I'm trying to use the invoke function with objects...

# announcements

[Sergio C.]

I'm trying to use the invoke function with objects that should be singletons, basically for having retrofit accept different base urls. If I call invoke("some url") in different places of the app how does that affect the singleton creation? Do I get the same object with the changed parameter , or is it creating a new object every time I call invoke?

[karelpeeters]

What are these "objects that should be singletons"? Are they Kotlin object {} things with an operator fun invoke?

[Sergio C.]

@karelpeeters yes exactly that

[karelpeeters]

If you just have a top level object then it'll be a singleton, and using that object in multiple places shouldn't be a problem, there will only be a single instance.

[karelpeeters]

(Unless there's reflection or serialization going on, that can break the singleton guarantee)

[Sergio C.]

No, not using reflection or serialization.
Ok. It'll be a singleton even if I call invoke() with different parameters in multiple places?

[Sergio C.]

I've been reading this, wonder if I need to write some code like this https://medium.com/@BladeCoder/kotlin-singletons-with-argument-194ef06edd9e

[karelpeeters]

There's nothing special about invoke in any way, it's just a normal function. I don't understand where the confusion comes from:

object Test {
    fun foo() {
        println("I'm $this")
    }
}

[karelpeeters]

Of course it's the same singleton every time, and it's the same for invoke.

[karelpeeters]

That blog post is for when you want a singleton that needs to be initialized, but that's not the usual case.

[Sergio C.]

object ApiController2 {

    private var baseUrl: String = "<https://jsonplaceholder.typicode.com/>"

    operator fun invoke(baseUrl: String?): AppAPI {
        if (baseUrl != null) this.baseUrl = baseUrl
        return controller
    }

    operator fun invoke(): AppAPI {
        return controller
    }

    private val dispatcher: Dispatcher by lazy { Dispatcher() }

    private val controller: AppAPI by lazy {

        dispatcher.maxRequests = 64

        val okHttpClient = OkHttpClient
            .Builder()
            .dispatcher(dispatcher)
            .connectionPool(ConnectionPool())
            .build()

        Retrofit.Builder()
            .baseUrl(baseUrl)
            .addConverterFactory(GsonConverterFactory.create())
            .client(okHttpClient)
            .build()
            .create(AppAPI::class.java)
    }

}

[Sergio C.]

yes indeed it always returns the same object

[karelpeeters]

That's a strange setup, having the lazy creation be influenced only by the first call to invoke.

[karelpeeters]

If you really want this then I'd do what the blogpost says instead.

[karelpeeters]

But I think these fake singletons are a bad idea, just pass actual instances around.

[Sergio C.]

it's a fake singleton?.. I'm just curious about the inner workings of this and in the future implement the best solution.

[Sergio C.]

yep that lazy int the controller might be useless

[karelpeeters]

ApiController2 is a real singleton, but you're using it to create a controller in a strange way, where the first call invoke(url1) creates a controller, all future calls invoke(url1) silently ignore the url parameter and just return the previously created object.

[karelpeeters]

What inner workings do you want to know about? It's just a combination of object, the property delegate lazy and operator overloading invoke.

[Sergio C.]

all inner workings 🙂

[karelpeeters]

Well that's a lot to type out in a slack comment simple smile

[Sergio C.]

thanks

[Sergio C.]

😀