Invoke.kt

interface GetTransactionsUseCase {     
    operator fun invoke(): Single<SimpleResult<List<Transaction>>> 
}

class MyViewModel(
    private val getTransactionUseCase: GetTransactionUseCase
) {
    fun doSomething() {
        //No need for invoke
        getTransactionUseCase().subscribeBy { 
            //do something with the result
        }
    }
}