JavaScript - tagasihelistamisest asünkroonimiseni / ootamiseni

JavaScript on sünkroonne. See tähendab, et see täidab teie koodiploki pärast tõstmist korraldusega. Enne koodi täitmist varja functiondeklaratsioonid tõstetakse nende reguleerimisala tippu.

See on sünkroonse koodi näide:

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

See kood logib usaldusväärselt "1 2 3".

Asünkroonsed taotlused ootavad taimeri lõpuleviimist või vastusetaotlust, kuni ülejäänud kood jätkab täitmist. Kui aeg on käes, kutsub need asünkroonsed taotlused tagasi.

See on näide asünkroonsest koodist:

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

See logib tegelikult "1 3 2", kuna "2" on peal, setTimeoutmis käivitatakse selle näite abil alles kahe sekundi pärast. Teie rakendus ei jää ootama, kuni kaks sekundit lõpeb. Selle asemel jätkab ta ülejäänud koodi täitmist ja kui aegumine on lõppenud, naaseb see väärtusele AfterSwoSeconds.

Võite küsida: "Miks see on kasulik?" või "Kuidas saada asünkroonkood sünkrooniks?". Loodetavasti saan teile vastuseid näidata.

"Probleem"

Oletame, et meie eesmärk on otsida GitHubi kasutaja ja hankida selle kasutaja kõik hoidlad. Asi on selles, et me ei tea kasutaja täpset nime. Seega peame loetlema kõik sarnase nimega kasutajad ja nende vastavad hoidlad.

Ei pea super välja mõtlema, midagi sellist

Nendes näidetes kasutab päringukood XHR-i (XMLHttpRequest). Võite selle asendada jQuery $.ajaxvõi uuema natiivse lähenemisviisiga fetch. Mõlemad annavad teile lubadustest läheneda väravast välja.

Seda muudetakse veidi sõltuvalt teie lähenemisviisist, kuid alustajana:

// 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(); }

Pidage meeles, et nendes näidetes pole oluline osa sellest, mis on koodi lõpptulemus. Selle asemel peaks teie eesmärk olema mõista lähenemisviiside erinevusi ja seda, kuidas saaksite neid oma arenguks kasutada.

Helista tagasi

JavaScripti kasutamisel saate muutuja funktsiooni viite salvestada. Seejärel saate neid hiljem kasutada mõne teise funktsiooni argumendina. See on meie “tagasihelistamine”.

Üks näide oleks:

// 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() }

Kasutades callbackprobleemi lahendamiseks saame teha midagi sellist requestfunktsiooniga, mille me varem määratlesime:

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(); }

Meie päringu funktsioon aktsepteerib nüüd a-ga, callbacknii et kui see requeston tehtud, kutsutakse seda vea korral ja edu korral.

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 }) }) })

Selle jaotamine:

  • Esitame taotluse kasutaja hoidlate saamiseks
  • Pärast päringu täitmist kasutame tagasihelistamist handleUsersList
  • Kui viga pole, siis analüüsime oma serveri vastuse objektiks JSON.parse
  • Seejärel kordame oma kasutajaloendit, kuna sellel võib olla rohkem kui üks

    Iga kasutaja jaoks palume nende hoidlate loendit.

    Esimeses vastuses kasutame kasutaja kohta naasnud URL-i

    Helistame repos_urljärgmiste taotluste URL-iks või esimesest vastusest

  • Kui taotlus on tagasihelistamise lõpetanud, helistame

    See käsitleb kas selle viga või vastus koos selle kasutaja hoidlate loendiga

Märkus . Vea esmalt parameetrina saatmine on tavaline tava, eriti Node.js kasutamisel.

"Terviklikum" ja loetavam lähenemisviis oleks teatud tõrkeotsing. Hoiame tagasihelistamise taotluse täitmisest eraldi.

Midagi sellist:

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) }

Sellega kaasnevad lõpuks probleemid nagu võidusõit ja veakäsitlus. Võidusõit toimub siis, kui te ei kontrolli, millise kasutaja saate esimesena. Juhul kui neid on rohkem kui üks, palume teavet kõigi nende kohta. Me ei arvesta korraldust. Näiteks võib kasutaja 10 olla esimene ja kasutaja 2 viimane. Meil on võimalik lahendus hiljem artiklis.

Tagasihelistamise peamine probleem on see, et hooldus ja loetavus võivad muutuda piinaks. See on omamoodi juba ja kood ei tee peaaegu midagi. Seda nimetatakse tagasihelistamise põrguks, mida saab järgmise lähenemisega vältida.

Lubadused

Lubab, et saate oma koodi paremini loetavaks muuta. Uus arendaja võib tulla koodibaasi ja näha teie koodi täitmise selget järjekorda.

Lubaduse loomiseks võite kasutada järgmist.

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) })

Lagundame selle:

  • Lubadus vormistatakse, millel functionon resolveja rejectavaldused
  • Sisestage asünkroonkood Promisefunktsiooni sisse

    resolve kui kõik juhtub vastavalt soovile

    Muidu reject

  • Kui resolveleitakse .thenmeetod, käivitatakse selle jaoks meetodPromise

    Kui reject leitakse a, .catch käivitatakse see

Mida tasub meeles pidada:

  • 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.

Andke mulle teada, mida teeksite iga lähenemise loetavamaks muutmiseks erinevatel ja erinevatel viisidel.

See on artikkel 11–30. See on osa projektist, mille eesmärk on avaldada artikkel vähemalt kord nädalas, alates tühikäigulistest mõtetest kuni õpetusteni. Jäta kommentaar, jälgi mind Diogo Spínola lehel ja mine siis tagasi oma hiilgava projekti juurde!