01_mvvm.kt

class MainViewModel(private val layoutCenterViewUsecase : LayoutCenterViewUsecase) {
  
  val centerY = ObservableInt()
  
  init {
    layoutCenterViewUsecase.observe()
      .subscribe { y : Int ->
        centerY.set(y)
      }
  }
  
}

02_LayoutCenterViewUsecase.kt

class LayoutCenterViewUsecase(layouts : () -> View) {
  private val layout by lazy(layouts)
  
  fun observe() : Observable<Int> {
    return Observable.create { emitter -> 
        emitter.onNext(layout.bottom - layout.top)
                    
        val callback = object : View.OnLayoutChangeListener {
           override fun onLayoutChange(v: View?, left: Int, top: Int, right: Int, bottom: Int, oldLeft: Int, oldTop: Int, oldRight: Int, oldBottom: Int) {
               emitter.onNext(bottom - top)
           }
        }
        layout.addOnLayoutChangeListener(callback)
                              
        emitter.setCancellable {
          layout.removeLayoutChangeListener(callback)
        }
     }.map { it / 2}
  }
}

03_MainActivity.kt

class MainActivity : AppCompatActivity() {
  fun onCreated(saved : Bundle?) {
    val vm = MainViewModel(LayoutCenterViewUsecase { findViewById<ViewGroup>(R.id.layout) } )
  }
}