Api.kt

package com.example.gabriel.mediatorlivedata

import android.arch.lifecycle.LiveData
import android.arch.lifecycle.MediatorLiveData
import android.arch.lifecycle.MutableLiveData
import android.os.Handler
import android.util.Log
import java.util.*


class ApiClass {

    val list = listOf("Gabriel", "Miguel")

    fun getApi() : LiveData<List<String>>{
        val mutableLiveData = MutableLiveData<List<String>>()
        mutableLiveData.value = Collections.emptyList()

        Handler().postDelayed({ mutableLiveData.value = list }, 3000)

        return mutableLiveData
    }


}

class Caller {

    fun caller() : MediatorLiveData<List<String>>{
        val apiClass = ApiClass()
        val mediatorLiveData = MediatorLiveData<List<String>>()

        mediatorLiveData.addSource(apiClass.getApi(), { Log.i("myTag", "Works") })
        return mediatorLiveData
    }

}

MainActivity.kt

package com.example.gabriel.mediatorlivedata

import android.arch.lifecycle.Observer
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.util.Log

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val caller = Caller().caller()

        caller.observe(this, Observer { Log.i("Activity", "zica") })

    }
}