Delightful Delegate Design (Kotlin Budapest User Group meetup - April)

When developing a library, designing an easy to use API while hiding unnecessary implementation details from clients is fundamental. This talk and article looks at some of the API design choices we’ve made for our library Krate, an Android SharedPreferences wrapper.

Talk recording: https://www.youtube.com/watch?v=jTmHNo48zs0

Transcript

1. Delightful Delegate Design
   Braun Márton Szabolcs
   zsmb.co zsmb13
   braun.marton@autsoft.hu
   

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3. val oneToOne: Pair = 1 to "one"
   

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4. with(user) {
   println(name)
   }
   val oneToOne: Pair = 1 to "one"
   

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5. val reader = BufferedReader(...)
   reader.use {
   val line = it.readLine()
   // ...
   }
   with(user) {
   println(name)
   }
   val oneToOne: Pair = 1 to "one"
   

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6. val pi: Double by lazy {
   val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
   sqrt(sum * 6.0)
   }
   println(pi)
   println(pi)
   val reader = BufferedReader(...)
   reader.use {
   val line = it.readLine()
   // ...
   }
   with(user) {
   println(name)
   }
   val oneToOne: Pair = 1 to "one"
   

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7. val pi: Double by
   val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
   sqrt(sum * 6.0)
   println(pi)
   println(pi)
   lazy
   }
   {
   

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8. val pi: Double by
   val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
   sqrt(sum * 6.0)
   println(pi)
   println(pi)
   lazy
   }
   {
   https://forums.swift.org/t/pitch-property-delegates/21895
   

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9. val pi: Double by
   val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
   sqrt(sum * 6.0)
   println(pi)
   println(pi)
   LazyDelegate(
   })
   {
   

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10. class Constants {
    private var pi_: Double? = null
    val pi: Double
    get() {
    if (pi_ == null) {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    pi_ = sqrt(sum * 6.0)
    }
    return pi_!!
    }
    private var e_: Double? = null
    val e: Double
    get() {
    if (e_ == null) {
    val sum = (0..20).sumByDouble { 1.0 / (1..it).fold(1, { a, x -> a * x }) }
    e_ = sum
    }
    return e_!!
    }
    }
    

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11. class Constants {
    private var pi_: Double? = null
    val pi: Double
    get() {
    if (pi_ == null) {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    pi_ = sqrt(sum * 6.0)
    }
    return pi_!!
    }
    private var e_: Double? = null
    val e: Double
    get() {
    if (e_ == null) {
    val sum = (0..20).sumByDouble { 1.0 / (1..it).fold(1, { a, x -> a * x }) }
    e_ = sum
    }
    return e_!!
    }
    }
    

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12. class Constants {
    private var pi_: Double? = null
    val pi: Double
    get() {
    if (pi_ == null) {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    pi_ = sqrt(sum * 6.0)
    }
    return pi_!!
    }
    private var e_: Double? = null
    val e: Double
    get() {
    if (e_ == null) {
    val sum = (0..20).sumByDouble { 1.0 / (1..it).fold(1, { a, x -> a * x }) }
    e_ = sum
    }
    return e_!!
    }
    }
    

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13. class Constants {
    private var pi_: Double? = null
    val pi: Double
    get() {
    if (pi_ == null) {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    pi_ = sqrt(sum * 6.0)
    }
    return pi_!!
    }
    private var e_: Double? = null
    val e: Double
    get() {
    if (e_ == null) {
    val sum = (0..20).sumByDouble { 1.0 / (1..it).fold(1, { a, x -> a * x }) }
    e_ = sum
    }
    return e_!!
    }
    }
    

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14. val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi)
    println(pi)
    

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15. val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi)
    println(pi)
    class LazyDelegate(private val initializer: () -> T) {
    }
    

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16. val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi)
    println(pi)
    class LazyDelegate(private val initializer: () -> T) {
    }
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    }
    

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17. val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi)
    println(pi)
    class LazyDelegate(private val initializer: () -> T) {
    }
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    }
    private var value: T? = null
    

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18. class LazyDelegate(private val initializer: () -> T) {
    }
    val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi)
    println(pi)
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    }
    private var value: T? = null
    if (value == null) { value = initializer() }
    return value!!
    

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19. val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    class LazyDelegate(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    

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20. val pi: Double by LazyDelegate({
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    })
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    class LazyDelegate(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    

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21. val pi: Double by LazyDelegate {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    }
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    class LazyDelegate(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    

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22. val pi: Double by lazy {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    }
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    class LazyDelegate(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    

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23. val pi: Double by lazy {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    }
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    class lazy(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    

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24. val pi: Double by lazy {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    }
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    class LazyDelegate(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    

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25. fun lazy(initializer: () -> T) = LazyDelegate(initializer)
    class LazyDelegate(private val initializer: () -> T) {
    private var value: T? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
    if (value == null) { value = initializer() }
    return value!!
    }
    }
    val pi: Double by lazy {
    val sum = (1..50_000).sumByDouble { 1.0 / (it * it) }
    sqrt(sum * 6.0)
    }
    println(pi) // 12597400 ns
    println(pi) // 72100 ns
    

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26. https://github.com/AutSoft/Krate
    

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27. krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    

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28. krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    abstract class SimpleKrate(context: Context) : Krate {
    override val sharedPreferences =
    PreferenceManager.getDefaultSharedPreferences(context)
    }
    

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29. krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    abstract class SimpleKrate(context: Context) : Krate {
    override val sharedPreferences =
    PreferenceManager.getDefaultSharedPreferences(context)
    }
    app
    

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30. app
    krate
    class MyKrate(context: Context) : SimpleKrate(context) {
    var onboarded by booleanPref("onboarded")
    var appOpenCount by intPref("appOpenCount")
    var username by stringPref("username")
    }
    myKrate.onboarded = true
    myKrate.appOpenCount++
    myKrate.username = "t1gg3r"
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    abstract class SimpleKrate(context: Context) : Krate {
    override val sharedPreferences =
    PreferenceManager.getDefaultSharedPreferences(context)
    }
    

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31. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    

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32. krate
    app
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    

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33. krate
    app
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    }
    

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34. krate
    app
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    

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35. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    

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36. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    }
    

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37. krate
    app
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    }
    

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38. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    }
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    

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39. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    

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40. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by ???
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    

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41. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by IntDelegate(sharedPreferences, "score")
    }
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    }
    println(myKrate.score) // 0
    myKrate.score = 34
    println(myKrate.score) // 34
    

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42. app
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by IntDelegate(sharedPreferences, "score")
    }
    fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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43. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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44. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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45. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    interface Krate {
    val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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46. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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47. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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48. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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49. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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50. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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51. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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52. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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53. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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54. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Any?, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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55. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Krate, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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56. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Krate, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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57. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Int {
    return sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Krate, property: KProperty<*>, value: Int) {
    sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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58. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val sharedPreferences: SharedPreferences,
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Int {
    return thisRef.sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Krate, property: KProperty<*>, value: Int) {
    thisRef.sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(sharedPreferences, key)
    }
    

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59. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Int {
    return thisRef.sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Krate, property: KProperty<*>, value: Int) {
    thisRef.sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): IntDelegate {
    return IntDelegate(key)
    }
    

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60. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    var score: Int by intPref("score")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class IntDelegate(
    private val key: String
    ) : ReadWriteProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Int {
    return thisRef.sharedPreferences.getInt(key, 0)
    }
    override operator fun setValue(thisRef: Krate, property: KProperty<*>, value: Int) {
    thisRef.sharedPreferences.edit().putInt(key, value).apply()
    }
    }
    public fun Krate.intPref(key: String): ReadWriteProperty {
    return IntDelegate(key)
    }
    

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61. app
    krate
    

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62. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    var users: List? by ???
    }
    myKrate.users = listOf(User("William"), User("Virginia"))
    println(myKrate.users)
    

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63. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    va users: List? by gsonPref("users")
    }
    myKrate.users = listOf(User("William"), User("Virginia"))
    println(myKrate.users)
    r
    

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64. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    va users: List? by gsonPref("users")
    }
    myKrate.users = listOf(User("William"), User("Virginia"))
    println(myKrate.users)
    l
    

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65. app
    myKrate.users = listOf(User("William"), User("Virginia"))
    println(myKrate.users)
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by gsonPref("users")
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    internal class GsonDelegate(
    private val key: String
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string, object : TypeToken() {}.type)
    }
    }
    

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66. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by gsonPref("users")
    }
    internal class GsonDelegate(
    private val key: String
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string, object : TypeToken() {}.type)
    }
    }
    public fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key)
    }
    

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67. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by gsonPref("users")
    }
    internal class GsonDelegate(
    private val key: String
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): Object? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string, object : TypeToken() {}.type)
    }
    }
    public fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key)
    }
    

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68. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by gsonPref("users")
    }
    internal class GsonDelegate(
    private val key: String
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string, object : TypeToken() {}.type)
    }
    }
    public fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key)
    }
    

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69. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by gsonPref("users")
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key)
    }
    object : TypeToken() {}.type)
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    internal class GsonDelegate(
    private val key: String
    

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70. app
    gsonPref("users")
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    object : TypeToken() {}.type)
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    internal class GsonDelegate(
    private val key: String
    

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71. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    }
    gsonPref("users")
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    

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72. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    GsonDelegate(key, object : TypeToken() {}.type)
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    } gsonPref("users")
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    

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73. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    }
    GsonDelegate( , object : TypeToken< >() {}.type)
    "users"
    gsonPref("users")
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    T
    

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74. app
    GsonDelegate( , object : TypeToken< >() {}.type)
    "users" List
    gsonPref("users")
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    }
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    

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75. app
    gsonPref("users")
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    }
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    

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76. app
    gsonPref("users")
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    }
    

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77. app
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by
    } gsonPref("users")
    GsonDelegate(...)
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string,
    }
    }
    ,
    private val type: Type
    type)
    internal class GsonDelegate(
    private val key: String
    

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78. app
    krate
    public interface Krate {
    public val sharedPreferences: SharedPreferences
    }
    class MyKrate(context: Context) : SimpleKrate(context) {
    val users: List? by gsonPref("users")
    }
    @PublishedApi
    internal class GsonDelegate(
    private val key: String,
    private val type: Type
    ) : ReadOnlyProperty {
    override operator fun getValue(thisRef: Krate, property: KProperty<*>): T? {
    val string = thisRef.sharedPreferences.getString(key, null)
    return Gson().fromJson(string, type)
    }
    }
    public inline fun Krate.gsonPref(
    key: String
    ): ReadOnlyProperty {
    return GsonDelegate(key, object : TypeToken() {}.type)
    }
    

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79. https://zsmb.co/maintaining-compatibility-in-kotlin-libraries/
    

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80. Krate*
    class MyKrate(context: Context) : SimpleKrate(context) {
    var onboarded
    var appOpenCount
    var username
    }
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    by booleanPref("onboarded"
    by intPref("appOpenCount"
    by stringPref("username"
    )
    )
    )
    

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81. class MyKrate(context: Context) : SimpleKrate(context) {
    var onboarded: Boolean by booleanPref("onboarded", defaultValue = false)
    var appOpenCount: Int by intPref("appOpenCount", defaultValue = 0)
    var username: String? by stringPref("username")
    }
    Krate*
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    

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82. class MyKrate(context: Context) : SimpleKrate(context) {
    // non-nullable, has default value
    var onboarded: Boolean by booleanPref("onboarded", defaultValue = false)
    var appOpenCount: Int by intPref("appOpenCount", defaultValue = 0)
    // nullable, null by default
    var username: String? by stringPref("username")
    }
    Krate*
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    

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83. class MyKrate(context: Context) : SimpleKrate(context {
    // non-nullable, has default value
    var onboarded: Boolean
    by booleanPref("onboarded", defaultValue = false)
    var appOpenCount: Int
    by intPref("appOpenCount", defaultValue = 0)
    // nullable, null by default
    var username: String?
    by stringPref("username")
    }
    Krate*
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    )
    

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84. Krate*
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    class MyKrate(context: Context) : SimpleKrate(context , AppSettings {
    // non-nullable, has default value
    override var onboarded: Boolean
    by booleanPref("onboarded", defaultValue = false)
    override var appOpenCount: Int
    by intPref("appOpenCount", defaultValue = 0)
    // nullable, null by default
    override var username: String?
    by stringPref("username")
    }
    )
    

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85. class MyKrate @Inject constructor(context: Context)
    // non-nullable, has default value
    override var onboarded: Boolean
    by booleanPref("onboarded", defaultValue = false)
    override var appOpenCount: Int
    by intPref("appOpenCount", defaultValue = 0)
    // nullable, null by default
    override var username: String?
    by stringPref("username")
    }
    : SimpleKrate(context), AppSetti
    Krate*
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    

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86. class MyKrate @Inject constructor(context: Context)
    // non-nullable, has default value
    override var onboarded: Boolean
    by booleanPref("onboarded", defaultValue = false)
    override var appOpenCount: Int
    by intPref("appOpenCount", defaultValue = 0)
    // nullable, null by default
    override var username: String?
    by stringPref("username")
    }
    Krate*
    myKrate.onboarded = true
    myKrate.appOpenCount++
    println(myKrate.username)
    : SimpleKrate(context), AppSettings {
    

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87. References
    • Krate
     https://github.com/AutSoft/Krate
    • Delightful Delegate Design
     https://blog.autsoft.hu/delightful-delegate-design/
    • Further reading
     Maintaining Compatibility in Kotlin libraries
     https://zsmb.co/maintaining-compatibility-in-kotlin-libraries/
     Swift Pitch: Property Delegates
     https://forums.swift.org/t/pitch-property-delegates/21895
     DSL Design
     https://zsmb.co/kotlin-dsl-design-with-village-dsl/
     Tips for writing a library
     https://www.kotlindevelopment.com/tips-for-writing-a-library-in-kotlin/
    

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88. Questions?
    zsmb.co zsmb13
    braun.marton@autsoft.hu
    Photo by Agnieszka Kowalczyk on Unsplash
    

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