Premise: Promises

A presentation at Motor City Cocoaheads in April 2018 in Detroit, MI, USA by Anne Cahalan

Slide 1

PREMISE: PROMISES

Slide 2

! HI, I'M ANNE.

Slide 3

WHY PROMISES?

Slide 4

BRACE YOURSELF, I'M GOING TO SAY SOMETHING NICE ABOUT JAVASCRIPT

Slide 5

FIRST STOP IS ALWAYS WIKIPEDIA

First used in 1976-77

Slide 6

BARBARA LISKOV IS KINDA RAD

First used in 1976-77

Pipelining/Chaining invented in 1988

Slide 7

Y2K REVIVAL

First used in 1976-77

Pipelining/Chaining invented in 1988

Resurgence of interest in 2000

Slide 8

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

Slide 9

SO I MADE AN APP

Slide 10

I HIT UP THE REST COUNTRIES API TO GET A LIST OF ALL THE COUNTRIES IN THE WORLD...

Slide 11

THEN I FOUND A WEATHER API AND A CURRENCY EXCHANGE API TO PLAY WITH

Slide 12

COUNTRYLIST VIEW CONTROLLER

Slide 13

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() }

Slide 14

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() } } }

Slide 15

SELECTED COUNTRY VIEW CONTROLLER

Slide 16

func fetchCurrentExchangeRate (currencyCode: String, handler: @escaping (ExchangeRate?) -> ()) {

guard

let currencyURL = URL (string: currencyConversionBaseURLString + currencyAccessKey + "&currencies=(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

let exchangeRate: ExchangeRate

self .decodeExchangeRateData(currencyData: responseData) else {

print ( " ! decoding error" )

return } handler(exchangeRate) }) task.resume() }

Slide 17

↩ SIDE TRIP! THIS IS WHY YOU GO TO MEETUPS!

Slide 18

FUN WITH CODABLE Botswana Pula Danish Krone Polish Z ł oty "USDBWP":9.591896 "USDDKK":6.04103 "USDPLN":3.388799

Slide 19

FUN WITH CODABLE struct Quote: Codable {

var conversion: String

var rate: Float

0.0 }

Slide 20

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) } } }

Slide 21

SELECTED COUNTRY VIEW CONTROLLER

Slide 22

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 } } } }

Slide 23

! SOME CONCERNS

Two separate network calls

That could end at two different times

A third network call that depends on one of the others

Slide 24

LET'S TRY PROMISES

Slide 25

RETURN A PROMISE OF A TYPE func fetchAllCountries (handler: @escaping ([Country]?) -> ()) vs func promiseFetchAllCountries () -> Promise <[ Country ]>

Slide 26

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

let responseData = data {

let allCountries = self .decodeAllCountries(countryData: responseData) seal.fulfill(allCountries) } else

let requestError = error { seal.reject(requestError) } } task.resume() } }

Slide 27

↪ SIDE TRIP! THE MARCH OF PROGRESS VS. THE INTERNET IS FOREVER

Slide 28

PROMISEKIT 6.0 INCLUDED A MAJOR CHANGE IN THE PROMISE INITIALIZER

Slide 29

FROM Promise { fulfill, reject in

//… } TO Promise { seal in

// ... }

Slide 30

COUNTRYLIST VIEW CONTROLLER

Slide 31

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)" ) } }

Slide 32

! ENTER THE CIRCLE OF SHARING !

Slide 33

handler IS GARBAGE

Slide 34

I MEAN, LOOK AT THIS: func fetchAllCountries (handler: @escaping ([Country]?) -> ())

Slide 35

LOOK AT THIS MESS: func fetchAllCountries (handler: @escaping ([Country]?) -> ())

is this clean code?

Slide 36

AND WHAT ABOUT THIS: func duckBusiness () { doAThing { quackLikeADuck() } } func doAThing (handler: () -> ()) { doSomeStuff() doAnotherThing { handler() } } func doAnotherThing (handler: () ->()) {
doSomeMoreThings() handler() }

Slide 37

UGH. IT'S THE WORST: func fetchAllCountries (handler: @escaping ([Country]?) -> ())

is this clean code?

chaining is impossible

fuckingblocksyntax.com and

fuckingclosuresyntax.com

Slide 38

! THANK YOU FOR SHARING !

Slide 39

! PROMISES HANDLE handler

Slide 40

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)" ) }

Slide 41

THAT WAS EASY MODE. WHAT ABOUT THE HARD STUFF?

Slide 42

func promiseFetchCurrentExchangeRate (currencyCode: String) -> Promise < ExchangeRate

{ ... } func promiseFetchCapitalCityWeather (country: Country) -> Promise < Weather { ... } func promiseFetchWeatherIcon (iconCode: String) -> Promise < UIImage { ... }

Slide 43

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() }

Slide 44

WHEN when(fulfilled: networker.promiseFetchCurrentExchangeRate(currencyCode: currencyCode), networker.promiseFetchCapitalCityWeather(country: country) )

Slide 45

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 }

Slide 46

CATCH . catch { error in

print ( " ! some kind of error in getting the data for (String(describing: country.name)) -> (error)" ) }

Slide 47

FINALLY .finally {

self .setupExchangeRateUI()

self .setupWeatherUI()

self .activityIndicator.stopAnimating() }

Slide 48

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() }

Slide 49

WAIT A SECOND... self .networker.promiseFetchWeatherIcon(iconCode: iconCode) .done { weatherImage in

self .weatherIconImageView.image = weatherImage }

Slide 50

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() }