Consider I'm trying to implement delegation by storing properties in a Map instance, and one of the properties delegated is an array:
class Foo private constructor(map: Map<String, Any?>) {
constructor(value: Array<Byte>) : this(mapOf(Foo::value.name to value))
val value: Array<Byte> by map
}
object PropertyDelegationTest {
#JvmStatic
fun main(vararg args: String) {
val foo = Foo(arrayOf(42.toByte(), 127.toByte()))
println(foo.value[0]) // 42
println(foo.value[1]) // 127
}
}
The above compiles just fine and works as expected.
Now consider I want to enhance my property delegation mechanism by implementing a custom Map.getValue(thisRef: Any?, property: KProperty<*>) extension method (overriding the default extension):
import kotlin.reflect.KProperty
import kotlin.reflect.full.isSubtypeOf
import kotlin.reflect.full.starProjectedType
import kotlin.reflect.jvm.jvmName
// ...
operator fun <V, V1 : V> Map<in String, V>.getValue(thisRef: Any?, property: KProperty<*>): V1 {
val value = this[property.name]
?: throw NoSuchElementException("Key ${property.name} is missing in the map.")
val clazz = (value as Any)::class
#Suppress("UNCHECKED_CAST")
return when {
clazz.starProjectedType.isSubtypeOf(property.returnType) -> value as V1
else -> throw ClassCastException("${clazz.starProjectedType} (${clazz.jvmName}) cannot be cast to ${property.returnType}")
}
}
This fails at run time:
Exception in thread "main" java.lang.ClassCastException: kotlin.Array<*> ([Ljava.lang.Byte;) cannot be cast to kotlin.Array<kotlin.Byte>
at com.example.PropertyDelegationTestKt.getValue(PropertyDelegationTest.kt:30)
at com.example.Foo.getValue(PropertyDelegationTest.kt)
at com.example.PropertyDelegationTest.main(PropertyDelegationTest.kt:18)
Despite the effective JVM type is known ([Ljava.lang.Byte;), Kotlin-specific run time type of the value is Array<*> while Array<Byte> is required. Consistently, clazz.typeParameters[0].upperBounds[0] evaluates to kotlin.Any?, not kotlin.Byte?.
How do I implement my custom type checking which would also work correctly for arrays? Kotlin version is 1.2.71.
You could remove the explicit type check and instead do a safe cast to V1 and if that fails then throw the exception, e.g.
return value as? V1 ?: throw ClassCastException(...
Related
Abstract problem:
Consider the following:
data class Data(val i: Int, val s: String = "")
fun <A1, A2, D> make(ctor: KFunction2<A1, A2, D>, sideEffect: (D) -> D) =
{ a1: A1, a2: A2 -> sideEffect(ctor(a1, a2)) }
val makeData = make(::Data) {
it.also { println("Side effect: i=${it.i}, s=${it.s}") }
}
I can call makeData with all of the constructor arguments to obtain
an instance of Data and also call some side-effect. However, I
cannot omit parameters and use the constructor's default values instead:
val d1 = makeData(42, "hi")
val d2 = makeData2(42) // Error: No value passed for parameter 'p2'
How can I rewrite the generic make such that its output is a function with default values?
What I'm trying to achieve:
With Kotlin React, I declare properties like so:
external interface VideoListProps: RProps { var videos: List<Video> }
To use the component within render(), I can write:
child(VideoList::class) { attrs.videos = unwatchedVideos }
I don't like this for three reasons: (a) using a component is much more verbose than compared to TypeScript/TSX,
(b) properties need to be declared as var while they should be read-only (val), (c) there is no enforcement that a member is actually declared; props.videos may end up as null at runtime even though the Props interface does not declare it as nullable.
The tutorial for Kotlin React suggests to use lambdas with receivers like so:
fun RBuilder.videoList(handler: VideoListProps.() -> Unit): ReactElement {
return child(VideoList::class) {
this.attrs(handler)
}
}
Now I can use the component like so:
videoList { videos = unwatchedVideos }
While the calling site now looks much more concise, I now have to copy/paste and adapt the lambda snippet for each and every component I write. This is much more verbose than in TypeScript, where I don't need this construct. Moreover, the videoList lambda has no access to other members of the RBuilder as this now points to RProps. So if I need to define a React key, I'm at a loss and need to use the child(...) syntax from before. And lastly, the problem (c) from above still exists.
To make this better, I wrote the following helper constructs:
fun <P : RProps, A1> RBuilder.childWithProps(
klass: KClass<out Component<P, *>>,
ctor: KFunction1<A1, P>
) =
{ a1: A1, handler: RHandler<P> -> child(klass.rClass, ctor(a1), handler) }
fun <P : RProps, A1, A2> RBuilder.childWithProps(
klass: KClass<out Component<P, *>>,
ctor: KFunction2<A1, A2, P>
) =
{ a1: A1, a2: A2, handler: RHandler<P> -> child(klass.rClass, ctor(a1, a2), handler) }
// more for KFunction3, KFunction4, ... up to a reasonable amount of members, e.g. 16
I admit this is also kind of ugly, but I need to write this once only. Now I can declare props like this (note the val instead of var):
data class VideoListProps(val videos: List<Videos>) : RProps
The additional code for each component is now much more concise:
val RBuilder.videoList get() = childWithProps(VideoList::class, ::VideoListProps)
And use it like this:
videoList(watchedVideos) { /* you can set React key or children here if needed */ }
Kotlin now marks it as an error if I forget to set all properties.
Everything now looks concise, I write even slightly less code than in TypeScript/TSX. The only remaining problem is that now I always have to pass all members, since any default values of the data constructor will still be madatory in the wrapper. This is the actual question.
I have a helper class which creates instance of another class
class TestEnv {
val questionsController = new QuestionsController(...)
}
I am unit testing QuestionsController and have created a basic test case
class QuestionsControllerUnitSpec extends PlaySpec with BeforeAndAfterAll with BeforeAndAfterEach with OneAppPerSuiteWithComponents{
override def beforeEach() = {
println("------------new test -----------------")
}
override def components: BuiltInComponents = new BuiltInComponentsFromContext(context) with NoHttpFiltersComponents {
import play.api.mvc.Results
import play.api.routing.Router
import play.api.routing.sird._
lazy val router: Router = Router.from({
case GET(p"/") => defaultActionBuilder {
Results.Ok("success!")
}
})
}
"Question Controller " should {
"be created" in {
val testEnv = new TestEnv(components = components)
val qc:QuestionsController = testEnv.questionsController
qc mustBe defined //I get compilation error
}
}
}
I get the following compilation error
Error:(52, 10) could not find implicit value for parameter definition: org.scalatest.enablers.Definition[controllers.QuestionsController]
qc mustBe defined
Error:(52, 10) not enough arguments for method mustBe: (implicit definition: org.scalatest.enablers.Definition[controllers.QuestionsController])org.scalatest.Assertion.
Unspecified value parameter definition.
qc mustBe defined
I checked the definition of mustBe in MustMatchers.class. It is defined as def mustBe(right : org.scalatest.words.DefinedWord)(implicit definition : org.scalatest.enablers.Definition[T]) : org.scalatest.Assertion = { /* compiled code */ }
Why am I getting the error.
defined matcher syntax can be used with user defined types if we provide implicit implementation of Definition trait. For example, say we have a user defined class
class Foo {
val bar = 3
}
and we provide implicit definition
implicit val fooDefinition = new Definition[Foo] {
override def isDefined(foo: Foo): Boolean = foo.bar != null
}
then we can use defined syntax
(new Foo()) mustBe defined
If similar implicit implementation of Definition[QuestionsController] is provided, then the compiler error should be resolved.
I am happy to accept a different answer if it can provide more accurate answer. I suppose I am testing the wrong thing. What I am doing is similar to declaring an integer and checking if the integer exists! Instead I should be checking the value of the integer.
About matchers, more information is at http://doc.scalatest.org/3.0.1/#org.scalatest.MustMatchers. More information on Definition is on http://doc.scalatest.org/3.0.1/#org.scalatest.enablers.Definition
I am taking some inspiration from
https://marcosantadev.com/swift-arrays-holding-elements-weak-references/
and I want to be able to maintain an array holding weak references to its elements, so that in case those elements get released elsewhere in my code base, I don't retain them in my array.
I would like the implementation to be as type safe as possible, however should be reusable.
The strategy that I am using is declaring a Weak Reference container as so.
class WeakRefContainer<T> where T: AnyObject {
private(set) weak var value: T?
init(value: T?) {
self.value = value
}
}
Then I want to maintain an array of these WeakRefContainers, so I create an array extension:
extension Array where Element: WeakRefContainer<AnyObject> {
func compact() -> [WeakRefContainer<AnyObject>] {
return filter { $0.value != nil }
}
}
When calling the compact method, I am now able to clear up the array in case stuff needs to be cleaned up.
I am now having some compilation issues which am having trouble understanding.
Lets suppose I have a sample class
class SampleClass {
}
And I try to use everything as follows:
var weakReferencesArray = [WeakRefContainer<SampleClass>]()
let obj1 = WeakRefContainer.init(value: SampleClass())
let obj2 = WeakRefContainer.init(value: SampleClass())
weakReferencesArray.append(obj1)
weakReferencesArray.append(obj2)
weakReferencesArray.compact()
When I try to call compact I get the following error message:
MyPlayground.playground:29:21: 'WeakRefContainer<SampleClass>' is not a subtype of 'WeakRefContainer<AnyObject>'
Can anyone unblock me please? Thanks
Your code doesn't work because WeakRefContainer<SampleClass> is not a subclass of WeakRefContainer<AnyObject> because generics are invariant in Swift. Thus weakReferencesArray can't use the compact method added from the extension.
There is a workaround for this, via a protocol:
protocol WeakHolder {
var hasRef: Bool { get }
}
extension WeakRefContainer: WeakHolder {
var hasRef: Bool { return value != nil }
}
extension Array where Element: WeakHolder {
func compacted() -> [Element] {
return filter { $0.hasRef }
}
mutating func compact() {
self = compacted()
}
}
I also renamed compact to compacted, for better Swift semantics, and replaced the original compact by a mutating version.
You probably want the extension to apply to all [WeakRefContainer<T>] where T can be any type extending AnyObject.
extension Array where Element: WeakRefContainer<T> {
However, currently, parameterised extensions are not possible. See this proposal.
You can kind of work around this by making compact generic:
extension Array{
func compact<T>() -> [Element] where Element == WeakRefContainer<T> {
return filter { $0.value != nil }
}
}
From array.scala of scala-2.10.4, The Array is defined as
final class Array[T](_length: Int) extends java.io.Serializable with java.lang.Cloneable {
/** The length of the array */
def length: Int = throw new Error()
def apply(i: Int): T = throw new Error()
def update(i: Int, x: T) { throw new Error() }
override def clone(): Array[T] = throw new Error()
}
Please note, the apply method will throw an exception! And for the accompany object Arrry, I find the following codes:
def apply[T: ClassTag](xs: T*): Array[T] = {
val array = new Array[T](xs.length)
var i = 0
for (x <- xs.iterator) { array(i) = x; i += 1 }
array
}
I know there is an implicit parameter which is ClassTag[T], what make me surprised is how
new Array[T] (xs.length)
is compiled. By decompiling the Array.class, I find that line is translated to :
public <T> Object apply(Seq<T> xs, ClassTag<T> evidence$2)
{
// evidence$2 is implicit parameter
Object array = evidence$2.newArray(xs.length());
...
}
I am really confused by this kind of translation, what is the rule under the hood?
Thanks
Chang
The Scala Array Class is just a fake wrapper for the runtime so you can use arrays in Scala. You're probably confused because those methods on the Array class throw exceptions. The reason they did this is so that if you actually end up using the fake class it blows up since really it should be using the java runtime array, which does not have a proper container class like Scala. You can see how the compiler handles it here. When your using arrays in Scala you're probably also using some implicits from predef like ArrayOps and WrappedArray for extra helper methods.
TLDR: Scala compiler magic makes arrays work with the java runtime under the hood.
On the JVM arrays are exempt from type-erasure, e.g. at runtime instead of Array[_] there is a difference between Array[Int], Array[String] and Array[AnyRef] for example. Unlike Java, Scala can handle this mostly transparently, so
class Foo {
val foo = new Array[Int](123)
}
has a direct byte-code invocation for creating the integer array, whereas
class Bar[A](implicit ev: reflect.ClassTag[A]) {
val bar = new Array[A](123)
}
is solved by using the implicit type evidence parameter of type ClassTag[A] so that at runtime the JVM can still create the correct array. This is translated into the call you saw, ev.newArray(123).
How do you define a static extension method in Kotlin? Is this even possible? I currently have an extension method as shown below.
public fun Uber.doMagic(context: Context) {
// ...
}
The above extension can be invoked on an instance.
uberInstance.doMagic(context) // Instance method
but how do I make it static method like shown below.
Uber.doMagic(context) // Static or class method
To achieve Uber.doMagic(context), you can write an extension to the companion object of Uber (the companion object declaration is required):
class Uber {
companion object {}
}
fun Uber.Companion.doMagic(context: Context) { }
This is what the official documentation says:
Kotlin generates static methods for package-level functions. Kotlin
can also generate static methods for functions defined in named
objects or companion objects if you annotate those functions as
#JvmStatic. For example:
Kotlin static methods
class C {
companion object {
#JvmStatic fun foo() {}
fun bar() {}
}
}
Now, foo() is static in Java, while bar() is not:
C.foo(); // works fine
C.bar(); // error: not a static method
I actually had this exact question 30 minutes ago, so I started digging around and couldn't find any solution or workaround for this, BUT while searching I found this section on the Kotlinglang website that states that:
Note that extensions can be defined with a nullable receiver type. Such extensions can be called on an object variable even if its value is null.
So then I had the craziest idea ever, why not define an extension function with a nullable receiver (without actually using that receiver) and then call it on a null object!
So I tried that, and it worked pretty well, but it looked so ugly. It was like this:
(null as Type?).staticFunction(param1, param2)
So I went around that by creating a val in my extensions file of the receiver type that had a value of null and then use it in my other class.
So, as an example, here is how I implemented a "static" extension function for the Navigation class in Android:
In my NavigationExtensions.kt file:
val SNavigation: Navigation? = null
fun Navigation?.createNavigateOnClickListener(#IdRes resId: Int, args: Bundle? = null, navOptions: NavOptions? = null,
navigationExtras: Navigator.Extras? = null) : (View) -> Unit {
//This is just implementation details, don't worry too much about them, just focus on the Navigation? part in the method declaration
return { view: View -> view.navigate(resId, args, navOptions, navigationExtras) }
}
In the code that uses it:
SNavigation.createNavigateOnClickListener(R.id.action_gameWonFragment_to_gameFragment)
Obviously, this isn't a class name, it is just a variable of the class type that has a null value. This is obviously ugly on the extension maker side (because they have to create the variable) and on the developer side (because they have to use the SType format instead of the actual class name), but it is the closest that can be achieved right now compared to actual static functions. Hopefully, the Kotlin language makers will respond to the issue that was created and add that feature in the language.
Since I keep coming across this when searching, here's a different approach I haven't seen anyone mention that works in a static way and it works with generics!
Extension definitions:
// Extension function
fun <T> KClass<T>.doSomething() = /* do something */
// Extension Property
val <T> KClass<T>.someVal get() = /* something */
Usage:
MyType::class.doSomething()
MyType::class.someVal
As you can see, the trick is attaching the extension function to the KClass of the type instead since that can be referenced statically.
You can create a static method with using Companion object like:
class Foo {
// ...
companion object {
public fun bar() {
// do anything
}
}
}
and then you can call it like:
class Baz {
// ...
private fun callBar() {
Foo.bar()
}
}
Recomend you to look at this link. As you can see there, you just should declare method at the top-level of the package (file):
package strings
public fun joinToString(...): String { ... }
This is equal to
package strings;
public class JoinKt {
public static String joinToString(...) { ... }
}
With constans everything are the same. This declaration
val UNIX_LINE_SEPARATOR = "\n"
is equal to
public static final String UNIX_LINE_SEPARATOR = "\n";
I also required the ability to extend a Java object with a static method and found the best solution for me was to create a Kotlin object that extended the Java class and add my method there.
object Colour: Color(){
fun parseColor(r: Int?, g: Int?, b: Int?) = parseColor(String.format("#%02x%02x%02x", r, g, b))
}
invocation:
val colour = Colour.parseColor(62, 0, 100)
I'm also quite fond of having the possibility to add static extension methods in Kotlin. As a workaround for now I'm adding the exntension method to multiple classes instead of using one static extension method in all of them.
class Util
fun Util.isDeviceOnline(context: Context): Boolean {
val connMgr = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
val networkInfo = connMgr.activeNetworkInfo
return networkInfo != null && networkInfo.isConnected
}
fun Activity.isDeviceOnline(context: Context) = { Util().isDeviceOnline(context) }
fun OkHttpClient.isDeviceOnline(context: Context) = { Util().isDeviceOnline(context) }
To create an extension method in kotlin you have to create a kotlin file(not a class) then declare your method in the file
Eg:
public fun String.toLowercase(){
// **this** is the string object
}
Import the function in the class or file you are working on and use it.