【ANDROID STUDIO】Two Way Data Binding

package com.example.twowaybinding

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

class MainActivity : AppCompatActivity() {


    private lateinit var binding: ActivityMainBinding
    private lateinit var viewModel : MainActivityViewModel

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

        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        viewModel = ViewModelProvider(this).get(MainActivityViewModel::class.java)
        binding.viewModel = viewModel
        binding.lifecycleOwner = this

    }
}

package com.example.twowaybinding

import androidx.databinding.Bindable
import androidx.databinding.Observable
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MainActivityViewModel : ViewModel(), Observable {
    @Bindable
    val userName = MutableLiveData<String>()

    init {
        userName.value = "Edward"
    }

    override fun removeOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback?) {

    }

    override fun addOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback?) {

    }
}
<?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="viewModel"
            type="com.example.twowaybinding.MainActivityViewModel" />
    </data>
    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <TextView android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/text_view"
            android:text="@={viewModel.userName}"
            android:textSize="40sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintHorizontal_bias="0.525"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.663" />

        <EditText android:id="@+id/edit_text"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="textPersonName"
            android:textSize="40sp"
            android:text="@={viewModel.userName}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.271" />


    </androidx.constraintlayout.widget.ConstraintLayout>

</layout>