Writing your App Swiftly with Patterns

Writing your App
Swiftly
Sommer Panage
Chorus Fitness
@sommer

Today, in 4 short tales
• Schrödinger's Result
• The Little Layout Engine that Could
• Swiftilocks and the Three View States
• Pete and the Repeated Code

Code in a box
func getFilms(completion: @escaping ([Film]?, APIError?) -> Void) {
let url = SWAPI.baseURL.appendingPathComponent(Endpoint.films.rawValue)
let task = self.session.dataTask(with: url) { (data, response, error) in
if let data = data {
do {
let jsonObject = try JSONSerialization.jsonObject(with: data, options: [])
if let films = SWAPI.decodeFilms(jsonObject: jsonObject) {
completion(films, nil)
} else {
completion(nil, .decoding)
}
} catch {
completion(nil, .server(originalError: error))
}
} else {
completion(nil, .server(originalError: error!))
}
}
task.resume()
}

What's actually
happening…
override func viewDidLoad() {
super.viewDidLoad()
apiClient.getFilms() { films, error in
if let films = films {
// Show film UI
if let error = error {
// Log warning...this is weird
}
} else if let error = error {
// Show error UI
} else {
// No results at all? Show error UI I guess?
}
}
}

Result open source framework by Rob Rix
Model our server interaction as it actually is - success / failure!
public enum Result<T, Error: Swift.Error>: ResultProtocol {
case success(T)
case failure(Error)
}

New, improved code
func getFilms(completion: @escaping (Result<[Film], APIError>) -> Void) {
let task = self.session
.dataTask(with: SWAPI.baseURL.appendingPathComponent(Endpoint.films.rawValue)) { (data, response, error) in
let result = Result(data, failWith: APIError.server(originalError: error!))
.flatMap { data in
Result<Any, AnyError>(attempt: { try JSONSerialization.jsonObject(with: data, options: []) })
.mapError { _ in APIError.decoding }
}
.flatMap { Result(SWAPI.decodeFilms(jsonObject: $0), failWith: .decoding) }
completion(result)
}
task.resume()
}

New, improved code
super.viewDidLoad()
apiClient.getFilms() { result in
switch result {
case .success(let films): print(films) // Show my UI!
case .failure(let error): print(error) // Show some error UI
}
}
}

The Moral of the Story
Using the Result enum allowed us to
• Model the sucess/failure of our server interaction more
correctly
• Thus simplify our view controller code.

The Little Layout Engine that
Could

Old-school
override func layoutSubviews() {
super.layoutSubviews()
// WHY AM I DOING THIS?!?!
}

What about Storyboards and Xibs?
• Working in teams becomes harder because...
• XML diffs
• Merge conﬂicts?!
• No constants
• Stringly typed identiﬁers
• Fragile connections

Autolayout: iOS 9+ APIs
init() {
super.init(frame: .zero)
addSubview(tableView)
// Autolayout: Table same size as parent
tableView.translatesAutoresizingMaskIntoConstraints = false
tableView.topAnchor.constraint(equalTo: self.topAnchor).isActive = true
tableView.bottomAnchor.constraint(equalTo: self.bottomAnchor).isActive = true
tableView.leadingAnchor.constraint(equalTo: self.leadingAnchor).isActive = true
tableView.trailingAnchor.constraint(equalTo: self.trailingAnchor).isActive = true
}

Autolayout: Cartography by Robb Böhnke
init() {
super.init(frame: .zero)
addSubview(tableView)
// Autolayout: Table same size as parent
constrain(tableView, self) { table, parent in
table.edges == parent.edges
}
}

More Cartography
private let margin: CGFloat = 16
private let episodeLeftPadding: CGFloat = 8
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
contentView.addSubview(episodeLabel)
contentView.addSubview(titleLabel)
constrain(episodeLabel, titleLabel, contentView) { episode, title, parent in
episode.leading == parent.leading + margin
episode.top == parent.top + margin
episode.bottom == parent.bottom - margin
title.leading == episode.trailing + episodeLeftPadding
title.trailing <= parent.trailing - margin
title.centerY == episode.centerY
}
}

Using the Cartography framework harnesses Swift's
operator overloads to make programatic AutoLayout a
breeze!

Swiftilocks and the
Three View States

Swiftilocks and the Three View
States
LOADING

States
SUCCESS

States
ERROR

State management with bools
/// MainView.swift
var isLoading: Bool = false {
didSet {
errorView.isHidden = true
loadingView.isHidden = !isLoading
}
}
var isError: Bool = false {
didSet {
errorView.isHidden = !isError
loadingView.isHidden = true
}
}
var items: [MovieItem]? {
didSet {
tableView.reloadData()
}
}

/// MainViewController.swift
super.viewDidLoad()
title = "Star Wars Films"
mainView.isLoading = true
DispatchQueue.main.async {
switch result {
case .success(let films):
self.mainView.items = films
.map { MovieItem(episodeID: $0.episodeID, title: $0.title) }
.sorted { $0.0.episodeID < $0.1.episodeID }
self.mainView.isLoading = false
self.mainView.isError = false
case .failure(let error):
self.mainView.isLoading = false
self.mainView.isError = true
}
}
}
}

Enums to the rescue!
final class MainView: UIView {
enum State {
case loading
case loaded(items: [MovieItem])
case error(message: String)
}
init(state: State) { ... }
// the rest of my class...
}

var state: State {
didSet {
switch state {
case .loading:
items = nil
loadingView.isHidden = false
case .error(let message):
print(message)
items = nil
errorView.isHidden = false
case .loaded(let movieItems):
items = movieItems
}
}
}

super.viewDidLoad()
title = "Star Wars Films"
mainView.state = .loading
DispatchQueue.main.async {
switch result {
case .success(let films):
let items = films
.map { MovieItem(episodeID: $0.episodeID, title: $0.title) }
.sorted { $0.0.episodeID < $0.1.episodeID }
self.mainView.state = .loaded(items: items)
case .failure(let error):
self.mainView.state = .error(message: "Error: (error.localizedDescription)")
}
}
}
}

Modelling our view state with an enum with
associated values allows us to:
1. Simplify our VC
2. Avoid ambiguous state
3. Centralize our logic

Repeated code
var state: State {
didSet {
switch state {
case .loading:
text = nil
case .error(let message):
print(message)
text = nil
case .loaded(let text):
text = text
}
}
}

Protocols save the day!!
• A shared interface of methods and properties
• Addresses a particular task
• Types adopting protocol need not be related

protocol DataLoading {
associatedtype Data
var state: ViewState<Data> { get set }
var loadingView: LoadingView { get }
var errorView: ErrorView { get }
func update()
}

enum ViewState<Content> {
case loading
case loaded(data: Content)
case error(message: String)
}

Default protocol implementation
extension DataLoading where Self: UIView {
func update() {
switch state {
case .loading:
case .error(let error):
Log.error(error)
case .loaded:
}
}
}

Conforming to DataLoading
1. Provide an errorView variable
2. Provide an loadingView variable
3. Provide a state variable that take some sort of Data
4. Call update() whenever needed

DataLoading in our Main View
final class MainView: UIView, DataLoading {
let loadingView = LoadingView()
let errorView = ErrorView()
var state: ViewState<[MovieItem]> {
didSet {
update() // call update whenever we set our list of Movies
}
}

DataLoading in our Crawl View
class CrawlView: UIView, DataLoading {
let loadingView = LoadingView()
let errorView = ErrorView()
var state: ViewState<String> {
didSet {
update()
crawlLabel.text = state.data
}
}

Decomposing functionality that is shared by non-
related objects into a protocol helps us
• Avoid duplicated code
• Consolidate our logic into one place

Conclusion
• Result: easily differentiate our success/error pathways
• Cartography: use operator overloading to make code more
readable
• ViewState enum: never have an ambigous view state!
• Protocols: deﬁne/decompose shared behaviors in unrelated
types

THANK YOU
Contact Me:
@sommer on Twitter
me@sommerpanage.com

Writing your App Swiftly with Patterns

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