Droidcon SF: Reactive approach to delegation

PRESENTED AT:
Droidcon SF
https://www.sf.droidcon.com/speaker/Aida-Issayeva

DATE:
November 25, 2019

DESCRIPTION:
This talk about delegation and delegated properties in Kotlin and how to incorporate them into your custom views and to make them react immediately to new data. It showcases the usage of delegated properties in custom views
Repo to code sample: https://github.com/AidaIssayeva/Reactive-approach-to-delegation

MORE TALKS & ARTICLES FROM ME: https://cupsofcode.com/talks/

Transcript

1. Reactive approach to
   delegation in Kotlin
   Aida Issayeva
   @aida_isay
   

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2. has a chimney
   has a window
   has a door
   

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3. interface WindowActionListener {
   void openWindow();
   void closeWindow();
   }
   interface DoorActionListener {
   void closeDoor();
   void openDoor();
   }
   

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4. Delegate classes
   class Window implements WindowActionListener {
   @Override
   public void openWindow() { }
   @Override
   public void closeWindow() { }
   }
   class Door implements DoorActionListener {
   @Override
   public void closeDoor() { }
   @Override
   public void openDoor() { }
   }
   

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5. Delegation in Java
   class Room implements WindowActionListener, DoorActionListener {
   private Window window = new Window();
   private Door door = new Door();
   @Override
   public void openWindow() {
   window.openWindow();
   }
   @Override
   public void closeWindow() {
   window.closeWindow();
   }
   @Override
   public void closeDoor() {
   door.closeDoor();
   }
   @Override
   public void openDoor() {
   door.openDoor();
   }
   }
   

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6. Delegation in Java
   class LivingRoom implements WindowActionListener {
   private Window window = new Window();
   @Override
   public void openWindow() {
   window.openWindow();
   }
   @Override
   public void closeWindow() {
   window.closeWindow();
   }
   }
   

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7. “The Delegation pattern has proven to be a
   good alternative to implementation
   inheritance, and Kotlin supports it natively
   requiring zero boilerplate code”
   https://kotlinlang.org/docs/reference/delegation.html
   

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8. Delegation in Kotlin
   class Room : WindowActionListener by Window(),
   DoorActionListener by Door()
   class LivingRoom : WindowActionListener by Window()
   

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9. Delegation in Kotlin
   class Room : WindowActionListener by Window(),
   DoorActionListener by Door()
   class LivingRoom : WindowActionListener by Window()
   

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10. Implicit delegation
    Translation
    public final class Room implements WindowActionListener, DoorActionListener
    {
    // $FF: synthetic field
    private final Window $$delegate_0 = new Window();
    // $FF: synthetic field
    private final Door $$delegate_1 = new Door();
    public void closeWindow() {
    this.$$delegate_0.closeWindow();
    }
    public void openWindow() {
    this.$$delegate_0.openWindow();
    }
    public void closeDoor() {
    this.$$delegate_1.closeDoor();
    }
    public void openDoor() {
    this.$$delegate_1.openDoor();
    }
    }
    

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11. Implicit delegation
    Translation
    public final class LivingRoom implements
    WindowActionListener {
    // $FF: synthetic field
    private final Window $$delegate_0 = new Window();
    public void closeWindow() {
    this.$$delegate_0.closeWindow();
    }
    public void openWindow() {
    this.$$delegate_0.openWindow();
    }
    }
    Tools->Kotlin->Show Kotlin Bytecode
    -> “Decompile” button
    

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12. Delegated properties
    class User(val firstName: String, val lastName: String) {
    var userName: String by UserNameDelegate()
    }
    

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13. Create custom delegated properties
    //implement interfaces
    //for val
    class UserNameDelegate : ReadOnlyProperty {
    override fun getValue(thisRef: User, property: KProperty<*>):
    String {
    return "${thisRef.firstName}_${thisRef.lastName}"
    }
    }
    

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14. //implement interfaces
    //for var
    class UserNameDelegate : ReadWriteProperty {
    override fun getValue(thisRef: User, property: KProperty<*>):
    String {
    return "${thisRef.firstName}_${thisRef.lastName}"
    }
    override fun setValue(thisRef: User, property: KProperty<*>, value:
    String) {
    println("$value has been assigned to '${property.name}' in
    $thisRef.")
    }
    }
    Create custom delegated properties
    

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15. //or use `operator`
    //for val
    class UserNameDelegate {
    operator fun getValue(thisRef: User?, property: KProperty<*>):
    String {
    return "${thisRef?.firstName}_${thisRef?.lastName}"
    }
    }
    Create custom delegated properties
    

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16. //or use `operator`
    //for var
    class UserNameDelegate {
    operator fun getValue(thisRef: User?, property: KProperty<*>):
    String {
    return "${thisRef?.firstName}_${thisRef?.lastName}"
    }
    operator fun setValue(thisRef: User?, property: KProperty<*>,
    value: String) {
    println(“$value has been assigned to '${property.name}' in
    $thisRef.")
    }
    }
    Create custom delegated properties
    

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17. Delegated properties
    class CustomDelegatedPropertyActivity : AppCompatActivity()
    {
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity)
    val user = User("Aida", "Issayeva")
    println("username:" + user.userName)
    }
    }
    I/System.out: username:Aida_Issayeva
    

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18. 1. lazy
    

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19. 1. lazy delegate
    class AboutMeFragment : Fragment() {
    val component by lazy {
    val appComponent = DaggerHelper.getAppComponent(this)
    AboutMeComponent.builder()
    .appComponent(appComponent)
    .build()
    }
    }
    

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20. Lazy delegate
    class AboutMeFragment : Fragment() {
    val component by lazy {
    val appComponent = DaggerHelper.getAppComponent(this)
    AboutMeComponent.builder()
    .appComponent(appComponent)
    .build()
    }
    }
    

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21. Lazy delegate
    class AboutMeFragment : Fragment() {
    val component by lazy {
    val appComponent = DaggerHelper.getAppComponent(this)
    AboutMeComponent.builder()
    .appComponent(appComponent)
    .build()
    }
    }
    

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22. Lazy delegate
    class AboutMeFragment : Fragment() {
    val component by lazy {
    val appComponent = DaggerHelper.getAppComponent(this)
    AboutMeComponent.builder()
    .appComponent(appComponent)
    .build()
    }
    val viewModel by lazy {
    val factory = AboutMeViewModel.Factory(component)
    ViewModelProviders.of(this, factory)
    .get(AboutMeViewModel::class.java)
    }
    }
    

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23. init
    val
    init
    val
    init
    val
    ?
    LazyThreadSafetyMode
    SYNCHRONIZED PUBLICATION NONE
    

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24. lazy
    • Faster class init
    •No more useless null checks
    •Smart cast and thread sync
    •Not used property marked by compiler
    •Declared and init in single place
    

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25. 2. notNull
    

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26. 2. notNull delegate
    class AboutMeFragment : Fragment() {
    lateinit var age: Int
    }
    

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27. notNull delegate
    class AboutMeFragment : Fragment() {
    var age: Int by Delegates.notNull()
    }
    import kotlin.properties.Delegates.notNull
    class AboutMeFragment : Fragment() {
    var age: Int by notNull()
    }
    

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28. notNull
    •Later init
    •Primitive && top-level properties
    • No null checks
    

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29. 3. map
    

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30. 3. map delegate
    class Profile(map: Map) {
    val firstName: String by map
    val lastName: String by map
    val phoneNumber: String by map
    val income: Double by map
    }
    

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31. map delegate
    class Profile(map: Map) {
    val firstName: String by map
    val lastName: String by map
    val phoneNumber: String by map
    val income: Double by map
    }
    

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32. map delegate
    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    val map = mapOf(
    "firstName" to "Aida",
    "middleName" to null,
    "lastName" to "Issayeva",
    "phoneNumber" to "212-212-2121",
    "income" to 1000000000.00
    )
    val person = Profile(map)
    println(person.firstName)
    }
    

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33. map
    •Simplify access
    • Define expected structure
    • But have a power to access unexpected fields
    •Map key == property name
    

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34. 4. observable
    

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35. 4. observable delegate
    class AboutMeFragment : Fragment() {
    val disposables = CompositeDisposable()
    var isLoading by Delegates.observable(false) { p, old, new ->
    buttonDelete.isEnabled = !new
    buttonSubmit.isEnabled = !new
    progressBar.setVisibleOrGone(new)
    }
    . . .
    super.onViewCreated(view, savedInstanceState)
    buttonSubmit.setOnClickListener {
    isLoading = true
    viewModel.updateProfile("Aida", "Issayeva", “issayeva@aida.com“)
    .subscribe {
    isLoading = false
    }
    .addTo(disposables)
    }
    }
    }
    

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36. Observable delegate
    class AboutMeFragment : Fragment() {
    val disposables = CompositeDisposable()
    var isLoading by Delegates.observable(false) { p, old, new ->
    buttonDelete.isEnabled = !new
    buttonSubmit.isEnabled = !new
    progressBar.setVisibleOrGone(new)
    }
    . . .
    super.onViewCreated(view, savedInstanceState)
    buttonSubmit.setOnClickListener {
    isLoading = true
    viewModel.updateProfile("Aida", "Issayeva", “issayeva@aida.com”)
    .subscribe {
    isLoading = false
    }
    .addTo(disposables)
    }
    }
    }
    

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37. Observable delegate
    class AboutMeFragment : Fragment() {
    val disposables = CompositeDisposable()
    var isLoading by Delegates.observable(false) { p, old, new ->
    buttonDelete.isEnabled = !new
    buttonSubmit.isEnabled = !new
    progressBar.setVisibleOrGone(new)
    }
    . . .
    super.onViewCreated(view, savedInstanceState)
    buttonSubmit.setOnClickListener {
    isLoading = true
    viewModel.updateProfile("Aida", "Issayeva", “issayeva@aida.com”)
    .subscribe {
    isLoading = false
    }
    .addTo(disposables)
    }
    }
    }
    

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38. Observable delegate
    class AboutMeFragment : Fragment() {
    val disposables = CompositeDisposable()
    var isLoading by Delegates.observable(false) { p, old, new ->
    buttonDelete.isEnabled = !new
    buttonSubmit.isEnabled = !new
    progressBar.setVisibleOrGone(new)
    }
    . . .
    super.onViewCreated(view, savedInstanceState)
    buttonSubmit.setOnClickListener {
    isLoading = true
    viewModel.updateProfile("Aida", "Issayeva", “issayeva@aida.com”)
    .subscribe {
    isLoading = false
    }
    .addTo(disposables)
    }
    }
    }
    

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39. Adapters in Java
    public class NameAdapter extends
    RecyclerView.Adapter {
    public void updateNames(ArrayList names){
    this.list = names;
    notifyDataSetChanged();
    }
    }
    

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40. Adapters in Kotlin
    class NameAdapter(val context: Context) :
    RecyclerView.Adapter() {
    var list = emptyList()
    fun updateNames(names: ArrayList) {
    list = names
    notifyDataSetChanged()
    }
    }
    

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41. Adapters in Kotlin
    class NameAdapter(val context: Context) :
    RecyclerView.Adapter() {
    var list by observable(mutableListOf()) { _,
    old, new ->
    if (old != new) notifyDataSetChanged()
    }
    }
    

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42. Observable
    • Lambda is called after new value set
    •Have an access to old value
    •Notify ui immediately about changes in data
    • Advantage of partial ui update
    

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43. 5. vetoable
    

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44. Adapters in Java
    public class Adapter extends
    RecyclerView.Adapter {
    public void updateNames(ArrayList names){
    if(names.size() <= 5) {
    this.list = names;
    notifyDataSetChanged();
    }
    }
    }
    

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45. Adapters in Kotlin
    class NameAdapter(val context: Context) :
    RecyclerView.Adapter() {
    var list by
    Delegates.vetoable>(mutableListOf()) { _, old,
    new ->
    new.size <= 5
    }
    }
    

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46. Vetoable delegate
    class NameAdapter(val context: Context) :
    RecyclerView.Adapter() {
    var list by
    Delegates.vetoable>(mutableListOf()) { _, old,
    new ->
    if (new.size <= 5) {
    notifyDataSetChanged()
    return@vetoable true
    }
    return@vetoable false
    }
    }
    

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47. Vetoable delegate
    class MainActivity : Activity() {
    var firstName: String by Delegates.vetoable("")
    { prop, old, new ->
    when (new.length) {
    0 -> hideValidation()
    in 2..20 -> showValidData(new)
    else -> showError()
    }
    new.startsWith("a")
    }
    }
    

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48. vetoable
    •Lambda is called before new value set
    • To set or not to set
    •Have an access to old value
    • Notify ui immediately about changes in data
    • Advantage of partial ui update
    

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49. Delegated properties in custom views
    

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50. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    buttonLike.setOnClickListener { likePost() }
    buttonSend.setOnClickListener { sendComment() }
    editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start:
    Int, before: Int, count: Int) {
    when (count) {
    0 -> {
    buttonLike.visibility = View.VISIBLE
    buttonSend.visibility = View.GONE
    }
    1 -> {
    buttonLike.visibility = View.GONE
    buttonSend.visibility = View.VISIBLE
    }
    }
    }
    })
    }
    First approach:
    

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51. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    buttonLike.setOnClickListener { likePost() }
    buttonSend.setOnClickListener { sendComment() }
    editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start:
    Int, before: Int, count: Int) {
    when (count) {
    0 -> {
    buttonLike.visibility = View.VISIBLE
    buttonSend.visibility = View.GONE
    }
    1 -> {
    buttonLike.visibility = View.GONE
    buttonSend.visibility = View.VISIBLE
    }
    }
    }
    })
    }
    

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52. editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count:
    Int) {
    when (count) {
    0 -> {
    isInputEmpty = true
    buttonLike.setBackgroundResource(R.drawable.ic_favorite_gray_24dp)
    }
    1 -> {
    isInputEmpty = false
    buttonLike.setBackgroundResource(R.drawable.ic_send_24dp)
    }
    }
    }
    })
    buttonLike.setOnClickListener {
    if (isInputEmpty) likePost() else sendComment()
    }
    Second approach:
    

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53. editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count:
    Int) {
    when (count) {
    0 -> {
    isInputEmpty = true
    buttonLike.setBackgroundResource(R.drawable.ic_favorite_gray_24dp)
    }
    1 -> {
    isInputEmpty = false
    buttonLike.setBackgroundResource(R.drawable.ic_send_24dp)
    }
    }
    }
    })
    buttonLike.setOnClickListener {
    if (isInputEmpty) likePost() else sendComment()
    }
    

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54. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    private var showLike: Boolean = false
    init {
    showLike = true
    likeToSend = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_favorite_to_send)
    sendToLike = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_send_to_favorite)
    background = likeToSend
    }
    fun showLike() {
    if (!showLike) {
    morph()
    }
    }
    fun showSend() {
    if (showLike) {
    morph()
    }
    }
    }
    Third approach:
    

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55. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    private var showLike: Boolean = false
    init {
    showLike = true
    likeToSend = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_favorite_to_send)
    sendToLike = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_send_to_favorite)
    background = likeToSend
    }
    fun showLike() {
    if (!showLike) {
    morph()
    }
    }
    fun showSend() {
    if (showLike) {
    morph()
    }
    }
    }
    Third approach:
    

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56. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    private var showLike: Boolean = false
    init {
    showLike = true
    likeToSend = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_favorite_to_send)
    sendToLike = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_send_to_favorite)
    background = likeToSend
    }
    fun showLike() {
    if (!showLike) {
    morph()
    }
    }
    fun showSend() {
    if (showLike) {
    morph()
    }
    }
    }
    Third approach:
    

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57. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    private var showLike: Boolean = false
    . . .
    fun showLike() {
    if (!showLike) {
    morph()
    }
    }
    fun showSend() {
    if (showLike) {
    morph()
    }
    }
    private fun morph() {
    val drawable = if (showLike) likeToSend else sendToLike
    background = drawable
    drawable?.start()
    showLike = !showLike
    }
    }
    

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58. editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start:
    Int, before: Int, count: Int) {
    when (count) {
    0 -> buttonLike.showLike()
    1 -> buttonLike.showSend()
    }
    }
    })
    

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59. buttonLike.setOnClickListener { TODO("????") }
    editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start:
    Int, before: Int, count: Int) {
    when (count) {
    0 -> button_like.showLike()
    1 -> button_like.showSend()
    }
    }
    })
    

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60. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    private var showLike: Boolean = false
    init {
    showLike = true
    . . .
    }
    . . .
    }
    

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61. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    var showLike: Boolean = false
    init {
    showLike = true
    . . .
    }
    . . .
    }
    

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62. buttonLike.setOnClickListener {
    if (buttonLike.showLike) likePost() else sendComment()
    }
    editText.addTextChangedListener(object : TextWatcher {
    . . .
    override fun onTextChanged(s: CharSequence?, start:
    Int, before: Int, count: Int) {
    when (count) {
    0 -> buttonLike.showLike()
    1 -> buttonLike.showSend()
    }
    }
    })
    

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63. class SendLikeButton(context: Context) : Button(context) {
    private var sendToLike: AnimatedVectorDrawableCompat? = null
    private var likeToSend: AnimatedVectorDrawableCompat? = null
    var showLike: Boolean = false
    var isLiked: Boolean = false
    fun setLikedView(isLiked: Boolean) {
    this.isLiked = isLiked
    if (isLiked) {
    likeToSend = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_gray_favorite_to_send)
    sendToLike = AnimatedVectorDrawableCompat.create(context,
    R.drawable.avd_send_to_gray_favorite)
    }
    }
    }
    

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64. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    apiResponseLikeStatus
    .subscribe({
    buttonLike.setLikedView(it)
    }, {}).addTo(disposable)
    }
    

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65. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    buttonLike.setOnClickListener {
    when (buttonLike.isLiked && buttonLike.showLike) {
    true -> unlikePost()
    else -> if (buttonLike.showLike) likePost()
    else sendComment()
    }
    }
    }
    

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66. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    buttonLike.setOnClickListener {
    when (buttonLike.isLiked && buttonLike.showLike) {
    true -> unlikePost()
    else -> if (buttonLike.showLike) likePost()
    else sendComment()
    }
    }
    }
    

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67. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    buttonLike.setOnClickListener {
    when (buttonLike.isLiked && buttonLike.showLike) {
    true -> unlikePost()
    else -> if (buttonLike.showLike) likePost()
    else sendComment()
    }
    }
    }
    

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68. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    buttonLike.setOnClickListener {
    when (buttonLike.isLiked && buttonLike.showLike) {
    true -> unlikePost()
    else -> if (buttonLike.showLike) likePost()
    else sendComment()
    }
    }
    }
    

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69. sealed class ButtonState {
    object ShowLiked : ButtonState()
    object ShowUnLiked : ButtonState()
    object SendComment : ButtonState()
    }
    Reactive approach:
    

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70. class SendLikeButton(context: Context) : Button(context) {
    private val sendToLike by bindAnimation(R.drawable.avd_send_to_favorite)
    }
    

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71. class SendLikeButton(context: Context) : Button(context) {
    private val sendToLike by bindAnimation(R.drawable.avd_send_to_favorite)
    }
    fun View.bindAnimation(animateRes: Int) = lazy {
    AnimatedVectorDrawableCompat.create(context, animateRes)
    }
    

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72. class SendLikeButton(context: Context) : Button(context) {
    var state by
    Delegates.observable(ButtonState.ShowUnLiked)
    { p, oldValue, newValue ->
    when (newValue) {
    ButtonState.SendComment -> {
    when (oldValue) {
    ButtonState.ShowUnLiked -> morph(unlikeToSend)
    ButtonState.ShowLiked -> morph(likeToSend)
    }
    }
    . . .
    }
    }
    }
    

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73. class SendLikeButton(context: Context) : Button(context) {
    var state by
    Delegates.observable(ButtonState.ShowUnLiked)
    { p, oldValue, newValue ->
    when (newValue) {
    ButtonState.SendComment -> {
    when (oldValue) {
    ButtonState.ShowUnLiked ->
    morph(unlikeToSend)
    ButtonState.ShowLiked -> morph(likeToSend)
    }
    }
    ButtonState.ShowLiked -> . . .
    ButtonState.ShowUnLiked -> . . .
    }
    }
    

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74. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    RxView.clicks(buttonLike)
    .subscribe({
    when (buttonLike.state) {
    ButtonState.SendComment -> sendComment()
    ButtonState.ShowLiked -> unlikePost()
    ButtonState.ShowUnLiked -> likePost()
    }
    }, {}).addTo(disposable)
    val textChanged = RxTextView.textChanges(editText)
    Observable.combineLatest(apiResponseLikeStatus, textChanged,
    BiFunction { t1, t2 ->
    return@BiFunction when (t2.count() > 0) {
    true -> ButtonState.SendComment
    else -> if (t1) ButtonState.ShowLiked else
    ButtonState.ShowUnLiked
    }
    })
    .subscribe({
    buttonLike.state = it
    }, {}).addTo(disposable)
    }
    

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75. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    RxView.clicks(buttonLike)
    .subscribe({
    when (buttonLike.state) {
    ButtonState.SendComment -> sendComment()
    ButtonState.ShowLiked -> unlikePost()
    ButtonState.ShowUnLiked -> likePost()
    }
    }, {}).addTo(disposable)
    val textChanged = RxTextView.textChanges(editText)
    Observable.combineLatest(apiResponseLikeStatus, textChanged,
    BiFunction { t1, t2 ->
    return@BiFunction when (t2.count() > 0) {
    true -> ButtonState.SendComment
    else -> if (t1) ButtonState.ShowLiked else
    ButtonState.ShowUnLiked
    }
    })
    .subscribe({
    buttonLike.state = it
    }, {}).addTo(disposable)
    }
    

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76. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    RxView.clicks(buttonLike)
    .subscribe({
    when (buttonLike.state) {
    ButtonState.SendComment -> sendComment()
    ButtonState.ShowLiked -> unlikePost()
    ButtonState.ShowUnLiked -> likePost()
    }
    }, {}).addTo(disposable)
    val textChanged = RxTextView.textChanges(editText)
    Observable.combineLatest(apiResponseLikeStatus, textChanged,
    BiFunction { t1, t2 ->
    return@BiFunction when (t2.count() > 0) {
    true -> ButtonState.SendComment
    else -> if (t1) ButtonState.ShowLiked else
    ButtonState.ShowUnLiked
    }
    })
    .subscribe({
    buttonLike.state = it
    }, {}).addTo(disposable)
    }
    

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77. override fun onCreate(savedInstanceState: Bundle?) {
    . . .
    RxView.clicks(buttonLike)
    .subscribe({
    when (buttonLike.state) {
    ButtonState.SendComment -> sendComment()
    ButtonState.ShowLiked -> unlikePost()
    ButtonState.ShowUnLiked -> likePost()
    }
    }, {}).addTo(disposable)
    val textChanged = RxTextView.textChanges(editText)
    Observable.combineLatest(apiResponseLikeStatus, textChanged,
    BiFunction { t1, t2 ->
    return@BiFunction when (t2.count() > 0) {
    true -> ButtonState.SendComment
    else -> if (t1) ButtonState.ShowLiked else
    ButtonState.ShowUnLiked
    }
    })
    .subscribe({
    buttonLike.state = it
    }, {}).addTo(disposable)
    }
    

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78. class SendLikeButton(context: Context) : Button(context) {
    var state by Delegates.observable(ButtonState.ShowUnLiked) { p, oldValue,
    newValue ->
    when (newValue) {
    . . .
    }
    }
    private val sendToLike by bindAnimation(R.drawable.avd_send_to_favorite)
    . . .
    private val redLike by lazy { R.drawable.ic_favorite_red_24dp }
    . . .
    private fun morph(avd: AnimatedVectorDrawableCompat?) {
    background = avd
    avd?.start()
    }
    }
    fun View.bindAnimation(animateRes: Int) = lazy {
    AnimatedVectorDrawableCompat.create(context, animateRes)
    }
    sealed class ButtonState {
    . . .
    }
    

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79. References:
    https://kotlinlang.org/docs/reference/delegated-properties.html
    https://www.manning.com/books/kotlin-in-action
    https://github.com/AidaIssayeva/Reactive-approach-to-delegation
    

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80. Thank you
    Aida Issayeva
    @aida_isay
    

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