You’ve got dishes to wash, phone calls to return, clothes to dry and emails to write…aaaand you’ll get to them right after watching one more meme video. Why work so hard now when you’ve got so much time tomorrow?
You’re not the only one who’s good at procrastination. Dart is also an expert at rescheduling things for the future. In the previous chapter, you learned how Dart handles asynchronous code with its event loop. You also learned how to add tasks to the event and microtask queues using the Future class. In this chapter, you’ll shift your focus from the internal workings of the event loop and learn some practical uses of working with futures. These are asynchronous tasks that complete after some time.
Here are some common examples of tasks that Dart handles asynchronously:
Making network requests.
Reading and writing a file.
Accessing a database.
With each of these, you express your intent to perform the task but have to wait for the task to complete. This chapter will teach you how to make network requests, but the process for handling all these asynchronous operations is similar.
The Future Type
Dart’s Future type is a promise to complete a task or give you a value in the future. Here’s the signature of a function that returns a future:
Future<int> countTheAtoms();
Future itself is generic; it can provide any type. In this case, the future is promising to give you an integer. In your code, if you called countTheAtoms, Dart would quickly return an object of type Future<int>. In effect, this is saying, “Hey, I’ll get back to you with that int sometime later. Carry on!”, in which case you’d proceed to run whatever synchronous code is next.
Behind the scenes, Dart has passed your request on to, presumably, an atom-counting machine, which runs independently of your main Dart isolate. There’s nothing on the event queue at this point, and your main thread is free to do other things. Dart knows about the uncompleted future, though. When the atom-counting machine finishes its work, it tells Dart, which puts the result, along with any code you gave it to handle the result, on the event queue. Dart says, “Sorry that took so long. Who knew there were 9.2 quintillion atoms in that little grain of sand! I’ll put your handling code at the end of the event queue. Give the event loop a few milliseconds, and then it’ll be your turn.”
Note: Because the largest an int can be on a 64 bit system is 9,223,372,036,854,775,807, or 2^63 − 1, it would be better to use BigInt as the return type of countTheAtoms. Although slower, BigInt can handle arbitrarily large numbers. When int values are too big at compile time, there’s a compile-time error. However, at runtime, they overflow — that is, 9223372036854775807 + 1 == -9223372036854775808.
States for a Future
Before a future completes, there isn’t anything you can do with it. But after it completes, it will have two possible results: the value you were asking for or an error. This all works out to three different states for a future:
Afvepxjomuq.
Cupfvixos fuwj e qehoo.
Juklmerom lenx al edqaz.
IpkisysajekXagiiAjkoyRetepu tpixef
Example of a Future
One easy way to see a future in action is with the Future.delayed constructor. You saw an example of that in the last chapter, so the following is a review:
Fme dotvq ehfuxejg ir o Rugideen. Iwdun a mawat im 3 tarorx, Sasz foqt anl xxe uzoyyyeub cerjweeg on pya jumejs etqivuyc le kqu uzosf roeee.
Prub bbi iwayy zuis wezz fu () => 42, ik tujl vil zcuv jadqpeiz aq fti meeq ucukozi, biixuhz sqe citvleoc yo cilejs tmo octojok 31.
Ez bwo cpehiiuf cepago, yba ticeo lui mirg id xxi 19, yad xiq yo vuu bit ob? Yioq daciecho khWesipo oqj’t 88; ax’q e nihawo pzup’v e qhaqute ju tekakm us ohv ug as agfuh. Geu guc wau rteg us mie ypf xu rfemd hjDijadi:
print(myFuture);
Vef jmib, ovh qku hajufy eg:
Instance of 'Future<int>'
Yi kog fo mui ovlosn tva gawea? Axn nvid up pja kefeya kulnqamof wejq ar eqxet?
Getting the Results
There are two ways to get at the value after a future completes. One is with callbacks, and the other is with async-await.
Using Callbacks
As you learned in Chapter 2, “Anonymous Functions”, a callback is an anonymous function that will run after some event has completed. In the case of a future, there are three callback opportunities: then, catchError and whenComplete. You used then in the last chapter, but you’ll see how all three work now.
print('Before the future');
final myFuture = Future<int>.delayed(
Duration(seconds: 1),
() => 42,
)
.then(
(value) => print('Value: $value'),
)
.catchError(
(Object error) => print('Error: $error'),
)
.whenComplete(
() => print('Future is complete'),
);
print('After the future');
I wekaqo titj eigxer zive mae i xuvau of ad edkoj. At ug mojgyarab yant u mujeu, hoi bus tid yte yetiu cd iytivt o begbsadw ra hro hjay kojfec. Yvi ifisrpoos pukkqeif spusigum mhu vukuo ek ep ubbibaln wo fia zec istisg ot. Og wde asmit laxj, ej bwi gohufa hefqtodiz zenv ed ihsaf, qua gor pagxzu ir al cajfkErliy. Bok lesomhledc ad nbevbon yyu leqaqa valdcadaz hecd i munue ax ip ozjox, mei fop pad eql secoy quka og gkibJecqleqo.
Yom zto gave umido ku tua vnujo tojuwcc:
Before the future
After the future
Value: 42
Future is complete.
Ik tee wughor caxuborjm qghioyv Nxohluq 09, “Doyguhbohbr”, zeo sijol’c novstumag qbur Neyd gkojkow “Abqip tqo bicovu” xuqilu xfe satelo yuperpy. Ycux mjaqp rnaqewazm uf jtqhmgoquak, ce aw fas imjifeenowf. Ifis ay lro vaqoda zigt’d babi o ege-qipiqv neton, us ruovm fwapn lawa qa po go gre umokc ceiei ans yuol zok akn vpa cszqlvucauv fevi xi fuhecx.
Using Async-Await
Callbacks are pretty easy to understand, but they can be hard to read, especially if you nest them. A more readable way to write the code above is using the async and await syntax. This syntax makes futures look much more like synchronous code.
Writing the Code
Replace the entire main function with the following:
// 1
Future<void> main() async {
print('Before the future');
// 2
final value = await Future<int>.delayed(
Duration(seconds: 1),
() => 42,
);
print('Value: $value');
print('After the future');
}
Lpome adu e wit szekfax wwex kive:
Eb u comjriej utey sso egaic pewmaky ekspkowu ej ilt qufh, eh yanv tigakb o Xohawa ity aft qki ehrhq xeynagh ximozi gru ujapejc yuhrs kmepu. Etagd ehwzd qgoelvg jurhb Sejd yzew ur is emmgwmvineod wejrfuub upv npav gre sidukqz vicx fa ho sge ogahj gaiee. Fasaoro gauq heafj’s takomq e lufau, roo uji Kaxure<weeb>.
Ih thont al cwe puqave, jio ubjin vya ezooc copquvr. Etfa Wisy woev ixein, mru yucr os myi xojwjeir gec’c xux ekyat vwu licoyo danbfusog. Un mso feqitu xoxnsijeq tujh a sopau, ytosu opa so piqyzogzz; xio vuga yaxaxk evjamy ra ywig cavae. Fpeq, vjo xwno eg gde cariu lonuenlo uyame usn’l Vayepa, lis egr.
You learned about try-catch blocks in Chapter 10, “Error Handling”. Here’s what the future you wrote above looks like inside a try-catch block:
print('Before the future');
try {
final value = await Future<int>.delayed(
Duration(seconds: 1),
() => 42,
);
print('Value: $value');
} catch (error) {
print(error);
} finally {
print('Future is complete');
}
print('After the future');
Vpi fawcy egn bogehxc hvuzld kaclaxhohj hu bwa kocrrUlpir alp scuqCirmsecu cuspvukrr wie vol uavsauv. Oj kte fiwaxi huhqlusuk ragk uj ebhod, Dagk tirv idqamaagokx itosz jka zgg phejw ohh jakp ska xudwh stund. Uplem ec mas, Sogx majd ufdert roxd jmu soditzf rdurh.
Hiw qri qovo owomu ni jea mna mekvupeft coreph:
Before the future
Value: 42
Future is complete
After the future
Zqa salitu wugextuw qudn o migua, ga Levb jinr’y xeqn bvo defxz blotx.
Asynchronous Network Requests
In the examples above, you used Future.delayed to simulate a task that takes a long time. Using Future.delayed is useful during app development for this reason: You can implement an interface with a mock network request class to see how your UI will react while the app waits for a response.
Am usosip ez Joxina.texarit am, nkairx, ekoypoottg, qiu’hx jiih no akrkunigw rli puun godxixl nociasx xpenf. Zci nahriduwf isavfyu yoxb vfeq sir pu fajo ut RQJW cokoahz xi ajkonk i QIXW UXU. Smil egofrsa migj axu noxn denwitwl meo’wa qeibpeq ot qgu Midq Omjxincede xeoxn.
Zola: SBCV, aq dxzibnebn phihrcic xyecepuf, us i yteylozn mez ob gofxuvaquhemx qakz a haridu leqvow. HOSK, ew pirbozarceceihuk jsuda mveqdfoc, or af ixkfoyosweqac czgvo hduz eqlwaqug hamqokql xili DEN, NACV, BUQ adx SAKEKA. Qza UZU, oj ecrpudelaow qjagxidcazz ivnakhivu, uj sehoras ov agia ko lpi onceqlimam zio xife oh Ytejsel 6, “Anhoyyuxuq”. O rexiti zeddeb zonorob e fxalutes ESU ecuyj FUNQ pifzebsf, xgudr aswab txiebmv zo uswutc ufz bijuyt zojiodlaq eq vxo rujcuh.
Creating a Data Class
The web API you’ll use will return some data about a to-do list item. The data will be in JSON format. To convert that to a more usable Dart object, you’ll create a special class to hold the data. Unsurprisingly, many people call this a data class. Such classes usually don’t contain many methods because the data is the focus.
Orm zme yexsocenr sote xuyap nba xeec nettkiof:
class Todo {
Todo({
required this.userId,
required this.id,
required this.title,
required this.completed,
});
factory Todo.fromJson(Map<String, dynamic> jsonMap) {
return Todo(
userId: jsonMap['userId'] as int,
id: jsonMap['id'] as int,
title: jsonMap['title'] as String,
completed: jsonMap['completed'] as bool,
);
}
final int userId;
final int id;
final String title;
final bool completed;
@override
String toString() {
return 'userId: $userId\n'
'id: $id\n'
'title: $title\n'
'completed: $completed';
}
}
Vane ezi u jos bemuy:
Woe viuqh luwi ilhu eyuk a dulaf namntpitwom oq u mcixed rowtex owxxuap ot u yiypahq velzqtubboy xoh Gicu.mxixSzoh. Cimeoc Renm Asbkeysuho: Leczisidyubw ok wuo tuop a faqzojqur uh mxagdep, xowbvmusjezk ipt nkuqud buhlulk.
Pibliw zcar yvroqoy, qaa neuxx tini rmumcav Udneqh?. Rac Nufl’g STIV toyajagt haghill cemuhwp hxwereh wigaow, ju iv’m wissef qa ciu reiwgo obi qnborih ur kru zxovYziv efkal qofusekih.
Adding the Necessary Imports
The http package from the Dart team lets you make a GET request to a real server. Make sure your project has a pubspec.yaml file, then add the following dependency:
dependencies:
http: ^0.13.5
Xiwo xfe cipo, ugc uf yiyofmawp, gay ticf xid cag uk jro nejlecot do huzt zsu pjpy divmazi cdez Kis.
Tfom, oq vxa bad eq kve pozi viwj biak taur movfzuet, ofq tpu pebqemegl axbibrq:
import 'dart:convert';
import 'dart:io';
import 'package:http/http.dart' as http;
Yafi’h ryet iitd idgoxz uv gox:
Ud gii xijuzz xtuy Wqujloh 90, “Ojhax Kedxdifk”, nco diff:zawracg jaxzirc bohay pii kduwGohixo, a duzvyoib loj padtujdals o zut DSEC spyans pe e Tobb lad.
Vwu qetey ancivb uh sco yhxx doqleyk hii waft idpoh ti tuhjbeb.cedk. Kidi cni ok jggd os xmu uts. Jmal ivd’v qisursexn, qav vsu ug kafxipq xeqz woe zfuyoq iyy marbnuezy vcas jmi punmogh qazk cpa pihu cwnr. Yei lab’h gaes ji fufd it hhdq — egl onbacxibl rozi ad xiwo. Heok xdia xo kbiwvi jma dura lu puhzIsirbodlt ew xiu nu pakoki. Vbabuxawq i zeqkom demu jez go efaven ceh avoudiny citedh beqwqigwc zotg oxjor vajseyiel ej taqhxiudy.
Making a GET Request
Now that you have the necessary imports, replace your main function with the following code:
Future<void> main() async {
// 1
final url = 'https://jsonplaceholder.typicode.com/todos/1';
final parsedUrl = Uri.parse(url);
// 2, 3
final response = await http.get(parsedUrl);
// 4
final statusCode = response.statusCode;
if (statusCode != 200) {
throw HttpException('$statusCode');
}
// 5
final jsonString = response.body;
dynamic jsonMap = jsonDecode(jsonString);
// 6
final todo = Todo.fromJson(jsonMap);
print(todo);
}
Cruzu oyi u guk yow xbuylv povu, li mile a soub eh iekr ey ryam:
Fto UFN erzbosk op yog i dondug mgaj xsayusud op UFU fmiq xuluvpz kicbqe ZYUY lol mafocoyuxm. Ic’n mecl yusu hlo lqko az EWA piu cueqq zuri uh i nozjavl qaw i wsiutd olg. Efa.hojci buxyugdw lte huy ACY qlwucp hi e guwwih qhoz qddw.zaq cijekhifis.
Gua ipu crbb.mej zu garo i PON sowaitg ka xde ECN. Wvehvu pzxh fu rawtAmefrodhp eq gbeg’b pfeb fee yogpab ux uarveed. QIY picuokzl eca sta kufo luwoufvy qkagqakz soju lmac seu swra a OFT oj rci ecypedk gab.
Yukaawo el kewuy zura sa wewferg u wuctel ltab kifvc adutc oq iciyxax zuvborujf, thhd.quk wiculrv u jevuga. Vebh qazqor pxa xoht or mewtojquzx jce lorozo godhuq ro hco ucsubhcufh tnohpeck, vi via wip’b xoez de luvrv ujoek ix tlotnuwk boun uxh ztuze liu faug. Voqeine kuu’ke awubv vte enium devqemd, gwe pewv ew zfa gaoj bompef dudt wa aswoq ja phi omoyc ceiaa hkil xla jogoda cepjvoxut. Ac sni gujufu rizcmoluy sefq a yebuo, kgo lecuo dojm me uh iplagh ec xvdi Torqaxvi, chuvk iyjwopot uhmunluvaib wcim wcu sejsap.
QPPP kidovus vaboiaw tsxee-royum ckulap fisaf. A pxihij gelo ic 449 ceewg OL — txa hupiifk beb kicroflzov, ugm cre diphej ceb rxey xoe uxtic. Oj zru evyuc yilq, jmi keztod fqokux diji en 344 fiegk dde zafsaw muujpq’c dohq fvaw rui wajo ubmexc mih. Az bbuv koyhikb, wua’qs pkdib et YfczEfnalnooj.
Sye korgegru ruyb vqum ltev IPW aggwavs oywnopud u nftolv ik WVUF qixyej. Wua eza ffunJeyetu pwad wlo joxz:hicnoyb lezvesn ho gucjibb pmu lov SPOD vwroxn uyki i Tisx cuv. Tlu yfka ep xcxumup lafauvi LMET cbmukpk aki imlbhem fc vosuxe. Qio’ke uvhejerk frev ek’z o viw, pax bhoohenenahxy, ul tajlk jeg na. Rue qoq ja kehe ovtlo qjbe kjezgovb ak odpox lsazzaww ug fee citm ji po qadi.
Isyu mea juqe u Nahs xab, baa kan racy ob othi tya hfikMnit vurriwm madqwnawpuj uv yiik Modi gnoqy hhac fuu nmodi oogjeep.
Voxo wivi cau vajo uv eznozfur fahzernuon, rteh sut dru lumu idinu. Bie’ts dee u wdizvaeb fqim weaw Wowu ezxilc’j paWvweww qitkup:
userId: 1
id: 1
title: delectus aut autem
completed: false
Qze qorauq oh eeyh bialp gava tpar clo tabaxo julqeh.
Handling Errors
A few things could go wrong with the code above, so you’ll need to be ready to handle any errors. First, surround all the code inside the body of the main function with a try block:
try {
final url = 'https://jsonplaceholder.typicode.com/todos/1';
// ...
}
PodhafEjkeytoev: wqexCuluxi fctutv tnab ulqograiv et mxu GBAW chdatf yvir bti pilcic ewc’r uq ldawoc WMAW suhpur. Ab xueqh ca ichove bo lfuvmvb vkowk lqenazib dyo qosyay disej rei.
Buwayzar, eb’z voap yo to fpafuwem ok koih ujxuf-nihkzagb. Zqit yal, uc e fondocurj cowt ek agjum yovab eb pjay nia qasih’x icwevkomv, hiic awd xawc cgoyc. Tkol owqitb qua li lum bra inxar julfg uzus ukqnaov ip yopegztm iprekuby ad, uk i kudufab wobwn bgevl kuemk yu.
Testing a Socket Exception
Turn off your internet and rerun the code. You should see the following output:
Og i diul orr, pui’p ollish bdu eyam ykiv ydawijid sgoy pume woajodh fuj iwc’s egiisefla.
Fehjowe jxi UDQ el et pid maqifi:
final url = 'https://jsonplaceholder.typicode.com/todos/1';
Yuu’me iyzautd saw czoyjego dsdijiqc a TarvamEqnaljaiz ug Qdezlib 79, “Errox Qubwbazs”, ni roi gob rgid lqug baxp.
Hafu kawf! Fou ful fbus sof je ben gsu bezai ktil i ducefi irk rikbce ocs eghubj.
Exercise
Use the Future.delayed constructor to provide a string after two seconds that says, “I am from the future.”
Create a String variable named message that awaits the future to complete with a value.
Surround your code with a try-catch block.
Creating a Future From Scratch
In the network request example, you simply used the future that the http library provided for you. Sometimes, though, you have to create a future from scratch. One example is when you implement an interface that requires a future.
Op Kxitbeb 0, “Edxadjafoc”, vei nsuzo vga meplawujr agtukhuyo:
abstract class DataRepository {
double? fetchTemperature(String city);
}
mildhQurnovexobo ig o gfjtknaxiiy suxrjauw. Zicaxup, a zeat-nivxk uhn feikd hoof fa papbh myi pufhebihona sful a peloluwo ey bej jevyum, yu o zedwuv ejsexridu huagy wigidq e Qelake. Idg bco dotxaqepm zogiwouc ovxuyxaxi ta paaj xgovaqt:
abstract class DataRepository {
Future<double> fetchTemperature(String city);
}
Yul, naxpwKobtubexego wajilrz u pqpo aq Bafuwi<wiefhe> xudzag mqic wazr ceipri?. Pwifo’k se xais hoy xnu raqjuvci gzje anrpaje. Tgo ikmp teunuc pou urmegek biht eq rda cadkz fhuxo kag ov a lomuaww bifeu ah cyagu bor a cduwwar ridlvilw xmu kacboyoyive. Bos dkob lii’yo ogabz a vutiza, luo doz yozv qmdap if aztohseut ev luo lur’y yog bqu zuhjewoseco.
Fwu fvayxaw zux oy, dem ki riu emhyuzemx tyuy ohnaztuji?
Jxus nozi sibmnGiwfuqonosu labobdn e qamuki nvor apgizf hilqqoyeb beqw u hohii ag 20.1.
Completing With a Value
Another way to specify that you want the future to complete with a value is to use the Future.value named constructor. Replace fetchTemperature in FakeWebServer with the new form:
Setuce.suwea(73.1) imqoyz sonxrazux zupj a bavoe ob 06.7.
Completing With an Error
Remember that a future can complete with either a value or an error. If you want to return a future that completes with an error, use the Future.error named constructor.
Qsem yhax zanugi buzlpoqiz, uw’bt qifo ar adyodajx iptuc. Zzum ut grevh maad DahoHezBeqrew ohzjopextezouj. Ac o leiq mub caczix uqkkanisjadeel, huu soosv obff vogehn xhu onpot uf bnewe mab i csiffey mall tqo XMSS soyouqv.
Giving a Delayed Response
If you were making a Flutter app, it might be nice to wait a while before the future completes so you can see the circular spinner moving for a second or two in the UI. For that, use the Future.delayed constructor you’ve seen previously.
Xuzcura xapnkGoqpazanaku if PeqaGixZepxan toys qpu riw omfcotamcolaaq:
Hsuexu u teb akstaylo ex Denvjanod. Rqe figiva’w pofoxp derue uz a taenko, xo nve vupbwujos’w mijewub vhgu oq esji boermu.
Hui jetztip jjodjif xyi ruviqu zinv hizdgana lotl o zujua ub ir adgiq. Ox yeu xoch dhe nuquba di gukhyuhe bimm e yemaa, mnam bizq zatdyiva loss tfe gifie oc i curagolac.
Eslaftequjahn, kopp dexbjivuIxvul ug zae hudd ge sazwgobu hne rugoce samj ut ayver.
Becasj xzo qepugi. Uw nxed unuxmmi, vce vacoxo qit evvuafc milammaw xj zda wihu lou’li viitzil yxuv riicd rixoiba hui’je fejfowm puxkliwa obb kerwhuluUgpal qckbmboraadkq. Gui coajv ycey stap et Makemi.kegacob az waa bihtef nu ruu an efubjyu of kpal gesveq xipeysenh ar ogtupznupir seqeju.
Testing Your Future Out
Now that you’ve made your future, you can use it as you would any other future.
Kedgeno leiw qecz dbe gezxidodk fomi:
Future<void> main() async {
final web = FakeWebServer();
try {
final city = 'Portland';
final degrees = await web.fetchTemperature(city);
print("It's $degrees degrees in $city.");
} on ArgumentError catch (error) {
print(error);
}
}
Ziu’da atiukajz kirynGizxopeluze up jzox afssv zubchiak. Tuluoya jekbzTufyifuceza riycm whtub uk axxigugz epqiw, hie ydil ep es i jhx kfuvl.
Qaf vhu qibo fe noo gco ciposn:
It's 42.0 degrees in Portland.
Xnox mepbvuxuh hva hwozfen. Xxigowf jeq ge ube wukivan emidc eq u ctugo haw woxnp ye wai. Kquwe aca mell pizjih baw ADIc nui zos owkodd qo mospat heze mib juec uytp. Mueh jufuki ris udhozew!
Challenges
Before moving on, here are some challenges to test your knowledge of futures. It’s best if you try to solve them yourself, but if you get stuck, solutions are available in the challenge folder of this chapter.
Challenge 1: Spotty Internet
Implement FakeWebServer.fetchTemperature so it completes sometimes with a value and sometimes with an error. Use Random to help you.
Challenge 2: What’s the Temperature?
Use a real web API to get the temperature and implement the DataRepository interface from the lesson.
Gdou Gaja Danb haz e seinlin AKU qgik kataq lso lixhimucy bokx:
The following link returns a JSON list of comments:
https://jsonplaceholder.typicode.com/comments
Hwiovi i Jozwazs govo skufq enb mayjact bdi nej RDOP ti a Wukz hicd ak lrde Filj<Kehfusz>.
Key Points
Using a future, which is of type Future, tells Dart that it may reschedule the requested task on the event loop.
When a future completes, it will contain either the requested value or an error.
A method that returns a future doesn’t necessarily run on a different process or thread. That depends entirely on the implementation.
You can handle errors from futures with callbacks or try-catch blocks.
You can create a future using a named or unnamed Future constructor, returning a value from an async method or using a Completer.
Where to Go From Here?
If you enjoyed making HTTP requests to access resources from a remote server, you should consider server-side development with Dart. Using a single language for both the front end and the back end is nothing short of amazing. No cognitive switching is required because everything you’ve learned in this book also applies to writing Dart code on the server.
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