Protocol-Oriented Programming in iOS: the Correct Way to Use Swift by JC Velasquez

Protocol-Oriented
Programming
The *correct* way to use Swift

Protocols in Swift
• Mostly the same with @protocols in Objective-C
• Protocols in Swift can be extended like a class extension in
Obj-C (protocol extension)
• Protocol extensions can have implementation code
• Made more powerful by generics in Swift

Advantages
• You solve problems related to inheritance
• You get rid of implicit sharing
• You end up with more readable code

Isn’t inheritance a good
thing?

Composition vs Inheritance Principle
class Dog {
func bark() {
print("Bark!")
}
}
class GermanShephard: Dog {
}
let myDog = GermanShephard()
myDog.bark() // prints "Bark!"
Taken from http://mikebuss.com/2016/01/10/interfaces-vs-inheritance/

class Dog {
func bark() {
print("Bark!")
}
}
class GermanShephard: Dog {
func sniffDrugs() {
if drugs {
bark()
}
}
}
class BelgianMalinois: Dog {
func sniffDrugs() {
if drugs {
bark()
}
}
}
class Poodle: Dog {
}

class Dog {
func bark() {
print("Bark!")
}
}
class DrugSniffingDog: Dog {
func sniffDrugs() {
if drugs {
bark()
}
}
}
class GermanShephard: DrugSniffingDog {
}
class BelgianMalinois: DrugSniffingDog {
}
class Poodle: Dog {
}

protocol Barker {
func bark()
}
protocol Swimmer {
func swim()
}
protocol DrugSniffer {
func sniffDrugs()
}
extension Barker {
func bark() {
print("Bark!")
}
}
extension Swimmer {
func swim() {
print("Splash!")
}
}
extension DrugSniffer {
func sniffDrugs() {
print("I found drugs!");
}
}
class GermanShephard: Barker, DrugSniffer {
}
class BelgianMalinois: Barker, Swimmer, DrugSniffer {
}
class Poodle: Barker, Swimmer {
}
let myDog = BelgianMalinois()
myDog.bark() // prints "Bark!"
myDog.swim() // prints "Splash!"
myDog.sniffDrugs() // prints "I found drugs!"

Bad sharing
class Temperature {
var celsius: Double = 0
var fahrenheit: Double {
get { return celsius * 9 / 5 + 32 }
set { celsius = (newValue - 32) * 5 / 9 }
}
}
Taken from https://developer.apple.com/videos/play/wwdc2015/414/

Bad sharing
let home = House()
let temp = Temperature()
temp.fahrenheit = 75
home.thermostat.temperature = temp
home.oven.temperature = temp
home.oven.bake()

Bad sharing
House
Oven
Thermostat
Temperature

Bad sharing
struct Temperature: Equatable {
var celsius: Double = 0
var fahrenheit: Double {
get { return celsius * 9 / 5 + 32 }
set { celsius = (newValue - 32) * 5 / 9 }
}
}
func ==(lhs: Temperature, rhs: Temperature) -> Bool {
return lhs.celsius == rhs.celsius
}

Bad sharing
let home = House()
var temp = Temperature()
home.thermostat.temperature = temp
home.oven.temperature = temp
home.oven.bake()

Bad sharing
House
Oven
Thermostat
Temperature
Temperature

–Heraclitus (Panta Rhei “Everything Flows”)
“Ever-newer waters ﬂow on those who
step into the same rivers.”

Protocol-Oriented Programming in iOS: the Correct Way to Use Swift by JC Velasquez

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