Pragmatic Kotlin

Talk given at Kotlin Everywhere Berlin, Android Chapter in September 2019.

Event:
https://www.eventbrite.com/e/kotlin-everywhere-android-chapter-tickets-70491789841

Summary:

Kotlin promises a lot of features, and while some features (like data classes, lambda functions, etc.) have gained more popularity than others, there are some features that are not so widely used.

In this talk, we will look at a couple of nice features that are not very widely used. Some of the features we will look at include typealiases and inline classes as well as property delegations and others and their practical applications to Android development. Wherever it applies, we will also discuss the technical trade-offs to consider when using these features.

At the end of the talk, we would have touched on some of these nice features, and how we can immediately start applying in our codebase to get instant benefits.

Resources:

https://kotlinlang.org/docs/reference/
https://gist.github.com/segunfamisa/716401e58ead9e6eb1c848a51451f927
https://android.googlesource.com/platform/frameworks/support/+/androidx-master-dev/activity/activity-ktx/src/main/java/androidx/activity/ActivityViewModelLazy.kt

Transcript

1. Pragmatic Kotlin
   Android Chapter
   September, 2019
   

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2. segunfamisa
   segunfamisa.com
   

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3. Introduction
   

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4. typealiases
   

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5. typealiases
   

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6. typealiases
   ● Nicknames for regular existing types
   

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7. // before
   fun applyDiscount(price: Double, discount: Double): Double =
   (100.0 - discount) / 100.0 * price
   

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8. // before
   fun applyDiscount(price: Double, discount: Double): Double =
   (100.0 - discount) / 100.0 * price
   Discount in €? Cents?
   Or Percentage??
   

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9. // before
   fun applyDiscount(price: Double, discount: Double): Double
   // fix #1
   

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10. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #1
    /**
    * @param price - original price in Euros
    * @param discount - discount in percentage
    */
    fun applyDiscount(price: Double, discount: Double): Double
    

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11. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #1
    /**
    * @param price - original price in Euros
    * @param discount - discount in percentage
    */
    fun applyDiscount(price: Double, discount: Double): Double
    KDoc comment
    

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12. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #1
    /**
    * @param price - original price in Euros
    * @param discount - discount in percentage
    */
    fun applyDiscount(price: Double, discount: Double): Double
    KDoc comment
    

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13. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #2
    

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14. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #2
    fun applyDiscount(price: Double, discountInPercentage: Double): Double
    

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15. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #2
    fun applyDiscount(price: Double, discountInPercentage: Double): Double
    Clearer param name
    

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16. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #3
    // create typealias
    typealias Percentage = Double
    

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17. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #3
    // create typealias
    typealias Percentage = Double
    // usage
    fun applyDiscount(price: Double, discount: Percentage): Double
    

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18. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #3
    // create typealias
    typealias Percentage = Double
    // usage
    fun applyDiscount(price: Double, discount: Percentage): Double
    

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19. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #3
    // create typealias
    typealias Percentage = Double
    // usage
    fun applyDiscount(price: Double, discount: Percentage): Double
    ● Short parameter name
    ● “type” of discount is clear
    ● No new type created
    

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20. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // fix #3
    // create typealias
    typealias Percentage = Double
    // usage
    fun applyDiscount(price: Double, discount: Percentage): Double
    ● Short parameter name
    ● “type” of discount is clear
    ● No new type created
    

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21. typealias
    ● Improves readability at no performance cost
    ● Does not create a new type
    ● Allows for easy refactoring
    

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22. inline classes
    

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23. inline classes
    ● New in Kotlin 1.3
    ● Still experimental
    ● Useful for creating “wrappers” for primitive types
    

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24. // before
    fun applyDiscount(price: Double, discount: Double): Double =
    (100.0 - discount) / 100.0 * price
    

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25. // before
    fun applyDiscount(price: Double, discount: Double): Double =
    (100.0 - discount) / 100.0 * price
    Same problems as before
    ● Easy to mess up the order of the
    parameters
    ● Not clear what discount is - in Euros or
    in percentage?
    

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26. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    class Price(val value: Double)
    class Percentage(val value: Double)
    

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27. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    class Price(val value: Double)
    class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    

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28. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    class Price(val value: Double)
    class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    

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29. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    class Price(val value: Double)
    class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    ● Less error-prone
    ● Clear intentions/expectations
    

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30. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    class Price(val value: Double)
    class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    ● Additional memory allocation
    

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31. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    class Price(val value: Double)
    class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    ● Additional memory allocation
    

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32. // before
    fun applyDiscount(price: Double, discount: Double): Double
    // wrapper classes
    inline class Price(val value: Double)
    inline class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    

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33. // wrapper classes
    inline class Price(val value: Double)
    inline class Percentage(val value: Double)
    // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    

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34. // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    

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35. // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    // Generated Java code
    public final double discountedPrice_lwIBXQA(
    double price,
    double discount
    ) {
    return Price.constructor-impl((100.0D - discount) / 100.0D * price);
    }
    

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36. // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    // Generated Java code
    public final double discountedPrice_lwIBXQA(
    double price,
    double discount
    ) {
    return Price.constructor-impl((100.0D - discount) / 100.0D * price);
    }
    

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37. // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    // Generated Java code
    public final double discountedPrice_lwIBXQA(
    double price,
    double discount
    ) {
    return Price.constructor-impl((100.0D - discount) / 100.0D * price);
    }
    Inline class variables are resolved to
    the primitive type they wrap!
    

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38. // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    // Generated Java code
    public final double discountedPrice_lwIBXQA(
    double price,
    double discount
    ) {
    return Price.constructor-impl((100.0D - discount) / 100.0D * price);
    }
    

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39. // Usage
    fun applyDiscount(price: Price, discount: Percentage): Price
    // Generated Java code
    public final double discountedPrice_lwIBXQA(
    double price,
    double discount
    ) {
    return Price.constructor-impl((100.0D - discount) / 100.0D * price);
    }
    “-” is not a valid character for functions in Java,
    so for now, inline classes don’t work in Java
    

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40. inline classes
    ● Don’t work with Java for now
    ● Kotlin compiler is not always able to inline
    ● inline classes cannot have more than one “param”
    

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41. Another example
    ● Android Resource wrapper
    inline class StringResource(@StringRes val resId: Int)
    

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42. inline classes
    typealiases
    

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43. inline classes
    typealiases
    No new type is introduced
    

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44. inline classes
    New type is introduced
    typealiases
    No new type is introduced
    

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45. inline classes
    typealiases
    Can assign typealias to
    underlying type:
    typealias Password = String
    fun login(password: String)
    val pwd: Password = "kisHuy8q172"
    login(password = pwd)
    

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46. inline classes
    Cannot assign inline class
    to the wrapped type:
    inline class Password(val value: String)
    fun login(password: String)
    val pwd = Password("kisHuy8q172")
    login(password = pwd) // won't compile
    typealiases
    Can assign typealias to
    underlying type:
    typealias Password = String
    fun login(password: String)
    val pwd: Password = "kisHuy8q172"
    login(password = pwd)
    

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47. property delegates
    

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48. property delegates
    ● Special kinds of properties that are implemented
    differently
    ● E.g lazy properties, observable properties etc.
    

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49. Lazy
    Initialized only once.
    

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50. fun computeExpensiveStringThatDoesntChange() = "stringggg"
    val expensiveValue : String by lazy {
    println("Log expensiveValue created")
    computeExpensiveStringThatDoesntChange()
    }
    

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51. fun computeExpensiveStringThatDoesntChange() = "stringggg"
    val expensiveValue : String by lazy {
    println("Log expensiveValue created")
    computeExpensiveStringThatDoesntChange()
    }
    // Usage
    println(expensiveValue)
    println(expensiveValue)
    

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52. fun computeExpensiveStringThatDoesntChange() = "stringggg"
    val expensiveValue : String by lazy {
    println("Log expensiveValue created")
    computeExpensiveStringThatDoesntChange()
    }
    // Usage
    println(expensiveValue)
    println(expensiveValue)
    // Output
    "Log expensiveValue created"
    "stringggg"
    "stringggg"
    

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53. fun computeExpensiveStringThatDoesntChange() = "stringggg"
    val expensiveValue : String by lazy {
    println("Log expensiveValue created")
    computeExpensiveStringThatDoesntChange()
    }
    // Usage
    println(expensiveValue)
    println(expensiveValue)
    // Output
    "Log expensiveValue created"
    "stringggg"
    "stringggg"
    First access to the variable prints
    this
    

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54. fun computeExpensiveStringThatDoesntChange() = "stringggg"
    val expensiveValue : String by lazy {
    println("Log expensiveValue created")
    computeExpensiveStringThatDoesntChange()
    }
    // Usage
    println(expensiveValue)
    println(expensiveValue)
    // Output
    "Log expensiveValue created"
    "stringggg"
    "stringggg" Subsequent access doesn’t go into the lazy block
    

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55. Another sample - viewModel creation
    class MyActivity : Activity {
    @Inject
    lateinit var viewModelFactory: ViewModelFactory
    private val viewModel by viewModels {
    viewModelFactory
    }
    }
    

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56. Another sample - viewModel creation
    class MyActivity : Activity {
    @Inject
    lateinit var viewModelFactory: ViewModelFactory
    private val viewModel by viewModels {
    viewModelFactory
    }
    }
    implementation "androidx.activity:activity-ktx:$libs.ktxActivityVersion"
    

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57. class ViewModelLazy (
    private val viewModelClass: KClass,
    private val storeProducer: () -> ViewModelStore,
    private val factoryProducer: () -> ViewModelProvider.Factory
    ) : Lazy {
    }
    

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58. class ViewModelLazy (
    private val viewModelClass: KClass,
    private val storeProducer: () -> ViewModelStore,
    private val factoryProducer: () -> ViewModelProvider.Factory
    ) : Lazy {
    }
    

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59. class ViewModelLazy (
    ...
    ) : Lazy {
    private var cached: VM? = null
    override val value: VM
    get() {
    val viewModel = cached
    return if (viewModel == null) {
    val factory = factoryProducer()
    val store = storeProducer()
    ViewModelProvider(store,factory)
    .get(viewModelClass.java).also { cached = it }
    } else {
    viewModel
    }
    }
    override fun isInitialized() = cached != null
    }
    

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60. Putting it all together:
    inline fun ComponentActivity.viewModels(
    noinline factoryProducer: (() -> Factory)? = null
    ): Lazy {
    val factoryPromise = factoryProducer ?: {
    val application = application ?: throw IllegalArgumentException()
    AndroidViewModelFactory.getInstance(application)
    }
    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
    }
    

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61. Putting it all together:
    inline fun ComponentActivity.viewModels(
    noinline factoryProducer: (() -> Factory)? = null
    ): Lazy {
    val factoryPromise = factoryProducer ?: {
    val application = application ?: throw IllegalArgumentException()
    AndroidViewModelFactory.getInstance(application)
    }
    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
    }
    Extension function to make access easier!
    

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62. Observable fields
    Get notified whenever value changes.
    Need to supply initial value
    

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63. Sample usage - recyclerview adapter update & diffutil
    https://gist.github.com/segunfamisa/716401e58ead9e6eb1c848a51451f927
    class ItemAdapter @Inject constructor() :
    RecyclerView.Adapter() {
    var items: List by observableList(initialValue = listOf())
    ...
    }
    

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64. Sample usage - recyclerview adapter update & diffutil
    class ItemAdapter @Inject constructor() :
    RecyclerView.Adapter() {
    var items: List by observableList(initialValue = listOf())
    ...
    }
    

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65. class ObservableListField(
    private val adapter: RecyclerView.Adapter,
    private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback,
    initialValue: List
    ) : ObservableProperty>(initialValue = initialValue)
    

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66. class ObservableListField(
    private val adapter: RecyclerView.Adapter,
    private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback,
    initialValue: List
    ) : ObservableProperty>(initialValue = initialValue) {
    override fun beforeChange(
    property: KProperty,
    oldValue: List,
    newValue: List
    ): Boolean {
    return true
    }
    override fun afterChange(property: KProperty, oldValue: List, newValue: List) {
    super.afterChange(property, oldValue, newValue)
    }
    }
    

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67. class ObservableListField(
    private val adapter: RecyclerView.Adapter,
    private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback,
    initialValue: List
    ) : ObservableProperty>(initialValue = initialValue) {
    private var result: DiffUtil.DiffResult? = null
    override fun beforeChange(
    property: KProperty,
    oldValue: List,
    newValue: List
    ): Boolean {
    val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue)
    result = DiffUtil.calculateDiff(diffUtilCallback)
    return true
    }
    override fun afterChange(property: KProperty, oldValue: List, newValue: List) {
    result?.dispatchUpdatesTo(adapter)
    super.afterChange(property, oldValue, newValue)
    }
    }
    

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68. class ObservableListField(
    private val adapter: RecyclerView.Adapter,
    private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback,
    initialValue: List
    ) : ObservableProperty>(initialValue = initialValue) {
    private var result: DiffUtil.DiffResult? = null
    override fun beforeChange(
    property: KProperty,
    oldValue: List,
    newValue: List
    ): Boolean {
    val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue)
    result = DiffUtil.calculateDiff(diffUtilCallback)
    return true
    }
    override fun afterChange(property: KProperty, oldValue: List, newValue: List) {
    result?.dispatchUpdatesTo(adapter)
    super.afterChange(property, oldValue, newValue)
    }
    }
    

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69. class ObservableListField(
    private val adapter: RecyclerView.Adapter,
    private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback,
    initialValue: List
    ) : ObservableProperty>(initialValue = initialValue) {
    private var result: DiffUtil.DiffResult? = null
    override fun beforeChange(
    property: KProperty,
    oldValue: List,
    newValue: List
    ): Boolean {
    val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue)
    result = DiffUtil.calculateDiff(diffUtilCallback)
    return true
    }
    override fun afterChange(property: KProperty, oldValue: List, newValue: List) {
    result?.dispatchUpdatesTo(adapter)
    super.afterChange(property, oldValue, newValue)
    }
    }
    

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70. class ObservableListField(
    private val adapter: RecyclerView.Adapter,
    private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback,
    initialValue: List
    ) : ObservableProperty>(initialValue = initialValue) {
    private var result: DiffUtil.DiffResult? = null
    override fun beforeChange(
    property: KProperty,
    oldValue: List,
    newValue: List
    ): Boolean {
    val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue)
    result = DiffUtil.calculateDiff(diffUtilCallback)
    return true
    }
    override fun afterChange(property: KProperty, oldValue: List, newValue: List) {
    result?.dispatchUpdatesTo(adapter)
    super.afterChange(property, oldValue, newValue)
    }
    }
    

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71. fun RecyclerView.Adapter.observableList(
    initialValue: List,
    diffUtilCallbackProducer: ((List, List) -> DiffUtil.Callback)
    ): ObservableProperty> {
    return ObservableListField(
    adapter = this,
    diffUtilCallbackProducer = diffUtilCallbackProducer,
    initialValue = initialValue
    )
    }
    Putting it all together!
    

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72. More Kotlin features
    ● Generics
    ● Kotlin native
    ● Annotations
    

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73. Resources
    

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74. Resources
    https://kotlinlang.org/docs/reference/
    https://gist.github.com/segunfamisa/716401e58ead9e6eb1c848a51451f927
    AndroidX Source
    

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75. Thank you!
    

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