This presentation is part of a workshop series. In this section you will learn about operators, inlining, a little bit about generics, named parameters, lambdas and closures, obtaining references to functions and passing them to other functions, and higher-order functions. License: This presentation is licensed under the Creative Commons, No Derivatives, Version 3.0 US: https://creativecommons.org/licenses/by-nd/3.0/us/legalcode

1. Kotlin for Android
May 24, 2018
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2. Functions
Operators, Extensions, Inline, Infix, Interop
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3. ● Almost every operator in Kotlin is mapped to a function
Operators
FUNCTIONS
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Expression Translates to
a + b a.plus(b)
a..b a.rangeTo(b)
a in b b.contains(a)
a !in b !b.contains(a)
a[i] a.get(i)
a[i] = b a.set(i, b)
a() a.invoke()
a(i_1, ..., i_n) a.invoke(i_1, ..., i_n)
a += b a.plusAssign(b)
a == b a?.equals(b) ?: (b === null)
a != b !(a?.equals(b) ?: (b === null))
a > b a.compareTo(b) > 0

4. ● A class can extend from a function
Functions Are Types
FUNCTIONS
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5. fun world(): String {
return "World"
}
fun world(): String = "World"
fun world() = "World"Basic Syntax
FUNCTIONS
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6. In depth
FUNCTIONS
fun world() = "World"
object World : () -> String {
override fun invoke(): String {
return "World"
}
}
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7. In depth
FUNCTIONS
fun world() = "World"
object World : () -> String {
override fun invoke(): String {
return "World"
}
}
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8. In depth
FUNCTIONS
fun world() = "World"
object World : () -> String {
override fun invoke() = "World"
}
fun hello(who: () -> String) {
println("Hello ${who()}!")
}
hello({ "World" })
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9. In depth
FUNCTIONS
fun world() = "World"
object World : () -> String {
override fun invoke() = "World"
}
fun hello(who: () -> String) {
println("Hello ${who()}!")
}
hello { "World" }
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10. In depth
FUNCTIONS
fun world() = "World"
object World : () -> String {
override fun invoke() = "World"
}
fun hello(who: () -> String) {
println("Hello ${who()}!")
}
hello { "World" }
hello(::world)
hello(World)
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11. In depth
FUNCTIONS
fun world() = "World"
object World : () -> String {
override fun invoke() = "World"
}
fun hello(who: () -> String) {
println("Hello ${who()}!")
}
hello({ "World" }::invoke::invoke)
hello(::world::invoke::invoke::invoke)
hello(World::invoke::invoke::invoke::invoke::invoke)
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12. typealias NameProvider = () -> String
typealias DeviceName = () -> String
fun hello(who: NameProvider, what: DeviceName) {
println("Hello ${who()} on ${what()}!")
}
Functions Are Types
typealias
FUNCTIONS
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13. Functions Are Types
typealias
FUNCTIONS typealias NameProvider = () -> String
typealias DeviceName = () -> String
fun hello(who: NameProvider, what: DeviceName) {
println("Hello ${who()} on ${what()}!")
}
object World : NameProvider {
override fun invoke() = "World"
}
object Device : DeviceName {
override fun invoke() = "Android"
}
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14. Functions Are Types
typealias Warning
FUNCTIONS typealias NameProvider = () -> String
typealias DeviceName = () -> String
fun hello(who: NameProvider, what: DeviceName) {
println("Hello ${who()} on ${what()}!")
}
object World : NameProvider {
override fun invoke() = "World"
}
object Device : DeviceName {
override fun invoke() = "Android"
}
hello(World, Device) // "Hello World on Android!"
hello(Device, World) // "Hello Android on World!"
hello(what = Device, who = World)
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15. hello(what = Device, who = World)
● You can reorder the arguments you supply when invoking a
function if you use their names
● Works only for functions and constructors defined in Kotlin
Named Parameters
FUNCTIONS
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16. ● Commonly known as extension functions
● Declares receiver type together with its name
● this in the body refers to an instance of the receiver
● Calling from Java the receiver is the first argument
Functions with
Receivers
FUNCTIONS
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17. Functions with
Receivers
FUNCTIONS
// Kotlin
fun ViewGroup.foo(): String
// Java
public static String foo(ViewGroup dis)
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18. Functions with
Receivers
FUNCTIONS
// Kotlin
fun ViewGroup.foo(bar: Baz): String
// Java
public static String foo(ViewGroup dis, Baz bar)
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19. Functions with
Receivers
FUNCTIONS
class Potato {
fun ViewGroup.foo(bar: Baz): String {
(this is ViewGroup) == true
(this@Potato is Potato) == true
}
}
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20. Functions with
Receivers
FUNCTIONS
val container: ViewGroup = ...
container += R.layout.scroll_view
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21. Functions with
Receivers
FUNCTIONS
val container: ViewGroup = ...
container += R.layout.scroll_view
operator fun ViewGroup.plusAssign(
@LayoutRes layout: Int
) {
LayoutInflater
.from(this.context)
.inflate(layout, this, true)
}
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22. Higher Order Functions
FUNCTIONS
fun hi(who: () -> String) = println("Hi ${who()}!")
hi { "World" } // prints Hi World!
fun hi(who: String, where: () -> String) {
println("Hi $who from ${where()}!")
}
hi("Gesh") { "Hamburg" } // prints Hi Gesh from Hamburg!
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● Improved syntax if the last argument of a function is a
function

23. Vararg Functions
FUNCTIONS
fun main(vararg args: String)
// Java
public void main(String... args)
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● vararg keyword
● Only one per function
● Usually last unless a higher order function

24. Array Spread Operator
FUNCTIONS
fun main(vararg args: String)
// Java
public void main(String... args)
val array = arrayOf(1, 2) // [1, 2]
val list1 = listOf(array) // [[1, 2]]
val list2 = listOf(*array) // [1, 2]
val list3 = listOf(0, array, 3) // [0, [1, 2], 3]
val list4 = listOf(0, *array, 3) // [0, 1, 2, 3]
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● vararg keyword
● Only one per function
● Usually last unless a higher order function
● Spread an existing array into multiple vararg arguments

25. Infix Functions
FUNCTIONS
infix fun <A, B> A.to(that: B) = Pair(this, that)
mapOf(
"a" to "b",
"a".to("b"),
Pair("a", "b")
)
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● Member or extension functions
● Exactly one argument

26. Inline Functions
FUNCTIONS
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● Compiler will inline the body at the call-site
● Cannot be used from Java

27. Inline Functions
FUNCTIONS
// Java
ViewModel model = mock(ViewModel.class)
// Kotlin
val model: ViewModel = mock(ViewModel::class.java)y/kotlin-workbook
● Compiler will inline the body at the call-site
● Cannot be used from Java
● Can use reified generics

28. Inline Functions
FUNCTIONS
// Java
ViewModel model = mock(ViewModel.class)
inline fun <reified T> mock(): T {
return Mockito.mock(T::class.java) as T
}
// Kotlin
val model: ViewModel = mock()
val model = mock<ViewModel>()
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● Compiler will inline the body at the call-site
● Cannot be used from Java
● Can use reified generics

29. Questions?
Next Up: Lunch
After Luch: Properties