Ejemplo de ViewModel en Kotlin

Ejemplo de uso de ViewModel

ViewModel Explained

Crear un  nuevo proyecto en Kotlin llamado Vista Modelo

ViewBinding en build.gradle

buildFeatures {
    viewBinding = true
}

dependencias

implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:2.7.0")

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="128dp"
        android:layout_height="57dp"
        android:text="0"
        android:textAlignment="center"
        android:textAppearance="@style/TextAppearance.AppCompat.Large"
        android:textSize="40sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.207" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add +1"
        android:textAppearance="@style/TextAppearance.AppCompat.Large"
        android:textSize="40sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:layout_constraintVertical_bias="0.129" />

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivityViewModel.kt

class MainActivityViewModel : ViewModel() {
    var number = 0


    fun addNumber() {
        number++
    }
}

MainActivity.kt

class MainActivity : AppCompatActivity(), View.OnClickListener {
    lateinit var viewmodel: MainActivityViewModel
    lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // setContentView(R.layout.activity_main)
        binding = ActivityMainBinding.inflate(layoutInflater)
        val view: View = binding.root
        setContentView(view)

        binding.button.setOnClickListener(this)
        viewmodel = ViewModelProvider(this).get(MainActivityViewModel::class.java)
        binding.textView.text = viewmodel.number.toString()
    }

    override fun onClick(view: View) {
        if (view === binding.button) {
            viewmodel.addNumber()
            binding.textView.text = viewmodel.number.toString()
        }
    }
}

Codelab:

Incorporate Lifecycle-Aware Components