Advanced Swift Generics

Advanced Swift Generics
Перейдем на <T>
Max Sokolov - iOS dev @ Avito
masokolov@avito.ru
@max_sokolov

Agenda
• The problem
• Generics in Swift
• Protocols with associated types
• Declarative type-safe UIKit
• Generics in practise
2

🤔
class Matrix<T: Choiceable where T.T == T, T: Learnable, T: Fateable>

Generic code enables you to write ﬂexible, reusable functions
and types that can work with any type, subject to requirements
that you deﬁne. You can write code that avoids duplication and
expresses its intent in a clear, abstracted manner.
Apple docs
What for?
4

Why?
• One team of 5 people
• 3 different apps with shared components (mostly Swift)
• 20 private CocoaPods
• Large code base (~100k)
8

Typical api request
API Resource
/users
Raw Data
NSData?
Parsed Object
[String: String]?
Model
User?
Request Parsing
ObjectMapper
10

Implementation
@implementation DataProvider
- (void)fetch:(NSString *)resources
mappingClass:(Class)mappingClass
completion:(void(^)(NSArray<id<Mappable>> *results))completion {
// Load data...
// Parse data...
if ([mappingClass conformsToProtocol:@protocol(Mappable)]) {
id result = [mappingClass initWithDictionary:response];
completion(@[result]);
}
}
@end
11

The problem?
@implementation DataProvider
- (void)fetch:(NSString *)resources
mappingClass:(Class)mappingClass
completion:(void(^)(NSArray<id<Mappable>> *results))completion {
// Load data...
// Parse data...
if ([mappingClass conformsToProtocol:@protocol(Mappable)]) {
id result = [mappingClass initWithDictionary:response];
completion(@[result]);
}
}
@end
12

What run time means?
DataProvider *provider = [DataProvider new];
[provider fetch:@"/users"
mappingClass:[NSString class] // compiler doesn’t warn you here...
completion:^(NSArray<User *> *results) {
}];
14

You are doing it wrong with Swift
let dataProvider = DataProvider()
dataProvider.fetch("/resource", mappingClass: User.self) { result in
// code smell...
guard let users = result.values as? [User] else {
return
}
}
15

Next generation table views
UITableView powered by generics

90% of our screens are table views

The problem with UITableView
• Boilerplate
• Painful when data sources are complex and dynamic
• Not type safe (dequeueReusableCellWithIdentiﬁer etc.)
22

What table actually needs?
Model ViewModel
UITableViewCell
Deadline ;)
23

Line of code
let rowBuilder = TableRowBuilder<String, MyTableViewCell>(items: ["1", "2", "3"])
26

Tablet.swift
Open Source
https://github.com/maxsokolov/Tablet.swift
🚀

Tablet.swift
import Tablet
let row = TableRowBuilder<String, MyTableViewCell>(items: ["1", "2", "3"])
.action(.click) { (data) in
}
.action(.willDisplay) { (data) in
}
let section = TableSectionBuilder(headerView: nil, footerView: nil, rows: [row])
tableDirector += section
29

NibLoadable
class MyView: UIView, NibLoadable {
}
31
let view = MyView.nibView() // view is MyView

NibLoadable / generic protocol
protocol NibLoadable {
associatedtype T: UIView
static func nibView() -> T?
}
32
extension NibLoadable where Self: UIView {
static func nibView() -> Self? {
return NSBundle(forClass: self)
.loadNibNamed(String(self), owner: nil, options: nil)
.first as? Self
}
}

NibLoadable / generic protocol
associatedtype T: UIView
static func nibView() -> T?
}
33
return NSBundle(forClass: self)
.loadNibNamed(String(self), owner: nil, options: nil)
.first as? Self
}
}

NibLoadable / generic func
static func nibView<T: UIView>(viewType type: T.Type) -> T?
}
34
static func nibView<T: UIView>(viewType type: T.Type) -> T? {
return NSBundle.mainBundle()
.loadNibNamed(String(type), owner: nil, options: nil)
.first as? T
}
return nibView(viewType: self)
}
}

NibLoadable / generic func
static func nibView<T: UIView>(viewType type: T.Type) -> T?
}
35
static func nibView<T: UIView>(viewType type: T.Type) -> T? {
return NSBundle.mainBundle()
.loadNibNamed(String(type), owner: nil, options: nil)
.first as! T
}
return nibView(viewType: self)
}
}

Resume
• Currently, SDK/API’s are not generic
• Compiler is not perfect
• Can’t be used with Objective-C
• Generics help to know about potential issues earlier
• Compile-time type-safety makes your code better
36

Links
37
Covariance and Contravariance
https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html
Protocols with Associated Types, and How They Got That Way
http://www.slideshare.net/alexis_gallagher/protocols-with-associated-types-and-how-they-got-that-way
Why Associated Types?
http://www.russbishop.net/swift-why-associated-types
Tablet.swift
https://github.com/maxsokolov/Tablet.swift

QUESTIONS?
Max Sokolov - iOS dev @ Avito
@max_sokolov
https://github.com/maxsokolov

Advanced Swift Generics

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