【ANDROID STUDIO】View Model With Data Binding

 package com.example.databinding

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import com.example.databinding.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.myViewModel = viewModel
        viewModel.count.observe(this, Observer {
            binding.countText.text = it.toString()
        })

    }
}

package com.example.databinding

import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MainActivityViewModel : ViewModel() {
    var count = MutableLiveData<Int>()

    init {
        count.value = 0
    }

    fun updateCount() {
        count.value = (count.value)?.plus(1)
    }
}

<?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">

    <data>
        <variable
            name="myViewModel"
            type="com.example.databinding.MainActivityViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <TextView android:id="@+id/count_text"
        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"
        android:onClick="@{()->myViewModel.updateCount()}"
    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>