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Reactive approach to
delegation in Kotlin
Aida Issayeva
@aida_isay
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has a chimney
has a window
has a door
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interface WindowActionListener {
void openWindow();
void closeWindow();
}
interface DoorActionListener {
void closeDoor();
void openDoor();
}
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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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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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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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“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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Delegation in Kotlin
class Room : WindowActionListener by Window(),
DoorActionListener by Door()
class LivingRoom : WindowActionListener by Window()
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Delegation in Kotlin
class Room : WindowActionListener by Window(),
DoorActionListener by Door()
class LivingRoom : WindowActionListener by Window()
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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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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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Delegated properties
class User(val firstName: String, val lastName: String) {
var userName: String by UserNameDelegate()
}
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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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//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
$thisRef.")
}
}
Create custom delegated properties
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//or use `operator`
//for val
class UserNameDelegate {
operator fun getValue(thisRef: User?, property: KProperty<*>):
String {
return "${thisRef?.firstName}_${thisRef?.lastName}"
}
}
Create custom delegated properties
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//or use `operator`
//for var
class UserNameDelegate {
operator fun getValue(thisRef: User?, property: KProperty<*>):
String {
return "${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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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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1. lazy
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1. lazy delegate
class AboutMeFragment : Fragment() {
val component by lazy {
val appComponent = DaggerHelper.getAppComponent(this)
AboutMeComponent.builder()
.appComponent(appComponent)
.build()
}
}
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Lazy delegate
class AboutMeFragment : Fragment() {
val component by lazy {
val appComponent = DaggerHelper.getAppComponent(this)
AboutMeComponent.builder()
.appComponent(appComponent)
.build()
}
}
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Lazy delegate
class AboutMeFragment : Fragment() {
val component by lazy {
val appComponent = DaggerHelper.getAppComponent(this)
AboutMeComponent.builder()
.appComponent(appComponent)
.build()
}
}
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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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init
val
init
val
init
val
?
LazyThreadSafetyMode
SYNCHRONIZED PUBLICATION NONE
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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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2. notNull
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2. notNull delegate
class AboutMeFragment : Fragment() {
lateinit var age: Int
}
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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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notNull
•Later init
•Primitive && top-level properties
• No null checks
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3. map
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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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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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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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map
•Simplify access
• Define expected structure
• But have a power to access unexpected fields
•Map key == property name
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4. observable
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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", “[email protected]“)
.subscribe {
isLoading = false
}
.addTo(disposables)
}
}
}
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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", “[email protected]”)
.subscribe {
isLoading = false
}
.addTo(disposables)
}
}
}
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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", “[email protected]”)
.subscribe {
isLoading = false
}
.addTo(disposables)
}
}
}
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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", “[email protected]”)
.subscribe {
isLoading = false
}
.addTo(disposables)
}
}
}
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Adapters in Java
public class NameAdapter extends
RecyclerView.Adapter {
public void updateNames(ArrayList names){
this.list = names;
notifyDataSetChanged();
}
}
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Adapters in Kotlin
class NameAdapter(val context: Context) :
RecyclerView.Adapter() {
var list = emptyList()
fun updateNames(names: ArrayList) {
list = names
notifyDataSetChanged()
}
}
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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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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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5. vetoable
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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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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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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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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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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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Delegated properties in custom views
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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override fun onCreate(savedInstanceState: Bundle?) {
. . .
apiResponseLikeStatus
.subscribe({
buttonLike.setLikedView(it)
}, {}).addTo(disposable)
}
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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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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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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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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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sealed class ButtonState {
object ShowLiked : ButtonState()
object ShowUnLiked : ButtonState()
object SendComment : ButtonState()
}
Reactive approach:
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class SendLikeButton(context: Context) : Button(context) {
private val sendToLike by bindAnimation(R.drawable.avd_send_to_favorite)
}
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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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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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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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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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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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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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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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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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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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Thank you
Aida Issayeva
@aida_isay