KOTLIN 102 GDG TORONTO ANDROID Jigar Brahmbhatt (@shaktiman_droid) 

WRITE CODE IN KOTLIN WAY 

Read popular open-sourced Kotlin-first code How? 

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

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? 

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

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

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()) } } 

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()) } } 

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()) } } 

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()) } } 

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()) } } 

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()) } } 

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() } } 

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()) } } 

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/ 

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 ? 

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() } 

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() } 

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/ 

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/ 

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

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

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

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() } } 

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() } } 

SOME INTERESTING FINDINGS 

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

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

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

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) } 

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

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] 

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) } 

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

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

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) } 

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) } 

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

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) } } 

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

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) } } 

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

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

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

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

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

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

Exhaustive `when` 

Exhaustive `when` 

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

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 `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 } } } 

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! 

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! 

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 } } } 

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 } } } 

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 } } } 

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/ 

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 } 

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 } 

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