Kotlin 102

In this talk, you'll learn about some tips on writing code in Kotlin way, along with interesting findings about Kotlin collection's mutability, exhaustiveness of when clause and a couple of other tricks.

Cross-border remote meetup event with GDG Toronto Android community and GDG Boston Android community
https://www.meetup.com/ToAndroidDev/events/270874680/

Youtube link: https://www.youtube.com/watch?v=O6EmGzAGnuk

Transcript

1. KOTLIN 102
   GDG TORONTO ANDROID
   Jigar Brahmbhatt (@shaktiman_droid)
   

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2. WRITE
   
   CODE
   
   IN
   
   KOTLIN
   
   WAY
   

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3. Read popular open-sourced
   
   Kotlin-first code
   How?
   

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4. Discuss idiomatic way to write
   Kotlin during PR reviews
   Read popular open-sourced
   
   Kotlin-first code
   How?
   

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5. Discuss idiomatic way to write
   Kotlin during PR reviews
   Ask yourself, is there a better
   
   way to write this Kotlin ?
   Read popular open-sourced
   
   Kotlin-first code
   How?
   

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6. 1
   https://play.google.com/store/apps/details?id=com.intersect.specly
   

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7. 1
   https://play.google.com/store/apps/details?id=com.intersect.specly
   

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8. 1
   private const val MAX_VALUE = 99
   
   private const val MIN_VALUE = 8
   
   fun setCurrentValue(newValue: Int) {
   
   if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
   
   val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
   
   textInputLayout.setText(valueToSet.toString())
   
   } else {
   
   textInputLayout.setText(newValue.toString())
   
   }
   
   }
   

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9. private const val MAX_VALUE = 99
   
   private const val MIN_VALUE = 8
   
   fun setCurrentValue(newValue: Int) {
   
   if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
   
   val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
   
   textInputLayout.setText(valueToSet.toString())
   
   } else {
   
   textInputLayout.setText(newValue.toString())
   
   }
   
   }
   

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10. private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    

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11. private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    

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12. private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    

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13. Can we improve this ?
    private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    

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14. private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    return
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    fun setCurrentValue(newValue: Int) {
    
    if (newValue in MIN_VALUE#..MAX_VALUE) {
    
    textInputLayout.text = newValue.toString()
    
    } else {
    
    textInputLayout.text = newValue.coerceIn(MIN_VALUE, MAX_VALUE).toString()
    
    }
    
    }
    

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15. fun setCurrentValue(newValue: Int) {
    
    if (newValue in MIN_VALUE#..MAX_VALUE) {
    
    textInputLayout.text = newValue.toString()
    
    } else {
    
    textInputLayout.text = newValue.coerceIn(MIN_VALUE, MAX_VALUE).toString()
    
    }
    
    }
    private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    return
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    

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16. Primives.kt
    fun Int.coerceIn(minimumValue: Int, maximumValue: Int): Int {
    
    if (minimumValue > maximumValue) throw IllegalArgumentException("Cannot
    coerce value to an empty range: maximum $maximumValue is less than minimum
    $minimumValue.")
    
    if (this < minimumValue) return minimumValue
    
    if (this > maximumValue) return maximumValue
    
    return this
    
    }
    https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.ranges/
    

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17. fun setCurrentValue(newValue: Int) {
    
    if (newValue in MIN_VALUE#..MAX_VALUE) {
    
    textInputLayout.text = newValue.toString()
    
    } else {
    
    textInputLayout.text = newValue.coerceIn(MIN_VALUE, MAX_VALUE).toString()
    
    }
    
    }
    private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    return
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    Can we improve this further ?
    

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18. fun setCurrentValue(newValue: Int) {
    
    if (newValue in MIN_VALUE#..MAX_VALUE) {
    
    textInputLayout.text = newValue.toString()
    
    } else {
    
    textInputLayout.text = newValue.coerceIn(MIN_VALUE, MAX_VALUE).toString()
    
    }
    
    }
    private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    return
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    fun setCurrentValue(newValue: Int) {
    
    val text = newValue
    
    .takeIf { newValue in MIN_VALUE#..MAX_VALUE }
    
    #?: newValue.coerceIn(MIN_VALUE, MAX_VALUE)
    
    textInputLayout.text = text.toString()
    
    }
    
    

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19. fun setCurrentValue(newValue: Int) {
    
    if (newValue in MIN_VALUE#..MAX_VALUE) {
    
    textInputLayout.text = newValue.toString()
    
    } else {
    
    textInputLayout.text = newValue.coerceIn(MIN_VALUE, MAX_VALUE).toString()
    
    }
    
    }
    private const val MAX_VALUE = 99
    
    private const val MIN_VALUE = 8
    
    fun setCurrentValue(newValue: Int) {
    
    if (newValue > MAX_VALUE #|| newValue < MIN_VALUE) {
    
    val valueToSet = if (newValue > MAX_VALUE) MAX_VALUE else MIN_VALUE
    
    textInputLayout.setText(valueToSet.toString())
    
    return
    
    } else {
    
    textInputLayout.setText(newValue.toString())
    
    }
    
    }
    fun setCurrentValue(newValue: Int) {
    
    val text = newValue
    
    .takeIf { newValue in MIN_VALUE#..MAX_VALUE }
    
    #?: newValue.coerceIn(MIN_VALUE, MAX_VALUE)
    
    textInputLayout.text = text.toString()
    
    }
    
    

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20. Standard.Kt
    /**
    
    * Returns `this` value if it satisfies the given [predicate]
    or `null`, if it doesn't.
    
    "*/
    
    public inline fun T.takeIf(predicate: (T) #-> Boolean): T? {
    
    contract {
    
    callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
    
    }
    
    return if (predicate(this)) this else null
    
    }
    https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/
    

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21. Standard.Kt
    /**
    
    * Returns `this` value if it satisfies the given [predicate]
    or `null`, if it doesn't.
    
    "*/
    
    public inline fun T.takeIf(predicate: (T) #-> Boolean): T? {
    
    contract {
    
    callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
    
    }
    
    return if (predicate(this)) this else null
    
    }
    https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/
    

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22. 2
    fun getUserId(): String? {
    
    return if (isValid) {
    
    userIdLocal
    
    } else {
    
    null
    
    }
    
    }
    

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23. 2
    fun getUserId(): String? {
    
    return userIdLocal.takeIf { isValid }
    
    }
    fun getUserId(): String? {
    
    return if (isValid) {
    
    userIdLocal
    
    } else {
    
    null
    
    }
    
    }
    

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24. 3
    fun getDeviceHash(): Int? {
    
    val deviceHash = prefs.getString(KEY_ENCRYPTED_HASH, null)
    
    return deviceHash#?.let { hash #->
    
    decrypt(hash)#?.toInt()
    
    }
    
    }
    

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25. 3
    fun getDeviceHash(): Int? {
    
    with(prefs.getString(KEY_ENCRYPTED_HASH, null)) {
    
    return this.takeUnless { this #== null }#?.toInt()
    
    }
    
    }
    
    fun getDeviceHash(): Int? {
    
    val deviceHash = prefs.getString(KEY_ENCRYPTED_HASH, null)
    
    return deviceHash#?.let { hash #->
    
    decrypt(hash)#?.toInt()
    
    }
    
    }
    

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26. 3
    fun getDeviceHash(): Int? {
    
    with(prefs.getString(KEY_ENCRYPTED_HASH, null)) {
    
    return this.takeUnless { this #== null }#?.toInt()
    
    }
    
    }
    
    fun getDeviceHash(): Int? {
    
    val deviceHash = prefs.getString(KEY_ENCRYPTED_HASH, null)
    
    return deviceHash#?.let { hash #->
    
    decrypt(hash)#?.toInt()
    
    }
    
    }
    

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27. SOME
    
    INTERESTING
    
    FINDINGS
    

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28. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    println(list)
    
    }
    

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29. Mutability in list
    Output: [123, 456]
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    println(list)
    
    }
    

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30. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    list.add("789") #// read-only
    
    println(list)
    
    }
    

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31. Mutability in list
    #//Java
    
    private static void setGiftCards(List cards) {
    
    cards.add("789");
    
    }
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    list.add("789") #// read-only
    
    println(list)
    
    }
    

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32. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    (list as MutableList).add("789")
    
    
    println(list)
    
    }
    

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33. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    (list as MutableList).add("789")
    
    
    println(list)
    
    }
    Output: [123, 456, 789]
    

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34. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    val newGiftCards = listOf("123", "456")
    
    setGiftCards(newGiftCards)
    
    fun setGiftCards(list: List) {
    
    (list as MutableList).add("789")
    
    println(list)
    
    }
    

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35. val giftCards = mutableListOf("123", "456")
    
    val newGiftCards = listOf("123", "456")
    
    setGiftCards(newGiftCards)
    
    fun setGiftCards(list: List) {
    
    (list as MutableList).add("789")
    
    println(list)
    
    }
    Mutability in list
    java.lang.UnsupportedOperationException
    
    at java.util.AbstractList.add(AbstractList.java:148)
    
    at java.util.AbstractList.add(AbstractList.java:108)
    

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37. Mutability in list
    implementation "org.jetbrains.kotlinx:kotlinx-collections-immutable:0.3.2"
    

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38. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    this.list = list.toImmutableList()
    println(list)
    
    }
    

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39. Mutability in list
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    this.list = list.toImmutableList()
    
    (list as MutableList).add("789")
    
    println(list)
    
    }
    

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40. Mutability in list
    java.lang.ClassCastException: kotlinx.collections.immutable.implementations.
    
    immutableList.SmallPersistentVector cannot be cast to kotlin.collections.MutableList
    val giftCards = mutableListOf("123", "456")
    
    setGiftCards(giftCards.toList())
    
    fun setGiftCards(list: List) {
    
    this.list = list.toImmutableList()
    
    (list as MutableList).add("789")
    
    println(list)
    
    }
    

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41. Requirements
    Analytics for product listing and
    product listing action should be
    sequential
    Execute Lambda in queue
    

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42. Execute Lambda in queue
    object PlaygroundSdk {
    
    private lateinit var retrofitService: RetrofitService
    
    fun sendProductListAnalytics() {
    
    GlobalScope.launch { putProductListAnalytics() }
    
    }
    
    private suspend fun putProductListAnalytics() {
    
    retrofitService.putAnalytics(data)
    
    }
    
    fun sendProductListActionAnalytics() {
    
    GlobalScope.launch { putProductListActionAnalytics() }
    
    }
    
    private suspend fun putProductListActionAnalytics() {
    
    retrofitService.putAnalytics(data)
    
    }
    
    }
    

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43. private val putCallQueue = PriorityQueue Unit>()
    
    
    Execute Lambda in queue
    

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44. private suspend fun enqueuePut(f: suspend () #-> Unit) {
    
    if (putCallQueue.isEmpty()) {
    
    putCallQueue.add(f)
    
    f.invoke()
    
    putCallQueue.apply {
    
    remove()
    
    poll()#?.let {
    
    enqueuePut(it)
    
    }
    
    }
    
    } else {
    
    putCallQueue.add(f)
    
    }
    
    }
    private val putCallQueue = PriorityQueue Unit>()
    Execute Lambda in queue
    

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45. private val putCallQueue = PriorityQueue Unit>()
    fun sendProductListAnalytics() {
    
    GlobalScope.launch {
    
    enqueuePut {
    
    putProductListAnalytics()
    
    }
    
    }
    
    }
    
    fun sendProductListActionAnalytics() {
    
    GlobalScope.launch {
    
    enqueuePut {
    
    putProductListActionAnalytics()
    
    }
    
    }
    
    }
    Execute Lambda in queue
    private suspend fun enqueuePut(f: suspend () #-> Unit) {
    
    if (putCallQueue.isEmpty()) {
    
    putCallQueue.add(f)
    
    f.invoke()
    
    putCallQueue.apply {
    
    remove()
    
    poll()#?.let {
    
    enqueuePut(it)
    
    }
    
    }
    
    } else {
    
    putCallQueue.add(f)
    
    }
    
    }
    

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46. Trick with type alias
    #// SDK
    
    class PlaygroundManeger {
    
    fun manage() {
    
    TODO()
    
    }
    
    }
    

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47. Trick with type alias
    #// SDK
    
    class PlaygroundManeger {
    
    fun manage() {
    
    TODO()
    
    }
    
    }
    #// App
    
    PlaygroundManeger().manage()
    

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48. Trick with type alias
    #// SDK
    
    class PlaygroundManeger {
    
    fun manage() {
    
    TODO()
    
    }
    
    }
    #// App
    
    PlaygroundManeger().manage()
    

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49. Trick with type alias
    #// SDK
    
    class PlaygroundManager {
    
    fun manage() {
    
    TODO()
    
    }
    
    }
    #// App
    
    PlaygroundManeger().manage()
    

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50. Trick with type alias
    #// SDK
    
    class PlaygroundManager {
    
    fun manage() {
    
    TODO()
    
    }
    
    }
    #// App
    
    PlaygroundManeger().manage()
    

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51. Trick with type alias
    #// SDK
    
    class PlaygroundManager {
    
    fun manage() {
    
    TODO()
    
    }
    
    }
    #// Kotlin app
    
    PlaygroundManeger().manage()
    typealias PlaygroundManeger = PlaygroundManager
    

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52. Exhaustive `when`
    

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53. Exhaustive `when`
    

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54. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    

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55. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    fun handleState(state: ViewState) {
    
    #// Not used as an expression i.e. no return
    
    when(state) {
    
    is Normal #-> {
    
    #// do something
    
    }
    
    is Loading #-> {
    
    #// do something
    
    }
    
    }
    
    }
    

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56. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    
    object Error: ViewState()
    fun handleState(state: ViewState) {
    
    #// Not used as an expression i.e. no return
    
    when(state) {
    
    is Normal #-> {
    
    #// do something
    
    }
    
    is Loading #-> {
    
    #// do something
    
    }
    
    }
    
    }
    

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57. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    
    object Error: ViewState()
    fun handleState(state: ViewState) {
    
    #// Not used as an expression i.e. no return
    
    when(state) {
    
    is Normal #-> {
    
    #// do something
    
    }
    
    is Loading #-> {
    
    #// do something
    
    }
    
    }
    
    }
    No errors!
    
    

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58. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    
    object Error: ViewState()
    fun handleState(state: ViewState) {
    
    #// Not used as an expression i.e. no return
    
    when(state) {
    
    is Normal #-> {
    
    #// do something
    
    }
    
    is Loading #-> {
    
    #// do something
    
    }
    
    }
    
    }
    No errors!
    
    

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59. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    fun handleState(state: ViewState) {
    
    #// Used as an expression
    
    val value = when(state) {
    
    is Normal #-> {
    
    #// Return something
    
    }
    
    is Loading #-> {
    
    #// Return something
    
    }
    
    }
    
    }
    

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60. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    fun handleState(state: ViewState) {
    
    #// Used as an expression
    
    val value = when(state) {
    
    is Normal #-> {
    
    #// Return something
    
    }
    
    is Loading #-> {
    
    #// Return something
    
    }
    
    }
    
    }
    

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61. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    fun handleState(state: ViewState) {
    
    #// Used as an expression
    
    val value = when(state) {
    
    is Normal #-> {
    
    #// Return something
    
    }
    
    is Loading #-> {
    
    #// Return something
    
    }
    
    }
    
    }
    

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62. Popular solution
    package io.plaidapp.core.util
    
    /**
    
    * Helper to force a when statement to assert all options are matched in a when statement.
    
    *
    
    * By default, Kotlin doesn't care if all branches are handled in a when statement. However, if you
    
    * use the when statement as an expression (with a value) it will force all cases to be handled.
    
    *
    
    * This helper is to make a lightweight way to say you meant to match all of them.
    
    *
    
    * Usage:
    
    *
    
    * ```
    
    * when(sealedObject) {
    
    * is OneType "-> "//
    
    * is AnotherType "-> "//
    
    * }.exhaustive
    
    "*/
    
    val T.exhaustive: T
    
    get() = this
    https:#//github.com/android/plaid/
    

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63. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    fun handleState(state: ViewState) {
    
    #// Not used as an expression i.e. no return
    
    when(state) {
    
    is Normal #-> {
    
    #// do something
    
    }
    
    is Loading #-> {
    
    #// do something
    
    }
    
    }.exhaustive
    
    }
    

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64. Exhaustive `when`
    sealed class ViewState
    
    data class Normal(val info: String): ViewState()
    
    object Loading: ViewState()
    fun handleState(state: ViewState) {
    
    #// Not used as an expression i.e. no return
    
    when(state) {
    
    is Normal #-> {
    
    #// do something
    
    }
    
    is Loading #-> {
    
    #// do something
    
    }
    
    }.exhaustive
    
    }
    

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65. THANK YOU
    HOPE YOU LEARNED SOMETHING!
    Jigar Brahmbhatt (@shaktiman_droid)
    

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