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I'd like to create a variable by delegation ```val username by Input { }``` Where ``` class Input ...
# getting-started
e
I'd like to create a variable by delegation
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val username by Input { }
Where
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class Input {
      companion object {
         operator fun invoke(..): Input {
            ..
         }
      }
   }
And
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internal operator fun Any.getValue(thisRef: Any?, property: KProperty<*>): Input {
    println("$thisRef, thank you for delegating '${property.name}' to me!")
    val pInput = property as KProperty0<Input>
    println(pInput)
    return pInput.get()
}
pInput
is a valid 
KProperty0
class, but the moment I call 
.get()
to retrieve 
Input
, then I get:
java.lang.UnsupportedOperationException: call/callBy are not supported for this declaration.
Why? How can I fix it? What is the way to retrieve the 
Input
instance produced by its own companion object?
j
Why declare 
invoke
on the companion instead of just using a constructor?
Also, where is 
getValue
declared here? Inside the 
Input
class or at the top level?
e
top level
I can also switch to a constructor, but I'm building a dsl, so that's kind of the builder which then later on will produce the resulting 
Input
j
I don't understand what you're trying to do here. Why would you define such an operator on 
Any
?
e
because in this way I can automatically retrieve the variable name (
username
) as well and use it to create 
Input
(it's one of the fields)
j
You are delegating 
username
so the delegate is the one that's supposed to provide the value
Regarding invoke on the companion, I still don't get why. The fact that you're using this for a DSL is unrelated to where you define the function
e
how would you do it?
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class Input {
   fun getValue(..)
}
?
btw, the delegate (companion object 
invoke
) is actually providing the value, or am I wrong?
I'm also delegating on 
Any
so that I can also have type inference
j
You seem to be confusing the delegate object and the value of the property. It looks like you want 
Input
to be the type of the delegate but you also want it to be the type of the value? I cannot really suggest anything at the moment because I didn't get what you're trying to do. What is the purpose of the delegate?
e
I'd like to initialize 
username: Input
in the lambda, this should include saving the variable name itself, that is 
username
inside the 
Input
instance itself
sort of Gradle does when you 
val task by tasks.creating { }
j
Then I don't believe you need a delegate at all, you can simply use 
val username = Input { ... }
e
I cant retrieve 
username
from inside 
Input{..}
in that way
username
is 
Input.id
I think I got it
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class Delegate(val block: Input.() -> Unit) {

    operator fun getValue(thisRef: Any?, property: KProperty<*>): Input {
        println("$thisRef, thank you for delegating '${property.name}' to me!")
        return Input().apply {
            block()
            id = property.name
        }
    }
}
j
Ah ok got it now, then indeed you can use a delegate for this, but in this case you need to separate the delegate type from the 
Input
type. The code you provided last can work, but then you'll create a new instance of 
Input
every time you access 
username
. You can use the 
provideDelegate
approach in order to store the value and return the same one everytime it's accessed
e
sorry for the confusion, I didn't get it right the difference between delegate class and return value class
what's the 
provideDelegate
approach?
j
Yes, that's it. It's one more level of indirection where a 
provideDelegate
function is called once when the class containing the delegated property is initialized. This function creates an instance of the delegate class and you already have access to the property name when doing so, so you can store the value in the delegate instance itself
Here is how I think you could use it: https://pl.kotl.in/MuYRhTYEs
Untitled.cpp
Untitled.cpp
e
cool, thanks a lot
I still don't get why a plan 
by
won't save the value in the variable by default, like 
by lazy
, instead of requiring all this boilerplate code, but anyway
I see that
One of the possible use cases of 
provideDelegate
is to check the consistency of the property upon its initialization.
I double checked though, the class gets initialized just once
ps: is there a way I can use the
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PropertyDelegateProvider
(at the end of the same doc link posted above) I guess no, because there isn't a way to pass the lambda
j
Hey sorry for the delay, I was AFK. I actually didn't know about 
PropertyDelegateProvider
and never thought of using SAM conversions like this! It is definitely possible to use by capturing the 
configure
lambda (and it would simplify the whole thing quite a bit):
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private fun Input(configure: Input.() -> Unit) = PropertyDelegateProvider { thisRef: Any?, prop ->
    val input = Input(prop.name).apply { configure() }
    ReadOnlyProperty<Any?, Input> { _, _ -> input }
}
So the whole thing is now: https://pl.kotl.in/n6KFhfDOp Thanks for making me learn this 🙂
I still don't get why a plan 
by
 won't save the value in the variable by default, like 
by lazy
I don't get your point here. The implementation of the lazy delegate does need to store the 
value
explicitly, look: https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/src/kotlin/util/Lazy.kt#L95 However, they don't need the additional indirection of 
provideDelegate
because they don't need information about the property itself (like the name of the property), unlike in your use case.
e
thank you for helping me out, appreciated
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