A presentation at Motor City Cocoaheads in in Detroit, MI, USA by Anne Cahalan
PREMISE: PROMISES
! HI, I'M ANNE.
WHY PROMISES?
BRACE YOURSELF, I'M GOING TO SAY SOMETHING NICE ABOUT JAVASCRIPT
FIRST STOP IS ALWAYS WIKIPEDIA
First used in 1976-77
BARBARA LISKOV IS KINDA RAD
First used in 1976-77
Pipelining/Chaining invented in 1988
Y2K REVIVAL
First used in 1976-77
Pipelining/Chaining invented in 1988
Resurgence of interest in 2000
WHAT IS A PROMISE AND WHY WOULD I WANT ONE?
represents the the eventual result of an asynchronous task
or the error if that task fails
a way of formalizing completion handlers to chain asynchronous tasks
SO I MADE AN APP
I HIT UP THE REST COUNTRIES API TO GET A LIST OF ALL THE COUNTRIES IN THE WORLD...
THEN I FOUND A WEATHER API AND A CURRENCY EXCHANGE API TO PLAY WITH
COUNTRYLIST VIEW CONTROLLER
func fetchAllCountries (handler: @escaping ([Country]?) -> ()) {
guard
let url = URL (string: allCountriesURLString) else { return }
let urlRequest = URLRequest (url: url)
let session = URLSession .shared
let task = session.dataTask(with: urlRequest, completionHandler: { data, response, error in
guard error == nil
else {
print ( " ! request error" )
return }
guard
let responseData = data else {
print ( " ! data response error" )
return }
let countryArray: [ Country ] = self .decodeAllCountries(countryData: responseData) handler(countryArray) }) task.resume() }
override
func viewDidLoad () {
super .viewDidLoad()
let networker = Networker () networker.fetchAllCountries { countries in
guard
let list = countries else { return }
self .countryList = list
DispatchQueue .main.async {
self .tableView.reloadData() } } }
SELECTED COUNTRY VIEW CONTROLLER
func fetchCurrentExchangeRate (currencyCode: String, handler: @escaping (ExchangeRate?) -> ()) {
guard
let currencyURL = URL (string: currencyConversionBaseURLString + currencyAccessKey + "¤cies=(currencyCode)&format=1" ) else {
print ( " ! currency url error" )
return }
let urlRequest = URLRequest (url: currencyURL)
let session = URLSession .shared
let task = session.dataTask(with: urlRequest, completionHandler: { data, response, error in
guard error == nil
else {
print ( " ! request error: (String(describing: error))" )
return }
guard
let responseData = data else {
print ( " ! data response error" )
return }
guard
self .decodeExchangeRateData(currencyData: responseData) else {
print ( " ! decoding error" )
return } handler(exchangeRate) }) task.resume() }
↩ SIDE TRIP! THIS IS WHY YOU GO TO MEETUPS!
FUN WITH CODABLE Botswana Pula Danish Krone Polish Z ł oty "USDBWP":9.591896 "USDDKK":6.04103 "USDPLN":3.388799
FUN WITH CODABLE struct Quote: Codable {
""
0.0 }
FUN WITH CODABLE extension Quote {
struct QuoteKeys: CodingKey {
var stringValue: String
var intValue: Int ?
init ?(stringValue: String ) {
self .stringValue = stringValue }
init ?(intValue: Int ) {
return
nil } }
public
init (from decoder: Decoder ) throws {
let container = try decoder.container(keyedBy: QuoteKeys . self )
for key in container.allKeys {
self .conversion = key.stringValue
self .rate = try container.decode( Float . self , forKey: key) } } }
SELECTED COUNTRY VIEW CONTROLLER
private
func vanillaNetworkingGetTheStuff () {
// guard some optional business networker.fetchCurrentExchangeRate(currencyCode: currencyCode) { rate in
self .exchangeRate = rate
DispatchQueue .main.async {
self .setupExchangeRateUI() } } networker.fetchCapitalCityWeather(country: country) { weather in
self .weather = weather
DispatchQueue .main.async {
self .setupWeatherUI() }
guard
let iconCode = self .weather?.conditions.first?.iconCode else {
print ( " ! error unwrapping icon code" )
return }
self .networker.fetchWeatherIcon(iconCode: iconCode) { weatherImage in
DispatchQueue .main.async {
self .weatherIconImageView.image = weatherImage } } } }
! SOME CONCERNS
Two separate network calls
That could end at two different times
A third network call that depends on one of the others
LET'S TRY PROMISES
RETURN A PROMISE OF A TYPE func fetchAllCountries (handler: @escaping ([Country]?) -> ()) vs func promiseFetchAllCountries () -> Promise <[ Country ]>
FULFILL OR REJECT func promiseFetchAllCountries () -> Promise <[ Country ]> {
// some url and session business
return
Promise { seal in
let task = session.dataTask(with: urlRequest) { data, _ , error in
if
let responseData = data {
let allCountries = self .decodeAllCountries(countryData: responseData) seal.fulfill(allCountries) } else
if
let requestError = error { seal.reject(requestError) } } task.resume() } }
↪ SIDE TRIP! THE MARCH OF PROGRESS VS. THE INTERNET IS FOREVER
PROMISEKIT 6.0 INCLUDED A MAJOR CHANGE IN THE PROMISE INITIALIZER
FROM Promise { fulfill, reject in
//… } TO Promise { seal in
// ... }
COUNTRYLIST VIEW CONTROLLER
override
func viewDidLoad () {
super .viewDidLoad()
let networker = Networker () firstly { networker.promiseFetchAllCountries() }.done { countryArray in
self .countryList = countryArray
self .tableView.reloadData() }. catch { error in
print ( " ! some kind of error listing all countries -> (error)" ) } }
! ENTER THE CIRCLE OF SHARING !
handler IS GARBAGE
I MEAN, LOOK AT THIS: func fetchAllCountries (handler: @escaping ([Country]?) -> ())
LOOK AT THIS MESS: func fetchAllCountries (handler: @escaping ([Country]?) -> ())
is this clean code?
AND WHAT ABOUT THIS:
func
duckBusiness
()
{
doAThing {
quackLikeADuck()
}
}
func
doAThing
(handler: ()
-> ()) {
doSomeStuff()
doAnotherThing {
handler()
}
}
func
doAnotherThing
(handler: ()
->()) {
doSomeMoreThings()
handler()
}
UGH. IT'S THE WORST: func fetchAllCountries (handler: @escaping ([Country]?) -> ())
is this clean code?
chaining is impossible
fuckingblocksyntax.com and
fuckingclosuresyntax.com
! THANK YOU FOR SHARING !
! PROMISES HANDLE handler
CHECK THIS OUT firstly { networker.promiseFetchAllCountries() }.done { countryArray in
self .countryList = countryArray
self .tableView.reloadData() }. catch { error in
print ( " ! some kind of error listing all countries -> (error)" ) }
THAT WAS EASY MODE. WHAT ABOUT THE HARD STUFF?
func promiseFetchCurrentExchangeRate (currencyCode: String) -> Promise < ExchangeRate
{ ... } func promiseFetchCapitalCityWeather (country: Country) -> Promise < Weather { ... } func promiseFetchWeatherIcon (iconCode: String) -> Promise < UIImage { ... }
firstly { when(fulfilled: networker.promiseFetchCurrentExchangeRate(currencyCode: currencyCode), networker.promiseFetchCapitalCityWeather(country: country)) }.done { exchangeRate, weather in
self .exchangeRate = exchangeRate
self .weather = weather
guard
let iconCode = weather.conditions.first?.iconCode else { return }
self .networker.promiseFetchWeatherIcon(iconCode: iconCode).done { weatherImage in
self .weatherIconImageView.image = weatherImage } }. catch { error in
print ( " ! error in getting the data for (String(describing: country.name)) -> (error)" ) }.finally {
self .setupExchangeRateUI()
self .setupWeatherUI()
self .activityIndicator.stopAnimating() }
WHEN when(fulfilled: networker.promiseFetchCurrentExchangeRate(currencyCode: currencyCode), networker.promiseFetchCapitalCityWeather(country: country) )
DONE .done { exchangeRate, weather in
self .exchangeRate = exchangeRate
self .weather = weather
guard
let iconCode = weather.conditions.first?.iconCode else { return }
self .networker.promiseFetchWeatherIcon(iconCode: iconCode).done { weatherImage in
self .weatherIconImageView.image = weatherImage }
CATCH . catch { error in
print ( " ! some kind of error in getting the data for (String(describing: country.name)) -> (error)" ) }
FINALLY .finally {
self .setupExchangeRateUI()
self .setupWeatherUI()
self .activityIndicator.stopAnimating() }
firstly { when(fulfilled: networker.promiseFetchCurrentExchangeRate(currencyCode: currencyCode), networker.promiseFetchCapitalCityWeather(country: country)) }.done { exchangeRate, weather in
self .exchangeRate = exchangeRate
self .weather = weather
guard
let iconCode = weather.conditions.first?.iconCode else { return }
self .networker.promiseFetchWeatherIcon(iconCode: iconCode).done { weatherImage in
self .weatherIconImageView.image = weatherImage } }. catch { error in
print ( " ! error in getting the data for (String(describing: country.name)) -> (error)" ) }.finally {
self .setupExchangeRateUI()
self .setupWeatherUI()
self .activityIndicator.stopAnimating() }
WAIT A SECOND... self .networker.promiseFetchWeatherIcon(iconCode: iconCode) .done { weatherImage in
self .weatherIconImageView.image = weatherImage }
firstly {
when(fulfilled:
networker.promiseFetchCurrentExchangeRate(currencyCode: currencyCode),
networker.promiseFetchCapitalCityWeather(country: country))
}.then { exchangeRate, weather
in
self .exchangeRate = exchangeRate
self .weather = weather
guard
let iconCode = weather.conditions.first?.iconcode else { return } networker.promiseFetchWeatherIcon(iconCode: iconCode) }.done { weatherImage in
self .weatherIconImageView.image = weatherImage }. catch { error in
print ( " ! error in getting the data for (String(describing: country.name)) -> (error)" ) }.finally {
self .setupExchangeRateUI()
self .setupWeatherUI()
self .activityIndicator.stopAnimating() }
❗ Ambigious reference to member 'firstly(execute:)'
!
➡
CHANGE YOUR TOOLS, CHANGE YOUR MIND
PROS & CONS
! SEEMS LIKE A LOT OF OVERHEAD FOR A SMALL PROJECT
! SEEMS LIKE A LOT OF OVERHEAD FOR A SMALL PROJECT ! SYNTAX IS CLEARLY MORE READABLE
! SEEMS LIKE A LOT OF OVERHEAD FOR A SMALL PROJECT ! SYNTAX IS CLEARLY MORE READABLE ! NEW(ISH) IDEA AROUND AN OLD PROBLEM
! SEEMS LIKE A LOT OF OVERHEAD FOR A SMALL PROJECT ! SYNTAX IS CLEARLY MORE READABLE ! NEW(ISH) IDEA AROUND AN OLD PROBLEM ! PROMISEKIT IS...OKAY
THE OTHER OPTIONS
Google Promises
THE OTHER OPTIONS
Google Promises
BrightFutures
THE OTHER OPTIONS
Google Promises
BrightFutures
Hydra
IF I HAD TO DO IT ALL OVER AGAIN...
QUESTIONS ❓
THANKS! ! @northofnormal ✉ northofnormal
github.com/northofnormal/PromisesPromises
The promise construct has been around since the late 70‘s, and iOS has had libraries for promises since at least 2015. Still, it seems like Swift has been slower to adopt promises than other languages. Promises can make tangled networking code more readable and can simplify complicated API business. Let’s take a look at how a tangle of calls to separate API’s, some using information dependent on previous calls, can be simplified into a series of promises fulfilled or rejected. We’ll investigate the pros and cons of various promise frameworks for Swift and of the promise pattern itself.
for free. You
can too.