Property Delegation with Kotlin for Android

What is property delegation in Kotlin?
How can you write your own custom delegates?
Examples of how we used delegation at sporttotal.tv and how it made Android databinding much simpler for us

Transcript

1. Property delegates
   Danny Preussler, Kotlin Meetup Berlin
   

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4. @PreusslerBerlin
   Property delegates?
   

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5. @PreusslerBerlin
   Delegated properties?
   

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7. @PreusslerBerlin
   Properties:
   val shareEnabled: Boolean
   get() = clip.sharable
   

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8. @PreusslerBerlin
   Delegation:
   class FirebaseFeatureFlags(
   defaults: DefaultFeatureFlags
   ) : FeatureFlags by defaults
   

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9. @PreusslerBerlin
   Example
   Property delegates?
   

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11. @PreusslerBerlin
    Property Delegates
    val label by lazy {
    resources.getString(R.string.featured)
    }
    

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12. @PreusslerBerlin
    Property Delegates
    public fun lazy(
    initializer: () -> T)
    : Lazy =
    SynchronizedLazyImpl(
    initializer
    )
    

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13. @PreusslerBerlin
    Property Delegates
    public fun lazy(
    initializer: () -> T)
    : Lazy =
    SynchronizedLazyImpl(
    initializer
    )
    

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14. @PreusslerBerlin
    Property Delegates
    public fun lazy(
    initializer: () -> T)
    : Lazy =
    SynchronizedLazyImpl(
    initializer
    )
    

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15. @PreusslerBerlin
    Property Delegates
    val tracker: Tracker by inject()
    

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16. @PreusslerBerlin
    Property Delegates
    inline fun ComponentCallbacks.inject(
    name: String = "",
    scope: Scope? = null,
    noinline parameters: ParameterDefinition =
    emptyParameterDefinition()
    ) = lazy { get(name, scope, parameters) }
    

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17. @PreusslerBerlin
    Property Delegates
    inline fun ComponentCallbacks.inject(
    name: String = "",
    scope: Scope? = null,
    noinline parameters: ParameterDefinition =
    emptyParameterDefinition()
    ) = lazy { get(name, scope, parameters) }
    

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18. @PreusslerBerlin
    Property Delegates
    val viewModel: ToolbarViewModel by viewModel()
    

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19. @PreusslerBerlin
    Property Delegates
    inline fun LifecycleOwner.viewModel(
    key: String? = null,
    name: String? = null,
    noinline parameters: ParameterDefinition =
    emptyParameterDefinition()
    ) = viewModelByClass(T::class, key, name, null, parameters)
    

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20. @PreusslerBerlin
    Property Delegates
    inline fun LifecycleOwner.viewModel(
    key: String? = null,
    name: String? = null,
    noinline parameters: ParameterDefinition =
    emptyParameterDefinition()
    ) = viewModelByClass(T::class, key, name, null, parameters)
    

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21. @PreusslerBerlin
    Property Delegates
    fun LifecycleOwner.viewModelByClass(
    clazz: KClass,
    key: String? = null,
    name: String? = null,
    from: ViewModelStoreOwnerDefinition? = null,
    parameters: ParameterDefinition =
    emptyParameterDefinition()
    ) = lazy { getViewModelByClass(clazz, key, name, from,
    parameters) }
    

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22. @PreusslerBerlin
    Property Delegates
    public interface Lazy {
    public val value: T
    public fun isInitialized(): Boolean
    }
    

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23. So only Lazy allowed?
    

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24. @PreusslerBerlin
    Property Delegates
    val something by 123
    

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25. @PreusslerBerlin
    Property Delegates
    val label by lazy {
    resources.getString(R.string.featured)
    }
    

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26. @PreusslerBerlin
    Property Delegates
    public inline operator fun Lazy.getValue(
    thisRef: Any?,
    property: KProperty
    ): T = value
    

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27. @PreusslerBerlin
    Property Delegates
    val something by 123
    operator fun Int.getValue(
    thisRef: Any?,
    property: KProperty
    ): Int = this
    

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28. @PreusslerBerlin
    Property Delegates
    val something by 123
    operator fun Int.getValue(
    thisRef: Any?,
    property: KProperty
    ): Int = 456
    

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30. @PreusslerBerlin
    A Property Delegate
    var message: String by censored()
    

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31. @PreusslerBerlin
    A Property Delegate
    fun censored(): String = ““
    operator fun String.getValue(
    thisRef: Any?,
    property: KProperty
    ): Int = this.replace(“sucks”, “is suboptimal”)
    operator fun String.setValue(
    thisRef: Any?,
    property: KProperty,
    String: value
    ) { ??? }
    

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32. @PreusslerBerlin
    A Property Delegate
    fun censored(message: String = ““)
    = CensorProperty(message)
    

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33. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(..): ReadWriteProperty{
    override fun setValue(..){..}
    override fun getValue(..){..}
    }
    

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34. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(..): ReadOnlyProperty{
    override fun getValue(..){..}
    }
    

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35. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(var message: String?) :
    ReadWriteProperty {
    override fun getValue(
    thisRef: Any?,
    property: KProperty) = message
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: String?
    ) {
    message = value?.replace(
    “sucks”, “is suboptimal”)
    }
    

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36. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(var message: String?) :
    ReadWriteProperty {
    override fun getValue(
    thisRef: Any?,
    property: KProperty) = message
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: String?
    ) {
    message = value?.replace(
    “sucks”, “is suboptimal”)
    }
    }
    

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37. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(var message: String?) :
    ReadWriteProperty {
    override fun getValue(
    thisRef: Any?,
    property: KProperty) = message
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: String?
    ) {
    message = value?.replace(
    “sucks”, “is suboptimal”)
    }
    }
    

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38. @PreusslerBerlin
    A Property Delegate
    package kotlin.reflect
    public interface KProperty : KCallable {
    @SinceKotlin("1.1")
    public val isLateinit: Boolean
    @SinceKotlin("1.1")
    public val isConst: Boolean
    public val getter: Getter
    …
    

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39. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(var message: String?) :
    ReadWriteProperty {
    override fun getValue(
    thisRef: Any?,
    property: KProperty) = message
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: String?
    ) {
    message = value?.replace(
    “sucks”, “is suboptimal”)
    }
    }
    

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40. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(var message: String?) :
    ReadWriteProperty {
    override fun getValue(
    thisRef: Any?,
    property: KProperty) = message
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: String?
    ) {
    message = value?.replace(
    “sucks”, “is suboptimal”)
    }
    }
    

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41. @PreusslerBerlin
    A Property Delegate
    class CensorProperty(var message: String?) :
    ReadWriteProperty {
    override fun getValue(
    thisRef: FrameLayout?,
    property: KProperty) = message
    override fun setValue(
    thisRef: FrameLayout?,
    property: KProperty,
    value: String?
    ) {
    message = value?.replace(
    “sucks”, “is suboptimal”)
    }
    }
    

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42. @PreusslerBerlin
    A Property Delegate
    var message by CensorProperty()
    

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43. @PreusslerBerlin
    A Property Delegate
    var message: String by censored()
    fun censored(message: String = ““)
    = CensorProperty(message)
    

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44. @PreusslerBerlin
    Your Property delegates
    •Extend
    ReadOnlyProperty, ReadWriteProperty
    or
    •Use extension operator functions:
    getValue/setValue
    

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45. Your Property delegates
    

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46. @PreusslerBerlin
    Your Property delegates
    •There is a third function that can be implemented:
    operator fun provideDelegate(
    thisRef: Any?,
    property: KProperty
    ): ReadOnlyProperty {
    …
    }
    One of the possible use cases of provideDelegate is to check
    property consistency when the property is created, not only in
    its getter or setter.
    

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47. @PreusslerBerlin
    Custom delegates
    

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48. @PreusslerBerlin
    Our delegates
    class PlayerLiveFragment : Fragment() {
    val factory: LiveGameViewModel.Provider by inject()
    val viewModel: LiveGameViewModel by unsafeLazy {
    factory(currentClip)
    }
    

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49. @PreusslerBerlin
    Our delegates
    fun unsafeLazy(initializer: () -> T)
    = lazy(LazyThreadSafetyMode.NONE, initializer)
    

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50. @PreusslerBerlin
    Our delegates
    fun unsafeLazy(initializer: () -> T)
    = lazy(LazyThreadSafetyMode.NONE, initializer)
    

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51. Show me more
    

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52. @PreusslerBerlin
    Our delegates
    val initialised by toggleOnFirstAccess(false)
    

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53. @PreusslerBerlin
    Our delegates
    val initialised by toggleOnFirstAccess(false)
    

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54. @PreusslerBerlin
    Our delegates
    val initialised by toggleOnFirstAccess(false)
    fun init() {
    if (not(initialised)) {
    subscribeToStore()
    }
    }
    

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55. @PreusslerBerlin
    Our delegates
    fun toggleOnFirstAccess(initial: Boolean) =
    object : ReadOnlyProperty {
    private val value = AtomicBoolean(initial)
    override fun getValue(
    thisRef: Any?,
    property: KProperty)
    = value.getAndSet(initial.not())
    }
    

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56. @PreusslerBerlin
    Our delegates
    fun toggleOnFirstAccess(initial: Boolean) =
    object : ReadOnlyProperty {
    private val value = AtomicBoolean(initial)
    override fun getValue(
    thisRef: Any?,
    property: KProperty)
    = value.getAndSet(initial.not())
    }
    

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57. @PreusslerBerlin
    Our delegates
    fun toggleOnFirstAccess(initial: Boolean) =
    object : ReadOnlyProperty {
    private val value = AtomicBoolean(initial)
    override fun getValue(
    thisRef: Any?,
    property: KProperty)
    = value.getAndSet(initial.not())
    }
    

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58. Show me more
    

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59. @PreusslerBerlin
    Our delegates
    var searchDisposable by disposable()
    

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60. @PreusslerBerlin
    Our delegates
    var searchDisposable by disposable()
    

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61. @PreusslerBerlin
    Our delegates
    var searchDisposable by disposable()
    searchDisposable = useCase.searchFor(query)
    

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62. @PreusslerBerlin
    Our delegates
    var searchDisposable by disposable()
    searchDisposable = useCase.searchFor(query)
    override fun onDestroy() {
    searchDisposable = null
    }
    

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63. @PreusslerBerlin
    Our delegates
    class DisposableProperty(
    private var disposable: Disposable?)
    :ReadWriteProperty {
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: Disposable?) {
    disposable?.dispose()
    disposable = value
    }
    override fun getValue(
    thisRef: Any?, property: KProperty)
    = disposable
    }
    

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64. @PreusslerBerlin
    Our delegates
    class DisposableProperty(
    private var disposable: Disposable?)
    :ReadWriteProperty {
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: Disposable?) {
    disposable?.dispose()
    disposable = value
    }
    override fun getValue(
    thisRef: Any?, property: KProperty)
    = disposable
    }
    

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65. @PreusslerBerlin
    Our delegates
    class DisposableProperty(
    private var disposable: Disposable?)
    :ReadWriteProperty {
    override fun setValue(
    thisRef: Any?,
    property: KProperty,
    value: Disposable?) {
    disposable?.dispose()
    disposable = value
    }
    override fun getValue(
    thisRef: Any?, property: KProperty)
    = disposable
    }
    

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66. Show me more
    

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67. @PreusslerBerlin
    Our delegates
    var query by observable(““)
    .doOnNext { usecase.search(it) }
    

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68. @PreusslerBerlin
    Our delegates
    var playingAd by observable(false)
    .distinctUntilChanged()
    .doOnTrue { onAdsStarted() }
    .doOnFalse { onAdsStopped() }
    

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69. @PreusslerBerlin
    Our delegates
    var playingAd by observable(false)
    .distinctUntilChanged()
    .doOnTrue { onAdsStarted() }
    .doOnFalse { onAdsStopped() }
    override fun onTimelineChanged(
    timeline: Timeline?, manifest: Any?, reason: Int)
    {
    playingAd = exoPlayer.isPlayingAd
    }
    

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71. @PreusslerBerlin
    Observable?
    •ReadOnlyProperty
    •ReadWriteProperty
    

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72. @PreusslerBerlin
    Observable?
    •ReadOnlyProperty
    •ReadWriteProperty
    •ObservableProperty
    

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73. @PreusslerBerlin
    Observable?
    var max: Int by observable(0) {
    property, oldValue, newValue ->. observed = true
    

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74. @PreusslerBerlin
    Observable?
    inline fun observable(
    initialValue: T,
    crossinline onChange: (
    property: KProperty,
    oldValue: T,
    newValue: T
    ) -> Unit
    ): ReadWriteProperty
    var max: Int by observable(0) {
    property, oldValue, newValue -> …
    }
    

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75. @PreusslerBerlin
    Observable?
    •We didn‘t use the Kotlin observable:
    •Dont care about previous value or kproperty
    •Would not work with the chaining
    

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76. @PreusslerBerlin
    Did you know…
    by vetoable
    

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77. @PreusslerBerlin
    Since Kotlin 1.3
    var max by vetoable(0) {
    property, old, new -> …
    }
    

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78. @PreusslerBerlin
    Since Kotlin 1.3
    public inline fun vetoable(
    initialValue: T,
    crossinline onChange: (
    property: KProperty,
    oldValue: T,
    newValue: T) -> Boolean)
    : ReadWriteProperty =
    object : ObservableProperty(initialValue) { }
    

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79. @PreusslerBerlin
    Since Kotlin 1.3
    public inline fun vetoable(
    initialValue: T,
    crossinline onChange: (
    property: KProperty,
    oldValue: T,
    newValue: T) -> Boolean)
    : ReadWriteProperty =
    object : ObservableProperty(initialValue) { }
    

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80. @PreusslerBerlin
    Since Kotlin 1.3
    public inline fun vetoable(
    initialValue: T,
    crossinline onChange: (
    property: KProperty,
    oldValue: T,
    newValue: T) -> Boolean)
    : ReadWriteProperty =
    object : ObservableProperty(initialValue) { }
    

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81. Show me more
    

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82. @PreusslerBerlin
    We love databinding!
    

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83. We love databinding!
    

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84. @PreusslerBerlin
    We love databinding
    android:text="@{viewModel.label}"
    name="viewModel"
    type="tv.sporttotal.android.LiveGamesViewModel" />
    

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85. @PreusslerBerlin
    We love databinding
    val label = ”some text"
    

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86. @PreusslerBerlin
    We love databinding
    val label = ObservableField("")
    val label = ”some text"
    

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87. @PreusslerBerlin
    We love databinding
    val label = ObservableField("")
    @Bindable
    var label: CharSequence = ""
    set(value) {
    field = value
    notifyPropertyChanged(BR.label)
    }
    val label = ”some text"
    

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88. @PreusslerBerlin
    We love databinding
    @Bindable
    var title: CharSequence = ""
    set(value) {
    field = value
    notifyPropertyChanged(BR.
    title)
    }
    @Bindable
    var showLoader: Boolean = false
    set(value) {
    field = value
    notifyPropertyChanged(BR.
    showLoader)
    }
    @Bindable
    var error: String? = null
    set(value) {
    field = value
    notifyPropertyChanged(BR.
    error)
    }
    

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89. @PreusslerBerlin
    We love databinding
    @get:Bindable
    var title: CharSequence by bindable(BR.title, "")
    @get:Bindable
    var showLoader: Boolean by bindable(BR.showLoader, false)
    @get:Bindable
    var error: String? by bindable(BR.error, null)
    

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90. @PreusslerBerlin
    We love databinding
    @get:Bindable
    var title: CharSequence by bindable("")
    @get:Bindable
    var showLoader: Boolean by bindable(false)
    @get:Bindable
    var error: String? by bindable(null)
    

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91. @PreusslerBerlin
    Our delegates
    @get:Bindable
    var chatRelatedVisibility by bindable(View.VISIBLE)
    private set
    

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92. @PreusslerBerlin
    Our delegates
    @get:Bindable
    var logoVisibility by bindable(View.VISIBLE)
    @get:Bindable("logoVisibility")
    val upVisibility: Int
    get() = logoVisibility.invertVisibility()
    

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93. @PreusslerBerlin
    Our delegates
    @get:Bindable
    var selected: Boolean by bindable(selected)
    .doOnNext { callback?.invoke(it) }
    

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94. @PreusslerBerlin
    @get:Bindable
    var currentMainScreen by bindable(Screen.Home.Main())
    .eager()
    .doOnNext {
    when (it) {
    is Home -> mainNavigator.goToStart()
    is Games -> mainNavigator.goToSports()
    is Profile -> mainNavigator.goToProfile()
    }
    }
    

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95. @PreusslerBerlin
    https://github.com/Aidanvii7/Toolbo
    x
    

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97. Thanks
    

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