The Principles of OOD in Kotlin 1.3.61

S.O.L.I.D.

• The Single Responsibility Principle
• The Open Closed Principle
• The Liskov Substitution Principle
• The Interface Segregation Principle
• The Dependency Inversion Principle

🔐 The Single Responsibility Principle

A class should have one, and only one, reason to change. (read more)

Example:

interface CanBeOpened {
    fun open()
}

interface CanBeClosed {
    fun closed()
}

// I'm the door. I have an encapsulated state and you can change it using methods.
class PodBayDoor : CanBeOpened, CanBeClosed {

    private enum class State {
        Open, Closed
    }

    private var state: State = State.Closed

    override fun open() {
        state = State.Open
    }

    override fun closed() {
        state = State.Closed
    }
}

// I'm only responsible for opening, no idea what's inside or how to close.
class DoorOpener(private val door: CanBeOpened) {

    fun execute() {
        door.open()
    }

}

// I'm only responsible for closing, no idea what's inside or how to open.
class DoorCloser(private val door: CanBeClosed) {

    fun execute() {
        door.closed()
    }

}

val door = PodBayDoor()

⚠ Only the DoorOpener is responsible for opening the door.

val doorOpener = DoorOpener(door)
doorOpener.execute()

⚠ If another operation should be made upon closing the door, like switching on the alarm, you don't have to change the DoorOpener class.

val doorCloser = DoorCloser(door)
doorCloser.execute()

✋ The Open Closed Principle

You should be able to extend a classes behavior, without modifying it. (read more)

Example:

interface CanShoot {
    fun shoot(): String
}

// I'm a laser beam. I can shoot.
class LaserBeam : CanShoot {
    override fun shoot(): String {
        return "Ziiiip!"
    }
}

// I have weapons and trust me I can fire them all at once. Boom! Boom! Boom!
class WeaponsComposite(var weapons: Array<CanShoot>) {
    fun shoot(): String {
        return weapons.map { it.shoot() }[0]
    }
}

val laser = LaserBeam()
var weapons = WeaponsComposite(weapons = arrayOf(laser))

weapons.shoot()

I'm a rocket launcher. I can shoot a rocket.

⚠️ To add rocket launcher support I don't need to change anything in existing classes.

class RocketLauncher : CanShoot {
    override fun shoot(): String {
        return "Whoosh!"
    }
}


val rocket = RocketLauncher()

weapons = WeaponsComposite(weapons = arrayOf(laser, rocket))
weapons.shoot()

👥 The Liskov Substitution Principle

Derived classes must be substitutable for their base classes. (read more)

Example:

private interface Bird {
    fun eat() {
        println("Eat!")
    }
}

private interface FlightBird : Bird {
    fun fly() {
        println("Fly!")
    }
}

private interface NonFlightBird : Bird {

}

private class Crow : FlightBird {

    override fun fly() {
        super.fly()

        println("It is Crow - it can fly")
    }

    override fun eat() {
        super.eat()
    }
}

private class Ostrich : NonFlightBird {
    override fun eat() {
        super.eat()
        println("It is Ostrich - it can eat but it can't fly")
    }
}

🍴 The Interface Segregation Principle

Make fine grained interfaces that are client specific. (read more)

Example:

interface LandingSiteHaving {
    var landingSite: String
}

interface Landing {
    fun landOn(on: LandingSiteHaving): String
}

interface PayloadHaving {
    var payload: String
}

class InternationalSpaceStation {

    fun fetchPayload(vehicle: PayloadHaving): String {
        return "Deployed ${vehicle.payload} at April 10, 2016, 11:23 UTC"
    }
}

class OfCourseIStillLoveYouBarge : LandingSiteHaving {
    override var landingSite = "a barge on the Atlantic Ocean"
}

class SpaceXCRS8 : Landing, PayloadHaving {

    override var payload = "BEAM and some Cube Sats"

    override fun landOn(on: LandingSiteHaving): String {
        return "Landed on ${on.landingSite} at April 8, 2016 20:52 UTC"
    }
}

val crs8 = SpaceXCRS8()
val barge = OfCourseIStillLoveYouBarge()
val spaceStation = InternationalSpaceStation()

spaceStation.fetchPayload(crs8)
crs8.landOn(barge)

🔩 The Dependency Inversion Principle

Depend on abstractions, not on concretions. (read more)

Example:

interface CoffeeMachine {
    fun brew(coffee:Coffee)
}

interface Coffee

class Arabica : Coffee


class Rabusta : Coffee

class BrewMachine : CoffeeMachine {
    override fun brew(coffee: Coffee) {
        println("Brew: $coffee")
    }
}

val brewMachine = BrewMachine()
brewMachine.brew(Arabica())
brewMachine.brew(Rabusta())

Info

📖 Descriptions from: ochococo/OOD-Principles-In-Swift Thank's