UI not updated when ViewModel value changed

I have created sample app to demo my question:

A TextView and a Button, text view visibility is bound to viewModel.bar

I want the button to switch the value of viewModel.bar when clicked and the UI to get updated as well.

However, this is not happening. The value is changed but the UI is not updated.

Layout File:

<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>
        <import type="android.view.View"/>
        <variable name="viewModel"
                  type="com.example.bindingone.MainViewModel"/>
    </data>

    <android.support.constraint.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:context=".MainActivity">

        <TextView
                android:id="@+id/text_view"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Hello World"
                android:visibility="@{viewModel.bar ? View.VISIBLE : View.GONE}"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintLeft_toLeftOf="parent"
                app:layout_constraintRight_toRightOf="parent"
                app:layout_constraintTop_toTopOf="parent"/>

        <Button
                android:text="Update UI"
                android:onClick="clicked"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"/>

    </android.support.constraint.ConstraintLayout>

</layout>

MainActivity File:

class MainActivity : AppCompatActivity() {

    private val viewModel = MainViewModel()

    private lateinit var binding: ActivityMainBinding

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

        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        binding.viewModel = viewModel
    }

    fun clicked(v: View) {
        viewModel.bar.value = viewModel.bar.value?.not() ?: false
    }
}

MainViewModel file:

class MainViewModel : ViewModel() {
    var bar = MutableLiveData<Boolean>()
}