I have a question about sealed class, generics and object.
Let's say I would like to model something like 3 finite cases with a sealed class something like this:
sealed class ChangeState<S> {
fun reduceState(state: S): S
}
data class SetState<S>(val newState: S) : ChangeState<S>() {
override fun reduce(state: S): S = newState
}
object NoStateChange : ChangeState<Nothing>() { // What do I specify here for ChangeState? Nothing?
override fun reduce(state: Nothing): Nothing {
throw Exception("This should never be called")
}
}
The goal is to provide a convenient way to define NoStateChange in a generic way that it can be used as following:
fun foo(i : Int) : ChangeState<Int> {
return if (i==0)
NoStateChange // Won't compile because return type is ChangeState<Nothing> but expected ChangeState<Int>
else
SetState(i)
}
Is there a way to do that with object and Generics somehow?
As pointed out by #Tenfour04 the issue is that out is needed but reduceState() would require in as well. However, reduceState() can be refactored out of the class hierarchy and moved to an extension function like that:
sealed class ChangeState<out S>
data class SetState<S>(val newState: S) : ChangeState<S>()
object NoStateChange : ChangeState<Nothing>()
fun <S> ChangeState<S>.reduce(state: S): S {
return when (val change = this) {
is SetState -> change.newState
is NoStateChange -> state
}
}
Related
Is it impossible to use generic on interface level as argument type for function?
I read about out and in keywords but as I understand they don't work for this case.
interface BaseB
open class ChildB1: BaseB
open class ChildB2: BaseB
abstract class BaseMapper<V: BaseB> {
open fun test(v: V) {
return
}
}
class TestMapper1: BaseMapper<ChildB1>() {
override fun test(v: ChildB1) {
return
}
}
class TestMapper2: BaseMapper<ChildB2>() {
override fun test(v: ChildB2) {
return
}
}
#Test
fun t() {
//ERROR
val mappers: List<BaseMapper<BaseB>> = listOf(TestMapper1(), TestMapper2())
mappers[0].test(ChildB1())
}
A BaseMapper<ChildB1> is not logically a BaseMapper<BaseB>. It consumes ChildB’s, so if you passed some other implementation of Base it would cause a ClassCastException if the compiler let you do that. There is no common subtype of your two subclasses besides Nothing, so the only way to put both of these types in the same list is to make it a List<BaseMapper<in Nothing>>.
Example of why it is not logically a BaseMapper<BaseB>:
open class ChildB1: BaseB {
fun sayHello() = println("Hello world")
}
class TestMapper1: BaseMapper<ChildB1>() {
override fun test(v: ChildB1) {
v.sayHello() // if v is not a ChildB1, this would be impossible
}
}
//...
val impossibleCast: BaseMapper<BaseB> = TestMapper1()
// TestMapper1 cannot call sayHello() because it's undefined for ChildB2.
// This is impossible:
impossibleCast.test(ChildB2())
// ...so the compiler prevents you from doing the impossible cast in the first place.
The Result<out R> is a sealed class which hold three subclass Success, Error and Loading.
The fun Greeting is #Composable.
By my design, I define queryList as Result class, and it is assigned as Loading first, then it will be Success or Error.
1: But the following code can't be compiled as the following error information, what's wrong with my Code?
2: Is there a better solution for my design?
Compile error
Property delegate must have a 'getValue(Nothing?, KProperty>)' method. None of the following functions are suitable.*
#Composable
fun Greeting(
name: String,
mViewMode:SoundViewModel= viewModel()
) {
Column() {
//The following code cause error.
val queryList by produceState(initialValue = Result<Flow<List<MRecord>>>.Loading ) {
value = mViewMode.listRecord()
}
when (queryList){
is Loading -> { ...}
is Error -> { ...}
is Success -> {...}
}
}
}
class SoundViewModel #Inject constructor(): ViewModel()
{
fun listRecord(): Result<Flow<List<MRecord>>>{
return aSoundMeter.listRecord()
}
}
sealed class Result<out R> {
data class Success<out T>(val data: T) : Result<T>()
data class Error(val exception: Exception) : Result<Nothing>()
object Loading : Result<Nothing>()
}
Since queryList is backed by a delegate, it can not be final.
This means in theory, each time you access it, it might hold a different value. The kotlin compiler is very pessimistic about this and assumes that between the time the is Result.Success branch of your when statement is selected and val mydata = queryList.data is executed, the value of queryList might have changed.
To solve this, you can assign the current value of queryList to a final variable and work with that one instead:
when (val currentList = queryList) {
is Result.Error -> {}
is Result.Loading -> {}
is Result.Success -> {
SomeComposable(currentList.data) //currentList is properly smart-cast to Result.Success
}
}
I am new to Kotlin and I was playing with it. I pretty much wanted to create a pretty basic event bus. So I came up with this
interface Event
interface EventListener<E : Event> {
fun handle(event: E)
}
interface EventBus {
fun <E : Event> registerListener(aClass: Class<E>, eventListener: EventListener<E>)
}
class MyBus() : EventBus {
private val eventListeners: MutableMap<String, MutableList<EventListener<out Event>>> = mutableMapOf()
constructor(listeners: List<Pair<Class<Event>, EventListener<Event>>>) : this() {
listeners.forEach {
registerListener(it.first, it.second)
}
}
override fun <E : Event> registerListener(aClass: Class<E>, eventListener: EventListener<E>) {
val key = aClass.name
val listeners: MutableList<EventListener<out Event>> = eventListeners.getOrPut(key) { mutableListOf() }
listeners.add(eventListener)
}
}
val bus = MyBus(
listOf(
MyEvent::class.java to MyEventListener()
)
)
class MyEvent : Event
class AnotherEvent : Event
class MyEventListener : EventListener<MyEvent> {
override fun handle(event: MyEvent) {
}
}
what happens is that when I try to create MyBus using the constructor accepting the list of pairs, I get
Type inference failed. Expected type mismatch: inferred type is List<Pair<Class<MyEvent>,MyEventListener>> but List<Pair<Class<Event>,EventListener<Event>>> was expected
But if I change the constructor to be something like
constructor(listeners: List<Pair<Class<out Event>, EventListener<out Event>>>) : this() {
listeners.forEach {
registerListener(it.first, it.second)
}
}
adding out pretty much everywhere, then the MyBus constructor works, but the invocation to registerListener(..) breaks for the same exact reason as before. So the only way to solve this is to add "out"s also on registerListener function.
I suspect I'm doing something wrong here, but I don't know what precisely. Any help?
If you want your EventListener to be able to consume Events, then its type has to be invariant or covariant (not declared out). If it let you pass your EventListener<MyEvent> as if it were an EventListener<Event>, then your MyBus class might call listener.handle(event) on it with some Event that is not a MyEvent, such as AnotherEvent. Then you will get a ClassCastException when it tries to cast this AnotherEvent to MyEvent.
To be able to store different types of invariant EventHandlers, you will have to remove the variance restrictions by using star projection, and cast them when you retrieve them from the map. So make the map keys into class objects instead of just Strings. Since you will not have the help of the compiler when working with the star-projected types, you need to be careful that you are only adding an item to your MutableMap that is of the same type as the Class key that's associated with it. Then when you retrieve items, only cast to an invariant type.
The other part of your issue is that your constructor needs a generic type. Right now it works exclusively with Event so it can't handle subtypes of Event. Kotlin doesn't (yet?) support generic types for constructors so you have to do this with a factory function.
Here's an example of all the above.
class MyBus() : EventBus {
private val eventListeners: MutableMap<Class<*>, MutableList<EventListener<*>>> = mutableMapOf()
override fun <E : Event> registerListener(aClass: Class<E>, eventListener: EventListener<E>) {
val listeners = retrieveListeners(aClass)
listeners.add(eventListener)
}
private fun <E: Event> retrieveListeners(aClass: Class<E>): MutableList<EventListener<E>> {
#Suppress("UNCHECKED_CAST")
return eventListeners.getOrPut(aClass) { mutableListOf() } as MutableList<EventListener<E>>
}
}
// Factory function
fun <E : Event> myBusOf(listeners: List<Pair<Class<E>, EventListener<E>>>): MyBus {
return MyBus().apply {
listeners.forEach {
registerListener(it.first, it.second)
}
}
}
And you might want to change the type of the factory parameter from a <List>Pair to a vararg Pair so it's easier to use.
Here's a stripped down example to explain the variance limitation.
Your interface for an Event consumer:
interface EventListener<E : Event> {
fun handle(event: E)
}
Two implementations of Event:
class HelloEvent: Event {
fun sayHello() = println("Hello world")
}
class BoringEvent: Event {}
A class implementing the interface:
class HelloEventListener: EventListener<HelloEvent> {
override fun handle(event: HelloEvent) {
event.sayHello()
}
}
Now you have an EventListener that can handle only HelloEvents. Try to treat it like an EventListener<Event>:
val eventListener: EventListener<Event> = HelloEventListener() // COMPILE ERROR!
Imagine the compiler did not prevent you from doing this and you do this:
val eventListener: EventListener<Event> = HelloEventListener()
eventListener.handle(BoringEvent()) // CLASS CAST EXCEPTION AT RUN TIME!
If this were allowed your HelloEventListener would try to call sayHello() on the BoringEvent, which doesn't have that function, so it will crash. This is what generics are here to protect you from.
Now suppose your HelloEventListener.handle() didn't call event.sayHello(). Well, then it could have safely handled a BoringEvent. But the compiler isn't doing that level of analysis for you. It just knows what you declared, that HelloEventListener cannot handle anything except HelloEvent.
I have a collection of objects which inherit Component and I want a function which finds an object by its concrete class and return it.
But Kotlin does not like the cast I do, and adding #Suppress("UNCHECKED_CAST") is ugly.
I have the following code:
open class GameObjectImpl : GameObject {
private val attachedComponents = mutableSetOf<Component>()
#Suppress("UNCHECKED_CAST")
override fun <TComponent : Component> getComponent(type: KClass<TComponent>): TComponent? {
return attachedComponents.find { type.isInstance(it) } as? TComponent
}
}
This should work for you:
open class GameObjectImpl : GameObject {
val attachedComponents = mutableSetOf<Component>()
override inline fun <reified TComponent : Component> getComponent(type: KClass<TComponent>): TComponent? {
return attachedComponents.filterIsInstance<TComponent>().firstOrNull()
}
}
Is there a way to specify the return type of a function to be the type of the called object?
e.g.
trait Foo {
fun bar(): <??> /* what to put here? */ {
return this
}
}
class FooClassA : Foo {
fun a() {}
}
class FooClassB : Foo {
fun b() {}
}
// this is the desired effect:
val a = FooClassA().bar() // should be of type FooClassA
a.a() // so this would work
val b = FooClassB().bar() // should be of type FooClassB
b.b() // so this would work
In effect, this would be roughly equivalent to instancetype in Objective-C or Self in Swift.
There's no language feature supporting this, but you can always use recursive generics (which is the pattern many libraries use):
// Define a recursive generic parameter Me
trait Foo<Me: Foo<Me>> {
fun bar(): Me {
// Here we have to cast, because the compiler does not know that Me is the same as this class
return this as Me
}
}
// In subclasses, pass itself to the superclass as an argument:
class FooClassA : Foo<FooClassA> {
fun a() {}
}
class FooClassB : Foo<FooClassB> {
fun b() {}
}
You can return something's own type with extension functions.
interface ExampleInterface
// Everything that implements ExampleInterface will have this method.
fun <T : ExampleInterface> T.doSomething(): T {
return this
}
class ClassA : ExampleInterface {
fun classASpecificMethod() {}
}
class ClassB : ExampleInterface {
fun classBSpecificMethod() {}
}
fun example() {
// doSomething() returns ClassA!
ClassA().doSomething().classASpecificMethod()
// doSomething() returns ClassB!
ClassB().doSomething().classBSpecificMethod()
}
You can use an extension method to achieve the "returns same type" effect. Here's a quick example that shows a base type with multiple type parameters and an extension method that takes a function which operates on an instance of said type:
public abstract class BuilderBase<A, B> {}
public fun <B : BuilderBase<*, *>> B.doIt(): B {
// Do something
return this
}
public class MyBuilder : BuilderBase<Int,String>() {}
public fun demo() {
val b : MyBuilder = MyBuilder().doIt()
}
Since extension methods are resolved statically (at least as of M12), you may need to have the extension delegate the actual implementation to its this should you need type-specific behaviors.
Recursive Type Bound
The pattern you have shown in the question is known as recursive type bound in the JVM world. A recursive type is one that includes a function that uses that type itself as a type for its parameter or its return value. In your example, you are using the same type for the return value by saying return this.
Example
Let's understand this with a simple and real example. We'll replace trait from your example with interface because trait is now deprecated in Kotlin. In this example, the interface VitaminSource returns different implementations of the sources of different vitamins.
In the following interface, you can see that its type parameter has itself as an upper bound. This is why it's known as recursive type bound:
VitaminSource.kt
interface VitaminSource<T: VitaminSource<T>> {
fun getSource(): T {
#Suppress("UNCHECKED_CAST")
return this as T
}
}
We suppress the UNCHECKED_CAST warning because the compiler can't possibly know whether we passed the same class name as a type argument.
Then we extend the interface with concrete implementations:
Carrot.kt
class Carrot : VitaminSource<Carrot> {
fun getVitaminA() = println("Vitamin A")
}
Banana.kt
class Banana : VitaminSource<Banana> {
fun getVitaminB() = println("Vitamin B")
}
While extending the classes, you must make sure to pass the same class to the interface otherwise you'll get ClassCastException at runtime:
class Banana : VitaminSource<Banana> // OK
class Banana : VitaminSource<Carrot> // No compiler error but exception at runtime
Test.kt
fun main() {
val carrot = Carrot().getSource()
carrot.getVitaminA()
val banana = Banana().getSource()
banana.getVitaminB()
}
That's it! Hope that helps.
Depending on the exact use case, scope functions can be a good alternative. For the builder pattern apply seems to be most useful because the context object is this and the result of the scope function is this as well.
Consider this example for a builder of List with a specialized builder subclass:
open class ListBuilder<E> {
// Return type does not matter, could also use Unit and not return anything
// But might be good to avoid that to not force users to use scope functions
fun add(element: E): ListBuilder<E> {
...
return this
}
fun buildList(): List<E> {
...
}
}
class EnhancedListBuilder<E>: ListBuilder<E>() {
fun addTwice(element: E): EnhancedListBuilder<E> {
addNTimes(element, 2)
return this
}
fun addNTimes(element: E, times: Int): EnhancedListBuilder<E> {
repeat(times) {
add(element)
}
return this
}
}
// Usage of builder:
val list = EnhancedListBuilder<String>().apply {
add("a") // Note: This would return only ListBuilder
addTwice("b")
addNTimes("c", 3)
}.buildList()
However, this only works if all methods have this as result. If one of the methods actually creates a new instance, then that instance would be discarded.
This is based on this answer to a similar question.
You can do it also via extension functions.
class Foo
fun <T: Foo>T.someFun(): T {
return this
}
Foo().someFun().someFun()