------------------------------------------------------------------------------------
      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/TextMenuItem"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Option Menu Example"
        android:textSize="24sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

------------------------------------------------------------------------------------
***      values/strings.xml
------------------------------------------------------------------------------------
<resources>
    <string name="app_name">ActionBarKotlin</string>
</resources>

------------------------------------------------------------------------------------
***      menu/actionbar_main.xml
------------------------------------------------------------------------------------
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto" >

    <item
        android:id="@+id/item01"
        app:showAsAction="ifRoom"
        android:icon="@android:drawable/ic_menu_search"
        android:title="Item 1" />
    <item
        android:id="@+id/item02"
        app:showAsAction="ifRoom"
        android:icon="@android:drawable/ic_menu_share"
        android:title="Item 2" />

    <item
        android:id="@+id/item03"
        app:showAsAction="ifRoom"
        android:title="Item 3">
        <menu>
            <item
                android:id="@+id/item3_1"
                app:showAsAction="ifRoom"
                android:title="Item 3-1" />
            <item
                android:id="@+id/item3_2"
                app:showAsAction="ifRoom"
                android:title="Item 3-2" />
        </menu>
    </item>

    <item
        android:id="@+id/item04"
        app:showAsAction="ifRoom"
        android:title="Item 4" />

</menu>

------------------------------------------------------------------------------------
 ***    values/themes.xml
------------------------------------------------------------------------------------
<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Base.Theme.ActionBarKotlin" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Customize your light theme here. -->
        <item name="colorPrimary">#3949AB</item>
        <item name="colorPrimaryDark">#4682B4</item>
        <item name="colorAccent">#FFC940</item>
        <item name="android:textColor">@color/black</item>
        <item name="android:textColorPrimary">#F4511E</item>
        <item name="android:textColorPrimaryInverse">#F4511E</item>
        <item name="android:windowBackground">#C0CA33</item>

    </style>

    <style name="Theme.ActionBarKotlin" parent="Base.Theme.ActionBarKotlin" />
</resources>

------------------------------------------------------------------------------------
    MainActivity.kt
------------------------------------------------------------------------------------
class MainActivity : AppCompatActivity(), PopupMenu.OnMenuItemClickListener {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // setContentView(R.layout.activity_main)

        val binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.btnMenu1.setOnClickListener { showPopup(binding.btnMenu1) }

    }

    //    팝업 메뉴 보여주는 커스텀 메소드
    private fun showPopup(v: View) {
        val popup = PopupMenu(this, v) // PopupMenu 객체 선언
        popup.menuInflater.inflate(R.menu.popup_menu, popup.menu) // 메뉴 레이아웃 inflate
        popup.setOnMenuItemClickListener(this) // 메뉴 아이템 클릭 리스너 달아주기
        popup.show() // 팝업 보여주기
    }

    // 팝업 메뉴 아이템 클릭 시 실행되는 메소드
    override fun onMenuItemClick(item: MenuItem?): Boolean {
        when (item?.itemId) { // 메뉴 아이템에 따라 동작 다르게 하기
            R.id.PopUP01 -> Toast.makeText(this, "첫번째 팝업 메뉴 클릭", Toast.LENGTH_LONG).show()
            R.id.PopUP02-> Toast.makeText(this, "두번째 팝업 메뉴 클릭", Toast.LENGTH_LONG).show()
            R.id.PopUP03 -> Toast.makeText(this, item.title, Toast.LENGTH_LONG).show()
        }

        return item != null // 아이템이 null이 아닌 경우 true, null인 경우 false 리턴
    }
}