Kotlin

Class

Open

By default Kotlin classes are final, meaning no classes can inherit from them. In order to make them not final declare them open

open class Vehicle() {}

Override

To override class functions they must also be declared open

open class Vehicle(val make: String, val model: String) {
    open fun drive() {} 
}

class Car(make: String, model: String, val seats: Int) : Vehicle(make, model) {
    override fun drive() {
    }
}

Super

To use a function from the parent class use super

open class Vehicle() {
    open fun drive() {
        println("original")
    } 
}

class Car() : Vehicle() {
    override fun drive() {
        println("some stuff")
        super.drive()   //will append "original"
    }
}

Lambdas

fun printMessage(message: String){
    println(message)
}

Can be expressed as lambda

val printMessage = { message: String -> println(message)}

val sumA = {x: Int, y: Int -> x + y}

Can also be written

val sumA : (Int, Int) -> Int = {x,y -> x + y}

Unit means returns nothing. In this example we do an async download:

    fun downloadData(url: String, completion: ()-> Unit) {
        // sent a download request
        // we got back data
        // there were no network errors
        completion()
    }

    // call downloadDataFunction
    downloadData("fakeUrl.com",{
        println("The code in this block, will only run" +
                "after the completion()")
    })

Can use "it" if we only have one variable passed, it substitutes for that variable name:

   fun downloadCarData(url: String, completion: (Car) -> Unit) {
        // send a download request
        // we got back car data
        val car = Car("Tesla", "ModelX","Blue")
        completion(car)
    }

    downloadCarData("fakeUrl.com"){ car ->
        println(car.color)
        println(car.make)
    }

    downloadCarData("fakeUrl.com"){
        println(it.color)
        println(it.make)
    }

   fun downloadTruckData(url: String, completion: (Truck?, Boolean) -> Unit) {
        // make the web request
        // we get the results back
        val webRequestSuccess = false
        if (webRequestSuccess) {
            // recieved truck data
            val truck = Truck("Ford", "F-150", 10000)
            completion(truck, true)
        } else {
            completion(null, false)
        }
    }

    downloadTruckData("fakeUrl.com") { truck, success ->
        if (success == true) {
            // do something with our truck
            truck?.tow()
        } else {
            // handle the web request failure
            println("Something went wrong!")
        }
    }

Instance State

Instance will be lost on rotation unless everything is saved and restored

class LeagueActivity : BaseActivity() {

    var player = Player("","")

    override fun onSaveInstanceState(outState: Bundle?) {
        super.onSaveInstanceState(outState)
        outState?.putParcelable(EXTRA_PLAYER,player)
    }

    override fun onRestoreInstanceState(savedInstanceState: Bundle?) {
        super.onRestoreInstanceState(savedInstanceState)
        if (savedInstanceState != null) {
            player = savedInstanceState.getParcelable(EXTRA_PLAYER)
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_league)
        leagueNextBtn.isEnabled = false
    }

In this case we need to override onSaveInstanceState and onRestoreInstanceState