How to zip few observables in Kotlin language with RxAndroid

Question

I have some problem. I'm a beginer in RxJava/RxKotlin/RxAndroid, and dont understand some features. For Example:

import rus.pifpaf.client.data.catalog.models.Category
import rus.pifpaf.client.data.main.MainRepository
import rus.pifpaf.client.data.main.models.FrontDataModel
import rus.pifpaf.client.data.product.models.Product
import rx.Observable
import rx.Single
import rx.lang.kotlin.observable
import java.util.*


class MainInteractor {

    private var repository: MainRepository = MainRepository()

    fun getFrontData() {

        val cats = getCategories()
        val day = getDayProduct()
        val top = getTopProducts()

        return Observable.zip(cats, day, top, MainInteractor::convert)
    }

    private fun getTopProducts(): Observable<List<Product>> {
        return repository.getTop()
                .toObservable()
                .onErrorReturn{throwable -> ArrayList() }

    }

    private fun getDayProduct(): Observable<Product> {
        return repository.getSingleProduct()
                .toObservable()
                .onErrorReturn{throwable -> Product()}

    }

    private fun getCategories(): Observable<List<Category>> {
        return repository.getCategories()
                .toObservable()
                .onErrorReturn{throwable -> ArrayList() }
    }

    private fun convert(cats: List<Category>, product: Product, top: List<Product>): FrontDataModel {

    }
}

Then I'm use MainInteractor::convert Android studio tell me next

I tried a lot of variant and tried to understand what does it want, but no success. Help me please... Best Regards.

Answer 1

Just replace function reference with lambda:

return Observable.zip(cats, day, top, { c, d, t -> convert(c, d, t) })

And don't forget to declare function's return type explicitly:

fun getFrontData(): Observable<FrontDataModel> {
    ...