Kotlin Delegates in practice - Kotlin community conf

Kotlin delegates
in practice
Fabio Collini
@fabioCollini

“”
“The ratio of time spent reading (code)

versus writing is well over 10 to 1

(therefore) making it easy to read

makes it easier to write

Robert C. Martin

“”
“The Principle of Least Astonishment states that

the result of performing some operation should be
obvious, consistent, and predictable, based upon
the name of the operation and other clues

https://wiki.c2.com/?PrincipleOfLeastAstonishment

Kotlin code is readable
Data classes
Extension functions
Sealed classes
Coroutines
Delegates
…

public class MyEquivalentJavaClass {
private final String property = slowMethod();
public String getProperty() {
return property;
}5
}6
class MyClass {
val property = slowMethod()
}1

private String property = slowMethod();
return property;
}
public void setProperty(String var1) {
this.property = var1;
}5
}6
class MyClass {
var property = slowMethod()
}1

private String property = slowMethod();
}6
class MyClass {
private var property = slowMethod()
}1

return slowMethod();
}5
}6
class MyClass {
val property get() = slowMethod()
}1

class MyClass {
}1
fun main() {
val obj = MyClass()
println(obj.property)A
println(obj.property)B
}2

class MyClass {
}1
fun main() {
println("starting")
val obj = MyClass()
println("obj created")
println("end")
}2
starting
slowMethod invoked
obj created
end

class MyClass {
val property get() = slowMethod()
}1
fun main() {
println("starting")
val obj = MyClass()
println("end")
}2
starting
obj created
slowMethod invoked
slowMethod invoked
end

class MyClass {
val property by lazy {
slowMethod()
}3
}1
fun main() {
println("starting")
val obj = MyClass()
println("end")
}2
starting
obj created
slowMethod invoked
end

class MyClass {
val property by lazy {
slowMethod()
}3
}1

private SimpleLazy lazy = new SimpleLazy(::slowMethod);
return lazy.getValue();
}
}

internal object UNINITIALIZED_VALUE
class SimpleLazy<T>(private val initializer: () -> T) {
private var value: Any? = UNINITIALIZED_VALUE
fun getValue(): T {
if (value == UNINITIALIZED_VALUE) {
value = initializer()
}4
return value as T
}3
}2

/**
* Specifies how a Lazy instance synchronizes initialization among multiple threads.
*/
public enum class LazyThreadSafetyMode {
/**
* Locks are used to ensure that only a single thread can initialize the Lazy instance.
*/
SYNCHRONIZED,
/**
* Initializer function can be called several times on concurrent access to
* uninitialized Lazy instance value,
* but only the first returned value will be used as the value of Lazy instance.
*/
PUBLICATION,
/**
* No locks are used to synchronize an access to the Lazy instance value;
* if the instance is accessed from multiple threads, its behavior is undefined.
*
* This mode should not be used unless the Lazy instance is guaranteed never to
* be initialized from more than one thread.
*/
NONE
}
Default value

class MyClass {
val notAGoodIdea by lazy {
2 + 2
}3
}1

class MyClass {
suspend val property by lazy {
slowMethod()
}1
}2
suspend fun slowMethod() = withContext(IO) {
//...
}3
Modifier 'suspend' is not applicable to 'member property with delegate'
Suspend function 'slowMethod' should be called only from a coroutine or another suspend function

class MyFragment : Fragment() {
private val appName by lazy { getString(R.string.app_name) }1
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
println("Starting $appName...")
}2
}3
Lazy

private lateinit var appName: String
appName = getString(R.string.app_name)
}4
}5
lateinit

}4
}5
lateinit
private val appName by lazy { getString(R.string.app_name) }1
}2
}3
Lazy

private lateinit var title: String
private lateinit var summary: String
private lateinit var text: String
title = getString(R.string.title)
summary = getString(R.string.summary)
text = "$appNamen$titlen$summary"
println(text)
}4
}5
lateinit class MyFragment2 : Fragment() {
private val appName by lazy { getString(R.string.app_name) }
private val title by lazy { getString(R.string.title) }
private val summary by lazy { getString(R.string.summary) }
private val text by lazy { "$appNamen$titlen$summary" }
println(text)
}2
}3
Lazy

operator fun getValue(thisRef: Any, property: KProperty<*>): T

/**
* Base interface that can be used for implementing property delegates of read-only properties.
*1
* This is provided only for convenience; you don't have to extend this interface
* as long as your property delegate has methods with the same signatures.
*2
* @param R the type of object which owns the delegated property.
* @param T the type of the property value.
*/
interface ReadOnlyProperty<in R, out T> {
/**
* Returns the value of the property for the given object.
* @param thisRef the object for which the value is requested.
* @param property the metadata for the property.
* @return the property value.
*/
operator fun getValue(thisRef: R, property: KProperty<*>): T
}3

class LogDelegate<T>(initialValue: T) : ReadOnlyProperty<Any, T> {
private var value: T = initialValue
override fun getValue(thisRef: Any, property: KProperty<*>): T {
println("get invoked on $thisRef.${property.name}")
return value
}1
}2

return value
}1
}2
class MyClass {
val property by LogDelegate("ABC")
}3

return value
}1
}2
class MyClass {
val property by LogDelegate("ABC")
}3
fun main() {
val obj = MyClass()
println(obj.property)
}4
get invoked on MyClass@87aac27.property
ABC

/**
* Base interface that can be used for implementing property delegates of read-write properties.
*
* This is provided only for convenience; you don't have to extend this interface
* as long as your property delegate has methods with the same signatures.
*
* @param R the type of object which owns the delegated property.
* @param T the type of the property value.
*/
interface ReadWriteProperty<in R, T> {
/**
* Returns the value of the property for the given object.
* @return the property value.
*/
operator fun getValue(thisRef: R, property: KProperty<*>): T
/**
* Sets the value of the property for the given object.
* @param value the value to set.
*/
operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
}

class LogDelegate<T>(initialValue: T) : ReadWriteProperty<Any, T> {
return value
}1
override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
println("set invoked on $thisRef.${property.name} with value $value")
this.value = value
}5
}2
class MyClass {
var property by LogDelegate("ABC")
}3
fun main() {
val obj = MyClass()
obj.property = "DEF"
}4
get invoked on MyClass@e9e54c2.property
ABC
set invoked on MyClass@e9e54c2.property with value DEF
get invoked on MyClass@e9e54c2.property
DEF

class TokenHolder(private val prefs: SharedPreferences) {
val token: String?
get() = prefs.getString(TOKEN, null)
val count: Int
get() = prefs.getInt(COUNT, 0)
fun saveToken(newToken: String) {
prefs.edit {
putString(TOKEN, newToken)
putInt(COUNT, count + 1)
}1
}2
companion object {
private const val TOKEN = "token"
private const val COUNT = "count"
}3
}4

fun main() {
val tokenHolder = TokenHolder(sharedPreferences())
tokenHolder.saveToken("ABC")
println("${tokenHolder.token} - ${tokenHolder.count}")
tokenHolder.saveToken("DEF")
println("${tokenHolder.token} - ${tokenHolder.count}")
}
ABC - 1
DEF - 2

fun SharedPreferences.int() =
object : ReadWriteProperty<Any, Int> {
override fun getValue(thisRef: Any, property: KProperty<*>) =
getInt(property.name, 0)
override fun setValue(thisRef: Any, property: KProperty<*>, value: Int) =
edit { putInt(property.name, value) }
}1

fun SharedPreferences.int(key: String, defaultValue: Int = 0) =
getInt(key, defaultValue)
edit { putInt(key, value) }
}1

fun SharedPreferences.int(key: String, defaultValue: Int = 0) =
getInt(key, defaultValue)
edit { putInt(key, value) }
}1
fun SharedPreferences.string(key: String, defaultValue: String? = null) =
object : ReadWriteProperty<Any, String?> {
getString(key, defaultValue)
override fun setValue(thisRef: Any, property: KProperty<*>, value: String?) =
edit { putString(key, value) }
}2

val token: String?
get() = prefs.getString(TOKEN,0null)1
val count: Int
prefs.edit {
}1
}2
companion object {
}3
}4

var token by prefs.string(TOKEN)1
private3set
val count: Int
prefs.edit {
}1
}2
companion object {
}3
}4

var token by prefs.string(TOKEN)1
private set
val count: Int
token = newToken
prefs.edit {
}1
}2
companion object {
}3
}4

var token by prefs.string("token")1
private set
val count: Int
get() = prefs.getInt(COUNT, 0)2
token = newToken
prefs.edit {
}1
}2
companion object {
}3
}4

var token by prefs.string("token")
private set
var count by prefs.int(COUNT)2
private set
token = newToken
prefs.edit {
}1
}2
companion object {
}3
}4

private set
var count by prefs.int(COUNT)2
private set
token = newToken
count++
}2
companion object {
}3
}4

private set
var count by prefs.int("count")2
private set
token = newToken
count++
}2
}4

class TokenHolder(prefs: SharedPreferences) {
private set
var count by prefs.int("count")
private set
token = newToken
count++
}2
}4
prefs.edit {
putInt("count", prefs.getInt("count", 0) + 1)
}

class DemoFragment : Fragment() {
private val component by lazy {
//...
}
private val viewModel by viewModelProvider {
component.myViewModel()
}
//...
}
https://proandroiddev.com/kotlin-delegates-in-android-development-part-2-2c15c11ﬀ438

class DemoFragment : Fragment() {
private var param1: Int by argument()
private var param2: String by argument()
companion object {
fun newInstance(param1: Int, param2: String): DemoFragment =
DemoFragment().apply {
this.param1 = param1
this.param2 = param2
}
}
}
https://proandroiddev.com/kotlin-delegates-in-android-1ab0a715762d

object AnalyticsLib {
fun trackEvent(event: Map<String, Any?>) {
println(event)
}1
}2
fun main() {
val event = mapOf(
"name" to "myEvent",
"value" to 123
)3
AnalyticsLib.trackEvent(event)
}4

fun main() {
val event = mapOf(
"name" to "myEvent",
"value" to 123
)3
}4

const val NAME = "name"
const val VALUE = "value"
fun main() {
val event = mapOf(
NAME to "myEvent",
VALUE to 123
)3
}4

const val NAME = "name"
const val VALUE = "value"
fun main() {
val event = mapOf(
NAME to "myEvent",
VALUE to "this should be an Int :("
)3
}4

class MyEvent {
val map: MutableMap<String, Any?> = mutableMapOf()
var name: String by map
var value: Int by map
}1
fun main() {
val event = MyEvent().apply {
name = "myEvent"
value = 123
}2
AnalyticsLib.trackEvent(event.map)
}3

data class MyEvent(val name: String, val value: Int) {
val map = mapOf(
"name" to name,
"value" to value
)
}1
fun main() {
val event = MyEvent(
name = "myEvent",
value = 123
)2
AnalyticsLib.trackEvent(event.map)
}3

object AbTestLib {
fun readValues() = mapOf<String, Any?>(
"featureEnabled" to true,
"delay" to 1000
)1
}2

object AbTestLib {
fun readValues() = mapOf<String, Any?>(
"featureEnabled" to true,
"delay" to 1000
)1
}2
fun main() {
val values = AbTestLib.readValues()
println(values["featureEnabled"] as Boolean)
println(values["delay"] as Int)
}3
true
1000

data class AbValues(private val map: Map<String, Any?>) {
val featureEnabled: Boolean by map
val delay: Int by map
}1
fun main() {
val values = AbValues(AbTestLib.readValues())
println(values.featureEnabled)
println(values.delay)
}2

data class AbValues(private val map: Map<String, Any?>) {
val featureEnabled: Boolean by map
val delay: Int by map
}1
fun main() {
val values = AbValues(
mapOf<String, Any?>(
"featureEnabled" to "true",
"delay" to "1000"
)
)
println(values.featureEnabled)
println(values.delay)
}2
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean

class User {
var name: String by Delegates.observable("<no name>") { prop, old, new ->
println("$old -> $new")
}
}
class User {
var name: String by Delegates.vetoable("<no name>") { prop, old, new ->
new.startsWith("f")
}
}
ObservableVetoable

class MyClass {
var myIntVar: Int by notNull()
fun onCreate() {
myIntVar = calculateValue()
}
}
notNull

Inheritance
Composition
Delegation

Inheritance
open class Class1 {
fun doSomething() = 123
}1
class Class2 : Class1()

Composition
class Class1 {
}
class Class2 {
private val wrapped = Class1()
fun doSomething() = wrapped.doSomething()
}

Delegation
interface Interface1 {
fun doSomething(): Int
}4
class Class1 : Interface1 {
override fun doSomething() = 123
}5
class Class2(private val wrapped: Class1 = Class1())
: Interface1 by wrapped

Delegation
}4
}5
class Class2 : Interface1 by Class1()
11

Delegation
}4
}5
class Class2 : Interface1 by Class1()
fun main() {
val obj = Class2()
println(obj.doSomething())
}m
Composition
class Class1 {
}2
class Class2 {
private val wrapped = Class1()
fun doSomething() =
wrapped.doSomething()
}3
Inheritance
open class Class1 {
}1
class Class2 : Class1()

interface HasMargin {
val marginBottom: Int
val marginTop: Int
val marginLeft: Int
val marginRight: Int
}1
class Style(
val backgroundColor: Int,
override val marginBottom: Int,
override val marginTop: Int,
override val marginLeft: Int,
override val marginRight: Int
) : HasMargin
class View(style: Style) : HasMargin by style {
fun draw() {
//...
}
}

[
{
"type": "student",
"name": "studentName",
"surname": "studentSurname",
"age": 20,
"university": "universityName"
},
{
//...
}
]
data class PersonJson(
val type: String,
val name: String,
val surname: String,
val age: Int,
val university: String?,
val company: String?
)

abstract class Person(
val name: String,
val surname: String,
val age: Int
)
class Student(
name: String,
surname: String,
age: Int,
val university: String
) : Person(name, surname, age)
class Worker(
name: String,
surname: String,
age: Int,
val company: String
) : Person(name, surname, age)

fun main() {
val json: List<PersonJson> = listOf(/* ... */)
val people = json.map {
if (it.type == "student")
Student(
it.name,
it.surname,
it.age,
it.university!!
)
else
Worker(
it.name,
it.surname,
it.age,
it.company!!
)
}
println(people.joinToString { "${it.name} ${it.surname}" })
}

abstract class Person(
val_name: String,
val_surname: String,
val_age: Int
)
class Student(
name: String,
surname: String,
age: Int,
) : Person(name, surname, age)_
class Worker(
name: String,
surname: String,
age: Int,
val company: String
) : Person(name, surname, age)_

abstract class Person {
abstract val_name: String
abstract val_surname: String
abstract val_age: Int
}1
data class Student(
override val name: String,
override val surname: String,
override val age: Int,
) : Person()_
data class Worker(
val company: String
) : Person()_

interface Person {
val name: String
val surname: String
val age: Int
}1
data class Student(
) : Person
data class Worker(
val company: String
) : Person

interface Person {
val name: String
val surname: String
val age: Int
}1

interface Person {
val name: String
val surname: String
val age: Int
}1
data class PersonData(
override val age: Int
) : Person

data class Student(
) : Person
data class Worker(
val company: String
) : Person

data class Student(
) : Person
data class Worker(
val company: String
) : Person
data class Student(
val data: PersonData,
) : Person by data
data class Worker(
val company: String
) : Person by data

fun main() {
val data = PersonData(
it.name,
it.surname,
it.age
)
Student(data, it.university!!)
else
Worker(data, it.company!!)
}
}

interface Person {
val name: String
val surname: String
val age: Int
}
data class Student(
) : Person
data class Worker(
val company: String
) : Person
fun main() {
Student(
it.name,
it.surname,
it.age,
it.university!!
)
else
Worker(
it.name,
it.surname,
it.age,
it.company!!
)
}
}
interface Person {
val name: String
val surname: String
val age: Int
}
override val age: Int
) : Person
data class Student(
) : Person by data
data class Worker(
val company: String
) : Person by data
fun main() {
it.name,
it.surname,
it.age
)
else
}
}
DelegationInheritance

interface Person {
val name: String
val surname: String
val age: Int
val address: String
val city: String
val zipCode: String
val nation: String
val telephoneNumber1: String
}
data class Student(
val university: String,
override val address: String,
override val city: String,
override val zipCode: String,
override val nation: String,
override val telephoneNumber1: String,
override val telephoneNumber3: String
) : Person
data class Worker(
val company: String,
) : Person
data class Unemployed(
) : Person
fun main() {
Student(
it.name,
it.surname,
it.age,
it.university!!,
it.address,
it.city,
it.zipCode,
it.nation,
it.telephoneNumber1,
it.telephoneNumber3
)
else if (it.type == "worker")
Worker(
it.name,
it.surname,
it.age,
it.company!!,
it.address,
it.city,
it.zipCode,
it.nation,
it.telephoneNumber3
)
else
Unemployed(
it.name,
it.surname,
it.age,
it.address,
it.city,
it.zipCode,
it.nation,
it.telephoneNumber3
)
}
}
interface Person {
val name: String
val surname: String
val age: Int
val address: String
val city: String
val zipCode: String
val nation: String
}
) : Person
data class Student(
) : Person by data
data class Worker(
val company: String
) : Person by data
data class Unemployed(
val data: PersonData
) : Person by data
fun main() {
it.name,
it.surname,
it.age,
it.address,
it.city,
it.zipCode,
it.nation,
it.telephoneNumber3
)
else if (it.type == "worker")
else
Unemployed(data)
}
}
DelegationInheritance

Wrappingup
“”
“The ratio of time spent reading (code)

versus writing is well over 10 to 1

(therefore) making it easy to read

makes it easier to write

Robert C. Martin

Wrappingup
“”
“The Principle of Least Astonishment states that

the result of performing some operation should be
obvious, consistent, and predictable, based upon
the name of the operation and other clues

https://wiki.c2.com/?PrincipleOfLeastAstonishment

Wrappingup
Delegates can be useful to simplify code
but there are pros and cons!

Links&contacts
Simpler Kotlin class hierarchies using class delegation
proandroiddev.com/simpler-kotlin-class-hierarchies-using-class-delegation-35464106fed5
Kotlin delegates in Android development — Part 1
medium.com/hackernoon/kotlin-delegates-in-android-development-part-1-50346cf4aed7
Kotlin delegates in Android development — Part 2
proandroiddev.com/kotlin-delegates-in-android-development-part-2-2c15c11ff438
@fabioCollini
linkedin.com/in/fabiocollini
github.com/fabioCollini
medium.com/@fabioCollini

Kotlin Delegates in practice - Kotlin community conf

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