Kotlin Advanced Tricks

Presentation about some advanced Kotlin tips, given on

- iMasters Kotlin Summit (September/2018)

Transcript

1. ADVANCED
   KOTLIN TRICKS
   Ubiratan Soares
   September / 2018
   

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2. STDLIB :
   THE FORGOTTEN
   FUNCTIONS
   

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3. Repeat
   fun severalPrints() =
   repeat(times = 10) {
   print("Hey!")
   }
   

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4. /!"
   * Calls the specified function [block] with `this` value as its
   * argument and returns `this` value.
   #$
   @kotlin.internal.InlineOnly
   @SinceKotlin("1.1")
   public inline fun T.also(block: (T) %& Unit): T {
   contract {
   callsInPlace(block, InvocationKind.EXACTLY_ONCE)
   }
   block(this)
   return this
   }
   Also
   

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5. fun validate(name: String) =
   name.length in 2'(20
   fun saveUser(name: String) =
   validate(name)
   .also { saveWith(name) }
   

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6. TakeIf
   /!"
   * Returns `this` value if it satisfies the
   * given [predicate] or `null`, if it doesn't.
   #$
   @kotlin.internal.InlineOnly
   @SinceKotlin("1.1")
   public inline fun T.takeIf(predicate: (T) %& Boolean): T? {
   contract {
   callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
   }
   return if (predicate(this)) this else null
   }
   

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7. TakeIf
   /!"
   * Returns `this` value if it satisfies the
   * given [predicate] or `null`, if it doesn't.
   #$
   @kotlin.internal.InlineOnly
   @SinceKotlin("1.1")
   public inline fun T.takeIf(predicate: (T) %& Boolean): T? {
   contract {
   callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
   }
   return if (predicate(this)) this else null
   }
   

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8. TakeUnless
   /!"
   * Returns `this` value if it _does not_ satisfy
   * the given [predicate] or `null`, if it does.
   #$
   @kotlin.internal.InlineOnly
   @SinceKotlin("1.1")
   public inline fun T.takeUnless(predicate: (T) %& Boolean): T? {
   contract {
   callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
   }
   return if (!predicate(this)) this else null
   }
   

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9. fun names() =
   if (Random().nextBoolean()) "Elvis"
   else "Presley"
   val shortName =
   names()
   .takeIf { it.length )* 5 }
   +, "Too long"
   val longName =
   names()
   .takeUnless { it.length )* 5 }
   +, "Too Short"
   

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10. INVOKING
    LIKE A PRO
    

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11. class DeepLinkParser {
    fun parse(uri : String) : ProductId {
    // Parse your URI and extract product id
    }
    }
    

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12. class DeepLinkHandler(
    val parser: DeepLinkParser) {
    operator fun invoke(url : String) : Intent {
    // Handle with parser and get your Intent
    }
    }
    

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13. class DeepLinkHandler(
    val parser: DeepLinkParser) {
    operator fun invoke(url : String) : Intent {
    // Handle with parser and get your Intent
    }
    }
    

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14. Invoking Instances
    val parser = DeepLinkParser()
    val handler = DeepLinkHandler(parser)
    val intent = handler("myapp:-.product/123456")
    // Launch your Activity
    

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15. Invoking Instances
    val parser = DeepLinkParser()
    val handler = DeepLinkHandler(parser)
    val intent = handler("myapp://product/123456")
    // Launch your Activity
    

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16. A tiny web DSL
    sealed class HTTPMethod
    object GET : HTTPMethod()
    object POST : HTTPMethod()
    class Status {
    var code: Int = 0
    var payload: String = ""
    }
    class HttpRequest {
    var method: HTTPMethod = GET
    var body: String = ""
    var query: String = ""
    }
    class HttpResponse {
    var internalMessage: String = ""
    lateinit var status: Status
    }
    

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17. class Endpoint {
    lateinit var request: HttpRequest
    lateinit var response: HttpResponse
    fun handle() {
    -. TODO
    }
    }
    

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18. fun endpoint(
    path: String,
    block: Endpoint.() %& Unit) =
    Endpoint().apply { block() }.handle()
    endpoint("/api/product") {
    }
    

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19. class Endpoint {
    lateinit var request: HttpRequest
    lateinit var response: HttpResponse
    fun request(block: HttpRequest.() %& Unit) {
    request = HttpRequest().apply { block() }
    }
    fun response(block: HttpResponse.() %& Unit) {
    response = HttpResponse().apply { block() }
    }
    fun handle() {
    -. TODO
    }
    }
    endpoint("/api/product") {
    request {
    method = POST,
    body = "{ Some json }"
    }
    response {
    }
    }
    

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20. endpoint("/api/product") {
    request {
    method = POST,
    body = "{ Some json }"
    }
    response {
    status {
    code = 200
    payload = "{ Some json }"
    }
    }
    }
    class HttpResponse {
    var internalMessage: String = ""
    lateinit var status: Status
    fun status(setup: Status.() %& Unit) {
    status = Status().apply { setup() }
    }
    }
    

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21. endpoint("/api/product") {
    request {
    method = POST,
    body = "{ Some json }"
    }
    response {
    status {
    code = 200
    payload = "{ Some json }"
    }
    }
    }
    Can we do better ?
    
    

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22. class Endpoint {
    lateinit var request: HttpRequest
    lateinit var response: HttpResponse
    fun request(block: HttpRequest.() %& Unit) {
    request = HttpRequest().apply { block() }
    }
    fun response(block: HttpResponse.() %& Unit) {
    response = HttpResponse().apply { block() }
    }
    fun handle() {
    -. TODO
    }
    }
    

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23. class Endpoint {
    lateinit var request: HttpRequest
    lateinit var response: HttpResponse
    fun request(block: HttpRequest.() %& Unit) {
    request = HttpRequest().apply { block() }
    }
    fun response(block: HttpResponse.() %& Unit) {
    response = HttpResponse().apply { block() }
    }
    fun handle() {
    -. TODO
    }
    }
    
    

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24. class Endpoint {
    lateinit var request: HttpRequest
    lateinit var response: HttpResponse
    fun request(block: HttpRequest.() %& Unit) {
    request = HttpRequest().apply { block() }
    }
    fun response(block: HttpResponse.() %& Unit) {
    response = HttpResponse().apply { block() }
    }
    fun handle() {
    -. TODO
    }
    }
    class HttpResponse {
    var internalMessage: String = ""
    lateinit var status: Status
    fun status(setup: Status.() %& Unit) {
    status = Status().apply { setup() }
    }
    }
    

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25. class Endpoint {
    lateinit var request: HttpRequest
    lateinit var response: HttpResponse
    fun request(block: HttpRequest.() %& Unit) {
    request = HttpRequest().apply { block() }
    }
    fun handle() {
    -. TODO
    }
    }
    class HttpResponse {
    var internalMessage: String = ""
    lateinit var status: Status
    operator fun invoke(
    setup: Status.() -> Unit) {
    status = Status().apply { setup() }
    }
    }
    

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26. endpoint("/api/product") {
    request {
    method = POST
    body = "{ Some json }"
    }
    response {
    code = 200
    payload = "{ Some json }"
    }
    }
    endpoint("/api/product") {
    request {
    method = POST,
    body = "{ Some json }"
    }
    response {
    status {
    code = 200
    payload = "{ Some json }"
    }
    }
    }
    Before After
    

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27. http://shop.oreilly.com/product/0636920052999.do
    

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28. AWESOME
    COMPANIONS
    

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30. interface FormValidator {
    fun validate(name: String?): Boolean
    }
    class FormPresenter {
    private companion object : FormValidator {
    override fun validate(name: String?) =
    name/0isNotEmpty()
    +, throw IllegalArgumentException("Invalid Name")
    }
    }
    

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31. interface FormValidator {
    fun validate(name: String?): Boolean
    }
    class FormPresenter {
    private companion object : FormValidator {
    override fun validate(name: String?) =
    name/0isNotEmpty()
    +, throw IllegalArgumentException("Invalid Name")
    }
    }
    

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32. Fake Constructors
    val sampa = Location("23.5505° S", "46.6333° W")
    data class Location(
    val latitude: Float,
    val longitude: Float
    )
    
    

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33. data class Location(
    val latitude: Float,
    val longitude: Float) {
    companion object {
    operator fun invoke(lat: String, long: String) =
    Location(parse(lat), parse(long))
    private fun parse(coordinate: String): Float {
    -. convert the String representation
    }
    }
    }
    

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34. data class Location(
    val latitude: Float,
    val longitude: Float) {
    companion object {
    operator fun invoke(lat: String, long: String) =
    Location(parse(lat), parse(long))
    private fun parse(coordinate: String): Float {
    -. convert the String representation
    }
    }
    }
    

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35. NOTHING
    ELSE
    MATTERS
    

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36. The Nothing Type
    https://en.wikipedia.org/wiki/Bottom_type
    " In type theory, a theory within mathematical logic,
    the bottom type is the type that has no values. It is also
    called the zero or empty type, and is sometimes denoted
    with falsum (⊥).
    A function whose return type is bottom cannot return any
    value. In the Curry–Howard correspondence, the bottom
    type corresponds to falsity. "
    

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37. /!"
    * The type with only one value: the Unit object.
    * This type corresponds to the `void` type
    * in Java.
    #$
    public object Unit {
    override fun toString() = "kotlin.Unit"
    }
    

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38. Kotlin Types
    Nothing
    SomeType
    Any
    Nothing?
    SomeType?
    Any?
    

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39. /!"
    * Always throws [NotImplementedError]
    * stating that operation is not implemented.
    #$
    @kotlin.internal.InlineOnly
    public inline fun TODO(): Nothing =
    throw NotImplementedError()
    

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41. Nothing and flow control
    inline fun guard(
    receiver: T?,
    block: () %& Nothing): T {
    if (receiver 12 null) { block() }
    return receiver
    }
    https://gist.github.com/hzsweers/463500043b1a9708ba4e26c7ad1862fd
    

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42. Nothing and flow control
    inline fun guard(
    receiver: T?,
    block: () -> Nothing): T {
    if (receiver 12 null) { block() }
    return receiver
    }
    https://gist.github.com/hzsweers/463500043b1a9708ba4e26c7ad1862fd
    

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43. fun guarding() {
    val conference: String? = "Kotlin Summit"
    }
    

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44. fun guarding() {
    val conference: String? = "Kotlin Summit"
    val guarded = guard(conference) { return }
    }
    

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45. fun guarding() {
    val conference: String? = "Kotlin Summit"
    val guarded = guard(conference) { return }
    val guardedAgain = guard(conference) {
    throw IllegalStateException("Null konf")
    }
    }
    

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46. fun guarding() {
    val conference: String? = "Kotlin Summit"
    val guarded = guard(conference) { return }
    val guardedAgain = guard(conference) {
    throw IllegalStateException("Null konf")
    }
    val invalid = guard(conference) {
    -.Compiler error because not
    -.returned or thrown an exception!
    }
    }
    

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47. Nothing and Generics
    sealed class Tree
    class Node(
    val left: Tree,
    val right: Tree) : Tree()
    class Leaf(val value: T) : Tree()
    object Empty : Tree()
    

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48. val tree = Node(
    Leaf("1"),
    Node(
    Leaf("2"),
    Empty
    )
    )
    1
    2
    

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49. sealed class Result
    data class Value(val value: V) :
    Result()
    data class Error(val error: E) :
    Result()
    

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50. ADVANCING ON
    DELEGATION
    

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51. interface Validation {
    fun validate(input: String): Boolean
    }
    class NameValidator : Validation {
    override fun validate(input: String) =
    input.isNotEmpty() 34 input.length 56 2
    }
    Class delegation
    

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52. val accepted = FormViewModel().validate("Bira")
    class FormViewModel : Validation by NameValidator()
    Class delegation
    

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53. class FormViewModel : FormValidator by Internal {
    companion object Internal : FormValidator {
    override fun validate(name: String?) =
    name/0isNotEmpty()+, false
    }
    }
    val rejected = FormViewModel().validate("Hi")
    

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54. class FormViewModel : FormValidator by Internal {
    companion object Internal : FormValidator {
    override fun validate(name: String?) =
    name/0isNotEmpty()+, false
    }
    }
    val rejected = FormViewModel().validate("Hi")
    

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55. interface HandleSuccess {
    fun showResult()
    }
    interface HandleError {
    fun showError()
    }
    interface LoginScreen : HandleSuccess, HandleError
    interface ProductScreen : HandleSuccess, HandleError
    

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56. interface HandleSuccess {
    fun showResult()
    }
    interface HandleError {
    fun showError()
    }
    interface LoginScreen : HandleSuccess, HandleError
    interface ProductScreen : HandleSuccess, HandleError
    

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57. Selective delegation
    fun performOperation(behavior: C)
    where C : HandleSuccess, C : HandleError {
    try {
    executeSomething()
    behavior.showResult()
    } catch (error: Throwable) {
    behavior.showError()
    }
    }
    

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58. Selective delegation
    fun performOperation(behavior: C)
    where C : HandleSuccess, C : HandleError {
    try {
    executeSomething()
    behavior.showResult()
    } catch (error: Throwable) {
    behavior.showError()
    }
    }
    

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59. PLUS,
    ULTRA !!!
    

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60. class CheckProfile {
    companion object : (Int) %& Boolean by ShouldBeLegal
    }
    val canDrink = CheckProfile(35)
    object ShouldBeLegal : (Int) %& Boolean {
    override fun invoke(age: Int) = age 5618
    }
    

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61. What if I told you
    that extensions are
    not only for objects
    ???
    

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62. val checkNotDead: (Int) %& Boolean = { it < 150 }
    fun ((Int) %& Boolean).report(age: Int) =
    this.invoke(age)
    .also { print((if (it) "Ok" else "Not OK")) }
    fun misjudge(age: Int) = checkNotDead.report(age)
    

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63. View Slide

64. Final Remarks
    Kotlin has it's secrets, go explore !!!
    Learn more hidden features for fun and profit
    

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65. https://speakerdeck.com/ubiratansoares
    

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66. UBIRATAN
    SOARES
    Computer Scientist by ICMC/USP
    Software Engineer, curious guy
    Google Developer Expert for Android / Kotlin
    Teacher, speaker, etc, etc
    

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67. THANK YOU
    @ubiratanfsoares
    ubiratansoares.github.io
    https://br.linkedin.com/in/ubiratanfsoares
    

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