It appears that class types in flow always refer to instances of that class and one uses typeof to refer to the actual class itself. So, if I want a variable to refer to a subclass (not an instance) of a base class, I can do:
class MyBaseClass {}
class MySubClass extends MyBaseClass {}
let a: $Subtype<MyBaseClass> = MySubClass; // fails
let b: $Subtype<MyBaseClass> = new MySubClass(); // works, but I don't want this.
let c: $Subtype<typeof MyBaseClass> = MySubClass; // works! Ok, we're good
However, I can't seem to do this with type parameters! For example, the following:
type GenericSubclass<T> = $Subtype<typeof T>;
// fails with `^ identifier `T`. Could not resolve name`
If I try the following Typescript trick (see Generic and typeof T in the parameters), it also fails:
type ValidSubclass<T> = { new(): T };
const c: ValidSubclass<BaseClass> = MySubClass;
// fails with: property `new`. Property not found in statics of MySubClass
Note that I tried new, __proto__ and constructor.
What gives? Is there a workaround?
typeof MyBaseClass
is
Class<MyBaseClass>
so you can do
type GenericSubclass<T> = $Subtype<Class<T>>;
Related
I've got a method that does the following:
val type = AClass::class.java.methods[0].parameters[0].type
val toDeserialise = SecondClass<type>()
My SecondClass is:
class SecondClass<T : Any> {
lateinit var p1: T
}
But this doesn't work. Basically, I want to take a type of a method's parameter and pass it into a SecondClass. Do you know how to implement this?
The AClass is:
class AClass{
fun myMethod(param1: String, param2: UUID)
}
So, I want the
val type
to be String, which I pass into SecondClass.
Reflection is resolved at runtime, and generic types are resolved at compile time.
The SecondClass is subject to type erasure, so what you want to do is essentially impossible.
Even if you try to work around it with a switch:
val type = AClass::class.java.methods[0].parameters[0].type
val toDeserialize = when(type){
String::class.java -> SecondClass<String>()
else -> SecondClass<Any>()
}
This will bring you no benefit since you won't be able to check the actual type of SecondClass.
I have read Access property delegate in Kotlin which is about accessing a delegate from an instance. One can use KProperty::getDelegate since Kotlin 1.1, however this will return the instance of the delegate and therefore needs an instance of the class first.
Now I want to get the type of the delegate without having an instance of the class. Consider a library with a custom delegate type CustomDelegate that want's to get all properties of a class that are delegated to an instance of CustomDelegate:
class Example
{
var nonDelegatedProperty = "I don't care about this property"
var delegatedProperty1 by lazy { "I don't care about this too" }
var delegatedProperty2 by CustomDelegate("I care about this one")
}
How can I, given I have KClass<Example>, but not an instance of Example, get all properties delegated to CustomDelegate?
How can I, given I have KClass<Example>, but not an instance of
Example, get all properties delegated to CustomDelegate?
You can do it in two ways depending on your needs.
First of all, you have to include the kotlin-reflect dependency in your build.gradle file:
compile "org.jetbrains.kotlin:kotlin-reflect:1.1.51"
In my opinion, you should use the first solution if you can, because it's the most clear and optimized one. The second solution instead, can handle one case that the first solution can't.
First
You can loop an the declared properties and check if the type of the property or the type of the delegate is CustomDelegate.
// Loop on the properties of this class.
Example::class.declaredMemberProperties.filter { property ->
// If the type of field is CustomDelegate or the delegate is an instance of CustomDelegate,
// it will return true.
CustomDelegate::class.java == property.javaField?.type
}
There's only one problem with this solution, you will get also the fields with type CustomDelegate, so, given this example:
class Example {
var nonDelegatedProperty = "I don't care about this property"
val delegatedProperty1 by lazy { "I don't care about this too" }
val delegatedProperty2 by CustomDelegate("I care about this one")
val customDelegate = CustomDelegate("jdo")
}
You will get delegatedProperty2 and customDelegate. If you want to get only delegatedProperty2, I found an horrible solution that you can use if you need to manage this case.
Second
If you check the source code of KPropertyImpl, you can see how a delegation is implemented. So, you can do something like this:
// Loop on the properties of this class.
Example::class.declaredMemberProperties.filter { property ->
// You must check in all superclasses till you find the right method.
property::class.allSuperclasses.find {
val computeField = try {
// Find the protected method "computeDelegateField".
it.declaredFunctions.find { it.name == "computeDelegateField" } ?: return#find false
} catch (t: Throwable) {
// Catch KotlinReflectionInternalError.
return#find false
}
// Get the delegate or null if the delegate is not present.
val delegateField = computeField.call(property) as? Field
// If the delegate was null or the type is different from CustomDelegate, it will return false.
CustomDelegate::class.java == delegateField?.type
} != null
}
In this case, you will get only delegatedProperty2 as result.
I'm trying to use websocket in my project.
to do so, I installed the package Microsoft Asp.Net SignalR, which consists of WebSocketHandler abstract class.
i defined a class inheriting WebSocketHandler, but then the compiler complains:
'Microsoft.AspNet.SignalR.WebSockets.WebSocketHandler' does not contain a constructor that takes 0 arguments'.
It seems wierd to me, because the definitioin of WebSocketHandler ctor gets a nullable value, which means the ctor could get no parameter,
the definition looks like this:
protected WebSocketHandler(int? maxIncomingMessageSize);
can anybody tell me what the problem is?
thanks.
It seems wierd to me, because the definitioin of WebSocketHandler ctor gets a nullable value, which means the ctor could get no parameter
No, it doesn't. There's a big difference between receiving a null value for a nullable type, and not receiving a value at all.
If the parameter were optional, that would be a different matter - but it's not. You have to supply an argument convertible to int? in the call. If you want to provide the null value for int?, do so:
var handler = new WebSocketHandler(null);
Or if you want to avoid accidentally using any other single-parameter constructor definitions which may be applicable with a null literal as the argument, you could use:
var handler = new WebSocketHandler((int?) null);
Or:
var handler = new WebSocketHandler(default(int?));
protected member is accessible by derived class instances and there's nothing special about it. Nothing special in the class itself, either # WebSocketHandler.cs.
It just mens you need to pass in a nullable type, it does not mean it can't get any arguments.
int? maxIncomingMessageSize = 0;
var socket = new WebSocketHandler(maxIncomingMessageSize);
In your derived class you could/should define a "constructor that takes 0 arguments".
public class MyHandler : WebSocketHandler
{
// not mandatory
public MyHandler()
:this(null)
{}
// mandatory
public MyHandler(int? maxIncomingMessageSize)
:base(maxIncomingMessageSize)
{}
}
I have class name : some.path.exampleclass and have to instantiate class (like getDefinitionByName()). Problem is that getDefinitionByName() makes class exampleclass and i need class with FULL name : some.path.exampleclass to pass it to custom component :
<tools:MyComp value="some.path.exampleclass"/>
Is there a quick solution for this? :)
If you're intent is to create an instance of an FXG element based on a string that represents it's fully qualified class name, you can do it like this:
var classNameString :String = // your code to get the classname;
var class :Class = getDefinitionByName( className ) as Class;
var newObject : MyObject = new class() as MyObject;
This should work perfectly fine with FXG Elements. I use it in my mobile game.
You mention that:
Setter method...need to be set with full path class
not only with one that getDefinitionByName gives me
getDefinitionByName returns an instance of Class. If your method is expecting a string that is why getDefinitionByName doesn't work.
For more specifics, if needed, you should provide the full source code for the method or property.
I'm in scala writing a serializer that saves an object (or Model) to the database (for app engine), and I need to treat some fields as special cases. For example, if the field is of type Array[Byte], I Save it as a blob. And I need to treat Enumerations as special cases too, but I can't find out how to know if a type is an enumeration.
For example:
object UserType extends Enumeration {
val Anonym, Registered, Admin, Super = Value
}
var value = UserType.Admin
value.isInstanceOf[Enumeration] // this returns false
Neither I can do value.isInstanceOf[Enumeration.Value] since Value is private... anyway I think that would return false too.
Any idea?
Thanks!
value.isInstanceOf[Enumeration$Value]
You could figure this out using these methods:
scala> value.getClass
res102: java.lang.Class[_] = class scala.Enumeration$Val
scala> value.getClass.getSuperclass
res103: java.lang.Class[_ >: ?0] = class scala.Enumeration$Value
scala> value.getClass.getSuperclass.getSuperclass
res104: java.lang.Class[_ >: ?0] = class java.lang.Object