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Kotlin 2 ½ Years Later A brief story about how Cody uses Kotlin.

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Started Android In 2011 Java and Eclipse were fun.

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Started Learning Kotlin At Yello Split my time about 25% Kotlin and 75% Java at the time.

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Working Exclusively In Kotlin At ActiveCampaign We’re hiring. Message me on engel.dev if you want to learn more.

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Time For Some Kotlin If you have questions during the talk, message me on engel.dev.

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val typeInference: Int = 1 What I’m Not Talking About

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val typeInference = 1 What I’m Not Talking About

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data class Speaker(val name: String = "Cody") val speaker = Speaker() val stringTemplates = """ Wow this is cool. Thanks, ${speaker.name} """.trimIndent() What I’m Not Talking About

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data class Speaker(val name: String = "Cody") val speaker = Speaker() val stringTemplates = """ Wow this is cool. Thanks, ${speaker.name} """.trimIndent() What I’m Not Talking About

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fun conditionalExpressions(a: Int, b: Int): Int { if (a > b) { return a } else { return b } } What I’m Not Talking About

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fun conditionalExpressions(a: Int, b: Int): Int { return if (a > b) { a } else { b } } What I’m Not Talking About

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fun conditionalExpressions(a: Int, b: Int) = if (a > b) a else b What I’m Not Talking About

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fun typeCasting(any: Any) { when (any) { is String -> any.length is Map<*, *> -> any.entries is Int -> any + 16 } } What I’m Not Talking About

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class DefaultValues(number: Int = 0) fun defaultValues(defaultValues: DefaultValues = DefaultValues()) { defaultValues() } What I’m Not Talking About

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Goodbye NPE if (nullDoesntExist != null) { } Kotlin greatly reduces the chance that you will inadvertently cause a NullPointerException in production. engel.dev

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if (nullDoesntExist != null) { // Why check null if it does // not exist in Kotlin? } Why Still Check For Null?

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package kotlin public open class KotlinNullPointerException : java.lang.NullPointerException { public constructor() { /* compiled code */ } public constructor(message: kotlin.String?) { /* compiled code */ } } Because NPEs Still Exist.

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val stringWithNothing: String? = null if (stringWithNothing != null) Which Do You Prefer? val stringWithNothing: String = “” stringWithNothing.isNotEmpty()

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val intWithNothing: Int? = null if (intWithNothing != null) Which Do You Prefer? val intWithNothing: Int = -1 if (intWithNothing != -1)

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val intWithNothing: Int? = null if (intWithNothing != null) Which Do You Prefer? val intWithNothing: Int = 0 if (intWithNothing != -1)

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if (possiblyNull == null) { // What do we do!?!?!? }

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val possiblyNull: String? = null val definitelyNotNull = possiblyNull ?: ""

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val possiblyNull: String? = null val definitelyNotNull = possiblyNull ?: ""

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val possiblyNull: String? = null val definitelyNotNull = possiblyNull?.run { println("Cool this wasn't null.") } ?: ""

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val possiblyNull: String? = null val definitelyNotNull = possiblyNull ?: { println("This used to be null.") "" }()

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val possiblyNull: String? = null val definitelyNotNull = possiblyNull?.let { println("Cool this wasn't null.") } ?: { println("This used to be null.") "" }()

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val possiblyNull: String? = null val definitelyNotNull = possiblyNull?.let { println("Cool this wasn't null.") } ?: { println("This used to be null.") "" }.invoke()

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Working With The NullPointerException data class LocalUser( val apiKey: String? = null, val username: String? = null, val userId: Long? = null )

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data class LocalUser( val apiKey: String? = null, val username: String? = null, val userId: Long? = null ) data class AuthenticationRecord( val apiKey: String, val username: String, val userId: Long )

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data class AuthenticationRecord( val apiKey: String, val username: String, val userId: Long ) val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey, username = localUser.username, userId = localUser.userId )

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data class AuthenticationRecord( val apiKey: String, val username: String, val userId: Long ) val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey, username = localUser.username, userId = localUser.userId ) Required: String, Found String?

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data class AuthenticationRecord( val apiKey: String, val username: String, val userId: Long ) val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey, username = localUser.username, userId = localUser.userId ) Required: String, Found String? Required: String, Found String?

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data class AuthenticationRecord( val apiKey: String, val username: String, val userId: Long ) val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey, username = localUser.username, userId = localUser.userId ) Required: String, Found String? Required: String, Found String? Required: String, Found String?

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val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey ?: "", username = localUser.username ?: "", userId = localUser.userId ?: -1 ) val authRecord = AuthenticationRecord( apiKey = localUser.apiKey, username = localUser.username, userId = localUser.userId ) Required: String, Found String? Required: String, Found String? Required: String, Found String?

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val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey!!, username = localUser.username!!, userId = localUser.userId!! ) val localUser = LocalUser() val authRecord = AuthenticationRecord( apiKey = localUser.apiKey ?: "", username = localUser.username ?: "", userId = localUser.userId ?: -1 )

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apiKey = localUser.apiKey!!, username = localUser.username!!, userId = localUser.userId!! ) val authRecord = try { AuthenticationRecord( apiKey = localUser.apiKey!!, username = localUser.username!!, userId = localUser.userId!! ) } catch (exception: KotlinNullPointerException) { // Provide an error to the user because they // aren't authenticated. }

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// Provide an error to the user because they // aren't authenticated. } val authRecord = if ( localUser.apiKey != null && localUser.username != null && localUser.userId != null) { AuthenticationRecord( localUser.apiKey, localUser.username, localUser.userId ) } else { // Provide an error to the user because they // aren't authenticated. }

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} else { // Provide an error to the user because they // aren't authenticated. } val authRecord = try { AuthenticationRecord( apiKey = localUser.apiKey!!, username = localUser.username!!, userId = localUser.userId!! ) } catch (exception: KotlinNullPointerException) { // Provide an error to the user because they // aren't authenticated. }

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This is Not Jake Wharton Approved… …so that was kind of

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Kotlin Defaults To Non-Null For Java APIs… …sorry.

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Data Classes data class Cat( val name: String = "Tina", val age: Int = 4 ) val tina = Cat() tina.name // Tina tina.age // 4 tina.equals(Cat(name = "Tina")) // true tina.hashCode() // some number val (name, age) = tina val tinaIsOlder = tina.copy(age = 5) // older Sometimes classes exist for the sole purpose of containing data. This is where data classes shine. engel.dev

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class Cat( val name: String = "Tina", val age: Int = 4 ) val tina = Cat() tina.name // Tina tina.age // 4 tina.equals(Cat(name = "Tina")) // true tina.hashCode() // some number val (name, age) = tina val tinaIsOlder = tina.copy(age = 5) // older Normal Boring Class.

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class Cat( val name: String = "Tina", val age: Int = 4 ) val tina = Cat() tina.name // Tina tina.age // 4 tina.equals(Cat(name = "Tina")) // false tina.hashCode() // some number val (name, age) = tina val tinaIsOlder = tina.copy(age = 5) // older Normal Boring Class.

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class Cat( val name: String = "Tina", val age: Int = 4 ) val tina = Cat() tina.name // Tina tina.age // 4 tina.equals(Cat(name = "Tina")) // false tina.hashCode() // some number, but calculated differently val (name, age) = tina val tinaIsOlder = tina.copy(age = 5) // older Normal Boring Class.

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class Cat( val name: String = "Tina", val age: Int = 4 ) val tina = Cat() tina.name // Tina tina.age // 4 tina.equals(Cat(name = "Tina")) // false tina.hashCode() // some number, but calculated differently val (name, age) = tina val tinaIsOlder = tina.copy(age = 5) // older Normal Boring Class.

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class Cat( val name: String = "Tina", val age: Int = 4 ) val tina = Cat() tina.name // Tina tina.age // 4 tina.equals(Cat(name = "Tina")) // false tina.hashCode() // some number, but calculated differently val (name, age) = tina val tinaIsOlder = tina.copy(age = 5) Normal Boring Class.

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class Cat( val name: String = "Tina", val age: Int = 4, currentlyHungry: Boolean ) Normal Boring Class.

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data class Cat( val name: String = "Tina", val age: Int = 4, currentlyHungry: Boolean ) Exciting Data Class.

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data class Cat( val name: String = "Tina", val age: Int = 4, currentlyHungry: Boolean ) " Exciting Data Class.

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data class Cat( val name: String = "Tina", val age: Int = 4, currentlyHungry: Boolean ) " Exciting Data Class. Data class primary constructor must have only property (val/var) parameters

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data class Cat( val name: String = "Tina", val age: Int = 4 ) Using Data Classes.

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data class Cat( val name: String = "Tina", val age: Int = 4 ) val shouldBeConsistent = tina.hashCode() Using Data Classes.

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data class Cat( val name: String = "Tina", val age: Int = 4 ) val shouldBeTrue = tina.equals(Cat(name = “Tina")) val shouldBeConsistent = tina.hashCode() Using Data Classes.

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data class Cat( val name: String = "Tina", val age: Int = 4 ) val tinaIsOlder = tina.copy(age = 5) val shouldBeTrue = tina.equals(Cat(name = “Tina")) val shouldBeConsistent = tina.hashCode() Using Data Classes.

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Sealed Classes sealed class DealCustomFieldValue( val fieldId: Long?, val value: String?, val title: String?, val order: Long?, val type: String?, val fieldValueId: Long? ) { data class StandardFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) data class NumberFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) data class CurrencyFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long?, val fieldCurrency: String?, val currencyPosition: String? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) } Used for representing restricted class hierarchies. They are like an extension on enum classes where sub- classes can have multiple instances available at a time. engel.dev

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sealed class DealCustomFieldValue( val fieldId: Long?, val value: String?, val title: String?, val order: Long?, val type: String?, val fieldValueId: Long? ) { data class StandardFieldValue(

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) { data class StandardFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long? ) : DealCustomFieldValue( _fieldId,

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private val _type: String?, private val _fieldValueId: Long? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId )

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data class CurrencyFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long?, val fieldCurrency: String?, val currencyPosition: String? ) : DealCustomFieldValue(

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val fieldCurrency: String?, val currencyPosition: String? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) }

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sealed class DealCustomFieldValue( val fieldId: Long?, val value: String?, val title: String?, val order: Long?, val type: String?, val fieldValueId: Long? ) { data class StandardFieldValue(

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sealed class DealCustomFieldValue { abstract val fieldId: Long? abstract val value: String? abstract val title: String? abstract val order: Long? abstract val type: String? abstract val fieldValueId: Long? data class StandardFieldValue( override val fieldId: Long?,

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data class StandardFieldValue( override val fieldId: Long?, override val value: String?, override val title: String?, override val order: Long?, override val type: String?, override val fieldValueId: Long? ) : DealCustomFieldValue() data class NumberFieldValue( override val fieldId: Long?,

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data class CurrencyFieldValue( override val fieldId: Long?, override val value: String?, override val title: String?, override val order: Long?, override val type: String?, override val fieldValueId: Long?, val fieldCurrency: String?, val currencyPosition: String? ) : DealCustomFieldValue()

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sealed class DealCustomFieldValue( val fieldId: Long?, val value: String?, val title: String?, val order: Long?, val type: String?, val fieldValueId: Long? ) { data class StandardFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) data class NumberFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) data class CurrencyFieldValue( private val _fieldId: Long?, private val _value: String?, private val _title: String?, private val _order: Long?, private val _type: String?, private val _fieldValueId: Long?, val fieldCurrency: String?, val currencyPosition: String? ) : DealCustomFieldValue( _fieldId, _value, _title, _order, _type, _fieldValueId ) } sealed class DealCustomFieldValue { abstract val fieldId: Long? abstract val value: String? abstract val title: String? abstract val order: Long? abstract val type: String? abstract val fieldValueId: Long? data class StandardFieldValue( override val fieldId: Long?, override val value: String?, override val title: String?, override val order: Long?, override val type: String?, override val fieldValueId: Long? ) : DealCustomFieldValue() data class NumberFieldValue( override val fieldId: Long?, override val value: String?, override val title: String?, override val order: Long?, override val type: String?, override val fieldValueId: Long? ) : DealCustomFieldValue() data class CurrencyFieldValue( override val fieldId: Long?, override val value: String?, override val title: String?, override val order: Long?, override val type: String?, override val fieldValueId: Long?, val fieldCurrency: String?, val currencyPosition: String? ) : DealCustomFieldValue() }

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fun mapCustomFieldResponse(field: DealCustomFieldValue): StandardFieldData { return field.run { when (this) { is DealCustomFieldValue.CurrencyFieldValue -> { createStandardField( title = title ?: "", body = currencyUtil.serverValueToDisplayValue( value?.toDouble(), currencyPosition, fieldCurrency ), leftIcon = R.drawable.ic_short_text ) } is DealCustomFieldValue.NumberFieldValue -> createStandardField( title = title ?: "", body = value.toDoubleFormatTwoDecimalPlaces(), leftIcon = R.drawable.ic_short_text ) is DealCustomFieldValue.StandardFieldValue -> createStandardField( title = title ?: “", body = value ?: “", leftIcon = R.drawable.ic_short_text ) } } }

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when (this) { is DealCustomFieldValue.CurrencyFieldValue -> {} is DealCustomFieldValue.NumberFieldValue -> {} is DealCustomFieldValue.StandardFieldValue -> {} }

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when (this) { is DealCustomFieldValue.CurrencyFieldValue -> {} is DealCustomFieldValue.NumberFieldValue -> {} is DealCustomFieldValue.StandardFieldValue -> {} else -> {} }

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when (this) { is DealCustomFieldValue.CurrencyFieldValue -> {} is DealCustomFieldValue.NumberFieldValue -> {} is DealCustomFieldValue.StandardFieldValue -> {} else -> { /* not necessary */ } }

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when (this) { is DealCustomFieldValue.CurrencyFieldValue -> {} is DealCustomFieldValue.NumberFieldValue -> { createStandardField( title = title ?: "", body = value.toDoubleFormatTwoDecimalPlaces(), leftIcon = R.drawable.ic_short_text ) } is DealCustomFieldValue.StandardFieldValue -> {} }

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when (this) { is DealCustomFieldValue.CurrencyFieldValue -> {} is DealCustomFieldValue.NumberFieldValue -> { createStandardField( title = title ?: "", body = value.toDoubleFormatTwoDecimalPlaces(), leftIcon = R.drawable.ic_short_text ) } is DealCustomFieldValue.StandardFieldValue -> { createStandardField( title = title ?: “", body = value ?: “", leftIcon = R.drawable.ic_short_text ) } }

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when (this) { is DealCustomFieldValue.CurrencyFieldValue -> { createStandardField( title = title ?: "", body = currencyUtil.serverValueToDisplayValue( value?.toDouble(), currencyPosition, fieldCurrency ), leftIcon = R.drawable.ic_short_text ) } is DealCustomFieldValue.NumberFieldValue -> { createStandardField( title = title ?: "", body = value.toDoubleFormatTwoDecimalPlaces(), leftIcon = R.drawable.ic_short_text ) }

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Collections.kt val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Cats grouped by their age. cats.random() // Random Cat from the list. The Collections framework that ships with Kotlin makes working with collections a breeze. engel.dev

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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val cats = listOf() cats.map { it.age } // List of Ages. cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. Collections.kt Abbreviated.

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cats.filter { it.age > 4 } // List of Cats older than 4. cats.last { it.name == "Tina" } // Last Cat named Tina. cats.slice(5..10) // Cats within the range of 5 to 10. cats.takeLast(15) // The last 15 Cats in the list. cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. cats .filter { it.age > 4 } .random() // Random Cat, older than 4.

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// Random Cat, from the last 20 elements, older than 4. cats .filter { it.age > 4 } .takeLast(20) .random() cats.groupBy { it.age } // Group Cats by their age. cats.random() // Random Cat from the list. cats .filter { it.age > 4 } .random() // Random Cat, older than 4.

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// Random Cat, from the last 20 elements, older than 4. cats .filter { it.age > 4 } .takeLast(20) .random() // Random Cat, older than 4, from the last 20 elements. cats .takeLast(20) .filter { it.age > 4 } .random()

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// Random Cat, from the last 20 elements, older than 4. cats .filter { it.age > 4 } .takeLast(20) .random() cats .takeLast(20) // New list of 20 cats. .filter { it.age > 4 } // New list containing only cats older than 4. .random()

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.takeLast(20) // New list of 20 cats. .filter { it.age > 4 } // New list containing only cats older than 4. .random() cats.reversed().asSequence() .take(20) .filter { it.age > 4 } .toList() .random()

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.takeLast(20) // New list of 20 cats. .filter { it.age > 4 } // New list containing only cats older than 4. .random() cats.reversed().asSequence() .take(20) .filter { it.age > 4 } .toList() .random()

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.takeLast(20) // New list of 20 cats. .filter { it.age > 4 } // New list containing only cats older than 4. .random() cats.reversed().asSequence() .take(20) .filter { it.age > 4 } .toList() .random()

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.takeLast(20) // New list of 20 cats. .filter { it.age > 4 } // New list containing only cats older than 4. .random() cats.reversed().asSequence() .take(20) .filter { it.age > 4 } .toList() .random()

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cats.reversed().asSequence() .take(20) .filter { it.age > 4 } .toList() .random() sealed class FieldValue { abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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abstract val value: T data class NumericField(override val value: Long) : FieldValue() data class MoneyField(override val value: Double) : FieldValue() data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues }

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data class TextField(override val value: String) : FieldValue() data class MultiSelect(override val value: List) : FieldValue>() } val fieldValues = listOf() val fieldGroup = mutableMapOf>() for (fieldValue in fieldValues) { val fieldGroupValues = fieldGroup[fieldValue::class] ?: mutableListOf() fieldGroupValues.add(fieldValue) fieldGroup[fieldValue::class] = fieldGroupValues } val fieldValues = listOf() val fieldGroup = fieldValues.groupBy { it::class }

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Extensions editText.setTextIfChanged(“blah") view.verticalPositionInParent moshi.toJson(EmailBody("subject", “body")) activeCampaignAnalytics.markComplete() intent.conversationUri In Java extending functionality on concrete classes require static Util classes. Kotlin allows us to add functionality to concrete classes through extensions. engel.dev

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editText.setTextIfChanged(“blah") view.verticalPositionInParent moshi.toJson(EmailBody("subject", “body")) activeCampaignAnalytics.markComplete() intent.conversationUri

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fun TextView.setTextIfChanged(text: CharSequence) { if (this.text.toString() != text) { setTextKeepState(text) } } editText.setTextIfChanged(“blah”) view.verticalPositionInParent moshi.toJson(EmailBody("subject", “body")) activeCampaignAnalytics.markComplete() intent.conversationUri

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editText.setTextIfChanged(“blah”) val View.verticalPositionInParent: Int get() { val offsetViewBounds = Rect() parentAsViewGroup .offsetDescendantRectToMyCoords( this, offsetViewBounds ) return offsetViewBounds.top } view.verticalPositionInParent moshi.toJson(EmailBody("subject", “body")) activeCampaignAnalytics.markComplete()

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.offsetDescendantRectToMyCoords( this, offsetViewBounds ) return offsetViewBounds.top } view.verticalPositionInParent inline fun Moshi.toJson(value: T): String { val adapter: JsonAdapter = adapter(T::class.java) return adapter.toJson(value) } moshi.toJson(EmailBody("subject", “body”)) activeCampaignAnalytics.markComplete() intent.conversationUri

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view.verticalPositionInParent inline fun Moshi.toJson(value: T): String { val adapter: JsonAdapter = adapter(T::class.java) return adapter.toJson(value) } moshi.toJson(EmailBody("subject", “body”)) fun ActiveCampaignAnalytics.markComplete() { sendEvent(completeConversationEvent) } activeCampaignAnalytics.markComplete() intent.conversationUri

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view.verticalPositionInParent inline fun Moshi.toJson(value: T): String { val adapter: JsonAdapter = adapter(T::class.java) return adapter.toJson(value) } moshi.toJson(EmailBody("subject", “body”)) fun ActiveCampaignAnalytics.markComplete() { sendEvent(completeConversationEvent) } activeCampaignAnalytics.markComplete() private val Intent.conversationUri: String get() = getStringExtra(EXTRA_CONVERSATION_URI) intent.conversationUri

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Class Delegation abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() { Delegation allows for code reuse in a manner that prefers composition over inheritance. engel.dev

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() {}

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() { override fun addDisposable(disposable: Disposable) {} override fun addDisposables(vararg disposables: Disposable) {} override fun clearDisposables() {} }

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() { override fun addDisposable(disposable: Disposable) { compositeDisposable.add(disposable) } override fun addDisposables(vararg disposables: Disposable) { compositeDisposable.addAll(*disposables) } override fun clearDisposables() { compositeDisposable.clear() } }

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() {} class DisposableHandlerReal : DisposableHandler { private val compositeDisposable = CompositeDisposable() override fun addDisposable(disposable: Disposable) { compositeDisposable.add(disposable) } override fun addDisposables(vararg disposables: Disposable) { compositeDisposable.addAll(*disposables) } override fun clearDisposables() { compositeDisposable.clear() } }

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abstract class AbstractViewModel : ViewModel(), DisposableHandler by DisposableHandlerReal() class DisposableHandlerReal : DisposableHandler { private val compositeDisposable = CompositeDisposable() override fun addDisposable(disposable: Disposable) { compositeDisposable.add(disposable) } override fun addDisposables(vararg disposables: Disposable) { compositeDisposable.addAll(*disposables) } override fun clearDisposables() { compositeDisposable.clear() } }

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() {}

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() { override fun showKeyboard(context: Context) {} override fun dismissKeyboard(context: Context, view: View) {} }

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() { override fun showKeyboard(context: Context) { val inputMethodManager = context .getSystemService(Context.INPUT_METHOD_SERVICE) inputMethodManager.toggleSoftInput( InputMethodManager.SHOW_FORCED, InputMethodManager.HIDE_IMPLICIT_ONLY ) } override fun dismissKeyboard(context: Context, view: View) { val inputMethodManager = context .getSystemService(Context.INPUT_METHOD_SERVICE) inputMethodManager.hideSoftInputFromWindow(view.windowToken, 0) } }

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abstract class AbstractActivity : AppCompatActivity(), DisposableHandler by DisposableHandlerReal(), KeyboardHandler by KeyboardHandlerReal() {} class KeyboardHandlerReal : KeyboardHandler { override fun showKeyboard(context: Context) { val inputMethodManager = context .getSystemService(Context.INPUT_METHOD_SERVICE) inputMethodManager.toggleSoftInput( InputMethodManager.SHOW_FORCED, InputMethodManager.HIDE_IMPLICIT_ONLY ) } override fun dismissKeyboard(context: Context, view: View) { val inputMethodManager = context .getSystemService(Context.INPUT_METHOD_SERVICE) inputMethodManager.hideSoftInputFromWindow(view.windowToken, 0) } }

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class AbstractFragment : Fragment(), KeyboardHandler by KeyboardHandlerReal() {} class KeyboardHandlerReal : KeyboardHandler { override fun showKeyboard(context: Context) { val inputMethodManager = context .getSystemService(Context.INPUT_METHOD_SERVICE) inputMethodManager.toggleSoftInput( InputMethodManager.SHOW_FORCED, InputMethodManager.HIDE_IMPLICIT_ONLY ) } override fun dismissKeyboard(context: Context, view: View) { val inputMethodManager = context .getSystemService(Context.INPUT_METHOD_SERVICE) inputMethodManager.hideSoftInputFromWindow(view.windowToken, 0) } }

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class Example { var property: String by Delegate() } Property Delegation

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class Example { var property: String by Delegate() } class Delegate { operator fun getValue(thisRef: Any?, property: KProperty<*>): String operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) }

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class Example { var property: String by Delegate() } class Delegate { operator fun getValue(thisRef: Any?, property: KProperty<*>): String { return "$thisRef, thank you for delegating '${property.name}' to me!" } operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) { println("$value has been assigned to '${property.name}' in $thisRef.") } } class Delegate { operator fun getValue(thisRef: Any?, property: KProperty<*>): String operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) }

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val lazyInitialization by lazy { """ This String Is Created When First Needed Then Cached And Reused In The Future """.trimIndent() } operator fun getValue(thisRef: Any?, property: KProperty<*>): String { return "$thisRef, thank you for delegating '${property.name}' to me!" } operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) { println("$value has been assigned to '${property.name}' in $thisRef.") } }

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""".trimIndent() } /* * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license * that can be found in the license/LICENSE.txt file. */ @file:kotlin.jvm.JvmName("LazyKt") @file:kotlin.jvm.JvmMultifileClass package kotlin /** * Creates a new instance of the [Lazy] that uses the specified initialization function [initializer] * and the default thread-safety mode [LazyThreadSafetyMode.SYNCHRONIZED]. * * If the initialization of a value throws an exception, it will attempt to reinitialize the value at next access. * * Note that the returned instance uses itself to synchronize on. Do not synchronize from external code on

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* If the initialization of a value throws an exception, it will attempt to reinitialize the value at next access. * * Note that the returned instance uses itself to synchronize on. Do not synchronize from external code on * the returned instance as it may cause accidental deadlock. Also this behavior can be changed in the future. */ public actual fun lazy(initializer: () -> T): Lazy = SynchronizedLazyImpl(initializer) /** * Creates a new instance of the [Lazy] that uses the specified initialization function [initializer] * and thread-safety [mode]. * * If the initialization of a value throws an exception, it will attempt to reinitialize the value at next access. * * Note that when the [LazyThreadSafetyMode.SYNCHRONIZED] mode is specified the returned instance uses itself * to synchronize on. Do not synchronize from external code on the returned instance as it may cause accidental deadlock.

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*/ public actual fun lazy(lock: Any?, initializer: () -> T): Lazy = SynchronizedLazyImpl(initializer, lock) private class SynchronizedLazyImpl(initializer: () -> T, lock: Any? = null) : Lazy, Serializable { private var initializer: (() -> T)? = initializer @Volatile private var _value: Any? = UNINITIALIZED_VALUE // final field is required to enable safe publication of constructed instance private val lock = lock ?: this override val value: T get() { val _v1 = _value if (_v1 !== UNINITIALIZED_VALUE) { @Suppress("UNCHECKED_CAST") return _v1 as T } return synchronized(lock) { val _v2 = _value if (_v2 !== UNINITIALIZED_VALUE) {

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return newValue } } @Suppress("UNCHECKED_CAST") return _value as T } override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE override fun toString(): String = if (isInitialized()) value.toString() else "Lazy value not initialized yet." private fun writeReplace(): Any = InitializedLazyImpl(value) companion object { private val valueUpdater = java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater( SafePublicationLazyImpl::class.java, Any::class.java, "_value" ) } }

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Higher Order Functions fun randomerNumber(randomNumber: () -> Long): () -> Long { return { randomNumber() * randomNumber() } } randomerNumber { Random.nextLong() }() Mostly used for passing a function into another function. However you can return a function from a function too. engel.dev

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Sandwich Code The why behind Higher Order Functions.

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Boring Boring

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Boring Boring Interesting

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val startTime = System.currentTimeMillis() for (i in 0..100000) { println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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val startTime = System.currentTimeMillis() for (i in 0..100000) { println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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val startTime = System.currentTimeMillis() for (i in 0..100000) { println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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val startTime = System.currentTimeMillis() for (i in 0..100000) { println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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val startTime = System.currentTimeMillis() for (i in 0..100000) { println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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fun benchmark(blockOfCode: () -> Unit): Long { val startTime = System.currentTimeMillis() blockOfCode.invoke() val endTime = System.currentTimeMillis() return endTime - startTime } println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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fun benchmark(blockOfCode: () -> Unit): Long { val startTime = System.currentTimeMillis() blockOfCode.invoke() val endTime = System.currentTimeMillis() return endTime - startTime } println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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fun benchmark(blockOfCode: () -> Unit): Long { val startTime = System.currentTimeMillis() blockOfCode.invoke() val endTime = System.currentTimeMillis() return endTime - startTime } println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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fun benchmark(blockOfCode: () -> Unit): Long { val startTime = System.currentTimeMillis() blockOfCode.invoke() val endTime = System.currentTimeMillis() return endTime - startTime } println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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fun benchmark(blockOfCode: () -> Unit): Long { val startTime = System.currentTimeMillis() blockOfCode() val endTime = System.currentTimeMillis() return endTime - startTime } println("Cat: ${Cat()}") } val endTime = System.currentTimeMillis() val runtime = endTime - startTime

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benchmark({ for (i in 0..100000) { println("Cat: ${Cat()}") } } val endTime = System.currentTimeMi return endTime - startTime }

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benchmark { for (i in 0..100000) { println("Cat: ${Cat()}") } } val endTime = System.currentTimeMi return endTime - startTime }

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fun generateCats() { for (i in 0..100000) { println("Cat: ${Cat()}") } } benchmark(::generateCats) } }

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fun generateCats() { for (i in 0..100000) { println("Cat: ${Cat()}") } } benchmark(::generateCats) } }

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fun generateCats() { for (i in 0..100000) { println("Cat: ${Cat()}") } } benchmark(::generateCats) } }

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interface MutableListViewModel : ListViewModel { var dataSetChanges: ((change: DataSetChange) -> Unit)? fun addItem(item: T) fun moveItem(item: T, newPosition: Int) fun removeItem(item: T) }

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interface MutableListViewModel : ListViewModel { var dataSetChanges: ((change: DataSetChange) -> Unit)? fun addItem(item: T) fun moveItem(item: T, newPosition: Int) fun removeItem(item: T) }

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class MutableListViewModelDelegate( initialItems: List = emptyList() ) : MutableListViewModel { private val items = mutableListOf() .apply { addAll(initialItems) } override var dataSetChanges: ((change: DataSetChange) -> Unit)? = null override val itemCount: Int = items.size override fun get(position: Int): T = items[position] override fun addItem(item: T) { items.add(item) dataSetChanges?.invoke(DataSetChange.Inserted(items.size, 1)) } override fun moveItem(item: T, newPosition: Int) { val from = items.indexOf(item) items.removeAt(from) items.add(newPosition, item) dataSetChanges?.invoke(DataSetChange.Moved(from, newPosition)) } override fun removeItem(item: T) { val index = items.indexOf(item) items.removeAt(index) dataSetChanges?.invoke(DataSetChange.Removed(index, 1)) } }

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class MutableListViewModelDelegate( initialItems: List = emptyList() ) : MutableListViewModel { private val items = mutableListOf() .apply { addAll(initialItems) } override var dataSetChanges: ((change: DataSetChange) -> Unit)? = null override val itemCount: Int = items.size override fun get(position: Int): T = items[position] override fun addItem(item: T) { items.add(item) dataSetChanges?.invoke(DataSetChange.Inserted(items.size, 1)) } override fun moveItem(item: T, newPosition: Int) { val from = items.indexOf(item) items.removeAt(from) items.add(newPosition, item) dataSetChanges?.invoke(DataSetChange.Moved(from, newPosition)) } override fun removeItem(item: T) { val index = items.indexOf(item) items.removeAt(index) dataSetChanges?.invoke(DataSetChange.Removed(index, 1)) } }

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class MutableListViewModelDelegate( initialItems: List = emptyList() ) : MutableListViewModel { override var dataSetChanges: ((change: DataSetChange) -> Unit)? = null override fun addItem(item: T) { dataSetChanges?.invoke(DataSetChange.Inserted(items.size, 1)) } override fun moveItem(item: T, newPosition: Int) { dataSetChanges?.invoke(DataSetChange.Moved(from, newPosition)) } override fun removeItem(item: T) { dataSetChanges?.invoke(DataSetChange.Removed(index, 1)) } }

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sealed class DataSetChange { object All : DataSetChange() data class Changed(val positionStart: Int, val itemCount: Int = 1) : DataSetChange() data class Inserted(val positionStart: Int, val itemCount: Int = 1) : DataSetChange() data class Moved(val from: Int, val to: Int) : DataSetChange() data class Removed(val positionStart: Int, val itemCount: Int = 1) : DataSetChange() }

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abstract class ListViewModelAdapter> : RecyclerView.Adapter() { protected abstract val listViewModel: ListViewModel val dataSetChanges: (change: DataSetChange) -> Unit = { when (it) { is DataSetChange.Inserted -> notifyItemRangeInserted(it.positionStart, it.itemCount) is DataSetChange.Moved -> notifyItemMoved(it.from, it.to) is DataSetChange.Removed -> notifyItemRangeRemoved(it.positionStart, it.itemCount) is DataSetChange.Changed -> notifyItemRangeChanged(it.positionStart, it.itemCount) is DataSetChange.All -> notifyDataSetChanged() } } override fun getItemCount(): Int = listViewModel.itemCount override fun onBindViewHolder(holder: VH, position: Int) { holder.bind(listViewModel[position]) } }

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abstract class ListViewModelAdapter> : RecyclerView.Adapter() { protected abstract val listViewModel: ListViewModel val dataSetChanges: (change: DataSetChange) -> Unit = { when (it) { is DataSetChange.Inserted -> notifyItemRangeInserted(it.positionStart, it.itemCount) is DataSetChange.Moved -> notifyItemMoved(it.from, it.to) is DataSetChange.Removed -> notifyItemRangeRemoved(it.positionStart, it.itemCount) is DataSetChange.Changed -> notifyItemRangeChanged(it.positionStart, it.itemCount) is DataSetChange.All -> notifyDataSetChanged() } } override fun getItemCount(): Int = listViewModel.itemCount override fun onBindViewHolder(holder: VH, position: Int) { holder.bind(listViewModel[position]) } }

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abstract class ListViewModelAdapter> : RecyclerView.Adapter() { val dataSetChanges: (change: DataSetChange) -> Unit = { when (it) { is DataSetChange.Inserted -> notifyItemRangeInserted(it.positionStart, it.itemCount) is DataSetChange.Moved -> notifyItemMoved(it.from, it.to) is DataSetChange.Removed -> notifyItemRangeRemoved(it.positionStart, it.itemCount) is DataSetChange.Changed -> notifyItemRangeChanged(it.positionStart, it.itemCount) is DataSetChange.All -> notifyDataSetChanged() } } }

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viewModelStub.dataSetChanges = adapter.dataSetChanges

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Unit Testing If you don’t want to ship Kotlin to production, then start with your CI/CD process. class LoginViewModelTest : ViewModelTest() { @MockK private lateinit var authenticationRepository: Authenti @MockK private lateinit var stringLoader: StringLoader @MockK private lateinit var telemetry: Telemetry @RelaxedMockK private lateinit var analytics: ActiveCampaignAnalytics private lateinit var subject: LoginViewModel private val viewStateAssertion = ViewStateAssertion( private val textChange by lazy { randomString() } private val errorMessage by lazy { randomString() } private val genericErrorMessage by lazy { randomString( private val invalidRequestErrorMessage by lazy { random private val noPermissionsErrorMessage by lazy { randomS engel.dev

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class LoginViewModelTest : ViewModelTest() { @MockK private lateinit var authenticationRepository: AuthenticationRepository @MockK private lateinit var stringLoader: StringLoader @MockK private lateinit var telemetry: Telemetry @RelaxedMockK private lateinit var analytics: ActiveCampaignAnalytics private lateinit var subject: LoginViewModel private val viewStateAssertion = ViewStateAssertion() private val accountTextChanges by lazy { EventObservable() } private val usernameTextChanges by lazy { EventObservable() } private val passwordTextChanges by lazy { EventObservable() } private val domainTextChanges by lazy { EventObservable() } private val loginClicks by lazy { EventObservable() } private val textChange by lazy { randomString() } private val errorMessage by lazy { randomString() } private val genericErrorMessage by lazy { randomString() } private val invalidRequestErrorMessage by lazy { randomString() } private val noPermissionsErrorMessage by lazy { randomString() } private val accountExpiredErrorMessage by lazy { randomString() } private val trace by lazy { mockk() }

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ss LoginViewModelTest : ViewModelTest() { @MockK private lateinit var authenticationRepository: AuthenticationRepository @MockK private lateinit var stringLoader: StringLoader @MockK private lateinit var telemetry: Telemetry @RelaxedMockK private lateinit var analytics: ActiveCampaignAnalytics private lateinit var subject: LoginViewModel private val viewStateAssertion = ViewStateAssertion() private val accountTextChanges by lazy { EventObservable() } private val usernameTextChanges by lazy { EventObservable() } private val passwordTextChanges by lazy { EventObservable() } private val domainTextChanges by lazy { EventObservable() } private val loginClicks by lazy { EventObservable() } private val textChange by lazy { randomString() } private val errorMessage by lazy { randomString() } private val genericErrorMessage by lazy { randomString() } private val invalidRequestErrorMessage by lazy { randomString() } private val noPermissionsErrorMessage by lazy { randomString() } private val accountExpiredErrorMessage by lazy { randomString() } private val trace by lazy { mockk() } MockKAnnotations.init(this)

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private val viewStateAssertion = ViewStateAssertion() private val accountTextChanges by lazy { EventObservable() } private val usernameTextChanges by lazy { EventObservable() } private val passwordTextChanges by lazy { EventObservable() } private val domainTextChanges by lazy { EventObservable() } private val loginClicks by lazy { EventObservable() } private val textChange by lazy { randomString() } private val errorMessage by lazy { randomString() } private val genericErrorMessage by lazy { randomString() } private val invalidRequestErrorMessage by lazy { randomString() } private val noPermissionsErrorMessage by lazy { randomString() } private val accountExpiredErrorMessage by lazy { randomString() } private val trace by lazy { mockk() } private fun prepareTelemetry() { every { telemetry.startTrace(any()) } returns trace every { trace.putAttribute(any(), any()) } just Runs every { telemetry.endTrace(trace) } just Runs }

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private val viewStateAssertion = ViewStateAssertion() private val accountTextChanges by lazy { EventObservable() } private val usernameTextChanges by lazy { EventObservable() } private val passwordTextChanges by lazy { EventObservable() } private val domainTextChanges by lazy { EventObservable() } private val loginClicks by lazy { EventObservable() } private val textChange by lazy { randomString() } private val errorMessage by lazy { randomString() } private val genericErrorMessage by lazy { randomString() } private val invalidRequestErrorMessage by lazy { randomString() } private val noPermissionsErrorMessage by lazy { randomString() } private val accountExpiredErrorMessage by lazy { randomString() } private val trace by lazy { mockk() } private fun prepareTelemetry() { every { telemetry.startTrace(any()) } returns trace every { trace.putAttribute(any(), any()) } just Runs every { telemetry.endTrace(trace) } just Runs }

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private val viewStateAssertion = ViewStateAssertion() private val accountTextChanges by lazy { EventObservable() } private val usernameTextChanges by lazy { EventObservable() } private val passwordTextChanges by lazy { EventObservable() } private val domainTextChanges by lazy { EventObservable() } private val loginClicks by lazy { EventObservable() } private val textChange by lazy { randomString() } private val errorMessage by lazy { randomString() } private val genericErrorMessage by lazy { randomString() } private val invalidRequestErrorMessage by lazy { randomString() } private val noPermissionsErrorMessage by lazy { randomString() } private val accountExpiredErrorMessage by lazy { randomString() } private val trace by lazy { mockk() } private fun prepareTelemetry() { every { telemetry.startTrace(any()) } returns trace every { trace.putAttribute(any(), any()) } just Runs every { telemetry.endTrace(trace) } just Runs }

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= ViewStateAssertion() by lazy { EventObservable() } s by lazy { EventObservable() } s by lazy { EventObservable() } by lazy { EventObservable() } y { EventObservable() } { randomString() } zy { randomString() } e by lazy { randomString() } rMessage by lazy { randomString() } Message by lazy { randomString() } rMessage by lazy { randomString() } ckk() } { ce(any()) } returns trace (any(), any()) } just Runs (trace) } just Runs /** * Generates a random string in the form of a [UUID]. */ fun randomString() = UUID.randomUUID().toString()

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@Test fun `initial state should have loginSuccessful set to false`() { viewStateAssertion.assertFirstViewState { assertThat(it.loginSuccessful).isFalse() } } @Test fun `initial state should have loginEnabled set to false`() { viewStateAssertion.assertFirstViewState { assertThat(it.loginEnabled).isFalse() } } @Test fun `initial state should have loginForm set to empty values`() { viewStateAssertion.assertFirstViewState { assertThat(it.loginForm).isEqualTo(LoginViewModel.LoginForm()) } }

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@Test fun `initial state should have loginSuccessful set to false`() { viewStateAssertion.assertFirstViewState { assertThat(it.loginSuccessful).isFalse() } } @Test fun `initial state should have loginEnabled set to false`() { viewStateAssertion.assertFirstViewState { assertThat(it.loginEnabled).isFalse() } } @Test fun `initial state should have loginForm set to empty values`() { viewStateAssertion.assertFirstViewState { assertThat(it.loginForm).isEqualTo(LoginViewModel.LoginForm()) } }

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statefulModel.state.removeObserver(currentObserver!!) } currentObserver = null } fun assertLastViewState(assertion: (actualState: S) -> Unit) { assertViewStateSeries { actualStates -> assertion(actualStates.last()) } } fun assertFirstViewState(assertion: (actualState: S) -> Unit) { assertViewStateSeries { actualStates -> assertion(actualStates.first()) } } fun assertViewStateSeries(assertion: (actualStates: List) -> Unit) { assertion(viewStates) } }

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statefulModel.state.removeObserver(currentObserver!!) } currentObserver = null } fun assertLastViewState(assertion: (actualState: S) -> Unit) { assertViewStateSeries { actualStates -> assertion(actualStates.last()) } } fun assertFirstViewState(assertion: (actualState: S) -> Unit) { assertViewStateSeries { actualStates -> assertion(actualStates.first()) } } fun assertViewStateSeries(assertion: (actualStates: List) -> Unit) { assertion(viewStates) } }

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class ViewStateAssertion { private val viewStates = mutableListOf() private var currentObserver: Observer? = null private lateinit var statefulModel: StatefulModel val numberOfStates: Int get() = viewStates.size fun start(statefulModel: StatefulModel) { this.statefulModel = statefulModel currentObserver = Observer { viewStates.add(it!!) } try { viewStates.clear() } catch (ex: Exception) { Assert.fail("Unable to complete ViewStateAssertion.start(). The followin } this.statefulModel.state.observeForever(currentObserver!!) }

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@Test fun `accountTextChanges should update the loginForm with new account value`() { prepareForFormInteractions() accountTextChanges.updateValue(textChange) viewStateAssertion.assertLastViewState { assertThat(it.loginForm.account).isEqualTo(textChange) } } @Test fun `usernameTextChanges should update the loginForm with new username value`() { prepareForFormInteractions() usernameTextChanges.updateValue(textChange) viewStateAssertion.assertLastViewState { assertThat(it.loginForm.username).isEqualTo(textChange) } }

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} this.statefulModel.state.observeForever(currentObserver!!) } fun end() { if (currentObserver != null) { statefulModel.state.removeObserver(currentObserver!!) } currentObserver = null } fun assertLastViewState(assertion: (actualState: S) -> Unit) { assertViewStateSeries { actualStates -> assertion(actualStates.last()) } } fun assertFirstViewState(assertion: (actualState: S) -> Unit) { assertViewStateSeries { actualStates -> assertion(actualStates.first()) } }

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@Test fun `accountTextChanges should update the loginForm with new account value`() { prepareForFormInteractions() accountTextChanges.updateValue(textChange) viewStateAssertion.assertLastViewState { assertThat(it.loginForm.account).isEqualTo(textChange) } } @Test fun `usernameTextChanges should update the loginForm with new username value`() { prepareForFormInteractions() usernameTextChanges.updateValue(textChange) viewStateAssertion.assertLastViewState { assertThat(it.loginForm.username).isEqualTo(textChange) } }

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Looking Ahead Kotlin is just getting started.

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Coroutines It’s like multi-threading without overhead and imperative.

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Contracts What if your functions could provide visibility into what they infer internally?

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Inline Classes Give meaning to things like Strings and Ints without overhead.

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Kotlin/Native Kotlin isn’t bound to the JVM.

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Questions? Let’s talk after or message me on engel.dev