【ANDROID STUDIO】View Model

 package com.example.viewmodeldemo

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.ViewModelProvider
import com.example.viewmodeldemo.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding
    private lateinit var viewModel: MainActivityViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        viewModel = ViewModelProvider(this).get(MainActivityViewModel::class.java)
        binding.countText.text = viewModel.getCurrentCount().toString()
        binding.button.setOnClickListener {

            binding.countText.text = viewModel.getUpdatedCount().toString()
        }
    }

}

package com.example.viewmodeldemo

import androidx.lifecycle.ViewModel

class MainActivityViewModel : ViewModel() {
    private var count = 0

    fun getCurrentCount():Int{
        return count
    }

    fun getUpdatedCount():Int{
        return ++count
    }
}
<?xml version="1.0" encoding="utf-8"?>
<layout 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">
    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <TextView android:id="@+id/countText"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="66sp"
            android:textStyle="bold"
            android:typeface="serif"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.5"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.262" />

        <Button android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Click Here"
            android:textSize="30sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.5"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>