Implement Observable ListView using kotlin

MainActivity.kt

import android.os.Bundle
import android.support.design.widget.FloatingActionButton
import android.support.v7.app.AppCompatActivity
import android.support.v7.widget.Toolbar
import android.view.Menu
import android.view.MenuItem
import com.github.ksoichiro.android.observablescrollview.ObservableListView
import com.github.ksoichiro.android.observablescrollview.ObservableScrollViewCallbacks
import com.github.ksoichiro.android.observablescrollview.ScrollState

class MainActivity : AppCompatActivity(), NavigationView.OnNavigationItemSelectedListener, ObservableScrollViewCallbacks {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val toolbar = findViewById(R.id.toolbar) as Toolbar
        setSupportActionBar(toolbar)

        val ab = supportActionBar
        ab!!.setHomeAsUpIndicator(R.drawable.ic_humberger_menu)
        ab.setDisplayHomeAsUpEnabled(true)

        val navigationView = findViewById(R.id.nav_view) as NavigationView
        navigationView.setNavigationItemSelectedListener(this)
    }

    override fun onUpOrCancelMotionEvent(scrollState: ScrollState?) {
        val ab = supportActionBar
        val fab = findViewById(R.id.fab) as FloatingActionButton
        if (scrollState === ScrollState.UP) {
            if (ab!!.isShowing)
                ab.hide()

            if (fab.isShown)
                fab.hide()
        } else if (scrollState === ScrollState.DOWN) {
            if (!ab!!.isShowing)
                ab.show()

            if (!fab.isShown)
                fab.show()
        }
    }

    override fun onScrollChanged(scrollY: Int, firstScroll: Boolean, dragging: Boolean) {
    }

    override fun onDownMotionEvent() {
    }


    override fun onBackPressed() {
        val drawer = findViewById(R.id.drawer_layout) as DrawerLayout
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START)
        } else {
            super.onBackPressed()
        }
    }
}