgang.

Løfter

Løfter du kan gøre din kode mere læselig. En ny udvikler kan komme til kodebasen og se en klar rækkefølge for udførelse af din kode.

For at skabe et løfte kan du bruge:

const myPromise = new Promise(function(resolve, reject) { // code here if (codeIsFine) { resolve('fine') } else { reject('error') } }) myPromise .then(function whenOk(response) { console.log(response) return response }) .catch(function notOk(err) { console.error(err) })

Lad os nedbryde det:

• Et løfte initialiseres med et functionder har resolveog rejectudsagn
• Lav din async-kode inde i Promisefunktionen

  resolve når alt sker som ønsket

  Ellers reject

• Når a resolveer fundet .then, udføres metoden for detPromise

  Når a reject er fundet .catch udløses viljen

Ting at huske på:

• resolve and reject only accept one parameter

  resolve(‘yey’, ‘works’) will only send ‘yey’ to the .then callback function

• If you chain multiple .then

  Add a return if you want the next .then value not to be undefined

• When a reject is caught with .catch if you have a .then chained to it

  It will still execute that .then

  You can see the .then as an “always executes” and you can check an example in this comment

• With a chain on .then if an error happens on the first one

  It will skip subsequent .then until it finds a .catch

• A promise has three states

  pending

• When waiting for a resolve or reject to happen

  resolved

  rejected

• Once it’s in a resolved or rejected state

  It cannot be changed

Note: You can create promises without the function at the moment of declarations. The way that I’m showing it is only a common way of doing it.

“Theory, theory, theory…I’m confused” you may say.

Let’s use our request example with a promise to try to clear things up:

function request(url) { return new Promise(function (resolve, reject) { const xhr = new XMLHttpRequest(); xhr.timeout = 2000; xhr.onreadystatechange = function(e) { if (xhr.readyState === 4) { if (xhr.status === 200) { resolve(xhr.response) } else { reject(xhr.status) } } } xhr.ontimeout = function () { reject('timeout') } xhr.open('get', url, true) xhr.send(); }) }

In this scenario when you execute request it will return something like this:

const userGet = `//api.github.com/search/users?page=1&q=daspinola&type=Users` const myPromise = request(userGet) console.log('will be pending when logged', myPromise) myPromise .then(function handleUsersList(users) { console.log('when resolve is found it comes here with the response, in this case users ', users) const list = JSON.parse(users).items return Promise.all(list.map(function(user) { return request(user.repos_url) })) }) .then(function handleReposList(repos) { console.log('All users repos in an array', repos) }) .catch(function handleErrors(error) { console.log('when a reject is executed it will come here ignoring the then statement ', error) })

This is how we solve racing and some of the error handling problems. The code is still a bit convoluted. But its a way to show you that this approach can also create readability problems.

A quick fix would be to separate the callbacks like so:

const userGet = `//api.github.com/search/users?page=1&q=daspinola&type=Users` const userRequest = request(userGet) // Just by reading this part out loud you have a good idea of what the code does userRequest .then(handleUsersList) .then(repoRequest) .then(handleReposList) .catch(handleErrors) function handleUsersList(users) { return JSON.parse(users).items } function repoRequest(users) { return Promise.all(users.map(function(user) { return request(user.repos_url) })) } function handleReposList(repos) { console.log('All users repos in an array', repos) } function handleErrors(error) { console.error('Something went wrong ', error) }

By looking at what userRequest is waiting in order with the .then you can get a sense of what we expect of this code block. Everything is more or less separated by responsibility.

This is “scratching the surface” of what Promises are. To have a great insight on how they work I cannot recommend enough this article.

Generators

Another approach is to use the generators. This is a bit more advance so if you are starting out feel free to jump to the next topic.

One use for generators is that they allow you to have async code looking like sync.

They are represented by a * in a function and look something like:

function* foo() { yield 1 const args = yield 2 console.log(args) } var fooIterator = foo() console.log(fooIterator.next().value) // will log 1 console.log(fooIterator.next().value) // will log 2 fooIterator.next('aParam') // will log the console.log inside the generator 'aParam'

Instead of returning with a return, generators have a yield statement. It stops the function execution until a .next is made for that function iteration. It is similar to .then promise that only executes when resolved comes back.

Our request function would look like this:

function request(url) { return function(callback) { const xhr = new XMLHttpRequest(); xhr.onreadystatechange = function(e) { if (xhr.readyState === 4) { if (xhr.status === 200) { callback(null, xhr.response) } else { callback(xhr.status, null) } } } xhr.ontimeout = function () { console.log('timeout') } xhr.open('get', url, true) xhr.send() } }

We want to have the url as an argument. But instead of executing the request out of the gate we want it only when we have a callback to handle the response.

Our generator would be something like:

function* list() { const userGet = `//api.github.com/search/users?page=1&q=daspinola&type=Users` const users = yield request(userGet) yield for (let i = 0; i<=users.length; i++) { yield request(users[i].repos_url) } }

It will:

• Wait until the first request is prepared
• Return a function reference expecting a callback for the first request

  Our request function accepts a url

  and returns a function that expects a callback

• Expect a users to be sent in the next .next
• Iterate over users
• Wait for a .next for each of the users
• Return their respective callback function

So an execution of this would be:

try { const iterator = list() iterator.next().value(function handleUsersList(err, users) { if (err) throw err const list = JSON.parse(users).items // send the list of users for the iterator iterator.next(list) list.forEach(function(user) { iterator.next().value(function userRepos(error, repos) { if (error) throw repos // Handle each individual user repo here console.log(user, JSON.parse(repos)) }) }) }) } catch (e) { console.error(e) }

We could separate the callback functions like we did previously. You get the deal by now, a takeaway is that we now can handle each individual user repository list individually.

I have mixed felling about generators. On one hand I can get a grasp of what is expected of the code by looking at the generator.

But its execution ends up having similar problems to the callback hell.

Like async/await, a compiler is recommended. This is because it isn’t supported in older browser versions.

Also it isn’t that common in my experience. So it may generate confusing in codebases maintained by various developers.

An awesome insight of how generators work can be found in this article. And here is another great resource.

Async/Await

This method seems like a mix of generators with promises. You just have to tell your code what functions are to be async. And what part of the code will have to await for that promise to finish.

sumTwentyAfterTwoSeconds(10) .then(result => console.log('after 2 seconds', result)) async function sumTwentyAfterTwoSeconds(value) { const remainder = afterTwoSeconds(20) return value + await remainder } function afterTwoSeconds(value) { return new Promise(resolve => { setTimeout(() => { resolve(value) }, 2000); }); }

In this scenario:

• We have sumTwentyAfterTwoSeconds as being an async function
• We tell our code to wait for the resolve or reject for our promise function afterTwoSeconds
• It will only end up in the .then when the await operations finish

  In this case there is only one

Applying this to our request we leave it as a promise as seen earlier:

function request(url) { return new Promise(function(resolve, reject) { const xhr = new XMLHttpRequest(); xhr.onreadystatechange = function(e) { if (xhr.readyState === 4) { if (xhr.status === 200) { resolve(xhr.response) } else { reject(xhr.status) } } } xhr.ontimeout = function () { reject('timeout') } xhr.open('get', url, true) xhr.send() }) }

We create our async function with the needed awaits like so:

async function list() { const userGet = `//api.github.com/search/users?page=1&q=daspinola&type=Users` const users = await request(userGet) const usersList = JSON.parse(users).items usersList.forEach(async function (user) { const repos = await request(user.repos_url) handleRepoList(user, repos) }) } function handleRepoList(user, repos) { const userRepos = JSON.parse(repos) // Handle each individual user repo here console.log(user, userRepos) }

So now we have an async list function that will handle the requests. Another async is needed in the forEach so that we have the list of repos for each user to manipulate.

We call it as:

list() .catch(e => console.error(e))

This and the promises approach are my favorites since the code is easy to read and change. You can read about async/await more in depth here.

A downside of using async/await is that it isn’t supported in the front-end by older browsers or in the back-end. You have to use the Node 8.

You can use a compiler like babel to help solve that.

“Solution”

You can see the end code accomplishing our initial goal using async/await in this snippet.

A good thing to do is to try it yourself in the various forms referenced in this article.

Conclusion

Depending on the scenario you might find yourself using:

• async/await
• callbacks
• mix

It’s up to you what fits your purposes. And what lets you maintain the code so that it is understandable to others and your future self.

Note: Any of the approaches become slightly less verbose when using the alternatives for requests like $.ajax and fetch.

Fortæl mig, hvad du ville gøre forskellige og forskellige måder, du fandt for at gøre hver tilgang mere læselig.

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