Kotlin 메뉴(Menu) : 배경색 변경 앱 - Kotlin 코드만 이용한 옵션 메뉴
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      activity_main.xml
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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/baseLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center_horizontal"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="오른쪽 위 메뉴 버튼을 누르세요"
        android:textSize="20dp" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="이건 버튼" />
</LinearLayout>

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    MainActivity.kt
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class MainActivity : AppCompatActivity() {
    lateinit var baseLayout : LinearLayout
    lateinit var button1 : Button

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

        title = "배경색 바꾸기"
        baseLayout = findViewById<LinearLayout>(R.id.baseLayout)
        button1 = findViewById<Button>(R.id.button1)
    }

    override fun onCreateOptionsMenu(menu: Menu?) : Boolean {
        super.onCreateOptionsMenu(menu)

        menu!!.add(0, 1, 0, "배경색 (빨강)")
        menu!!.add(0, 2, 0, "배경색 (초록)")
        menu!!.add(0, 3, 0, "배경색 (파랑)")

        var sMenu : SubMenu = menu.addSubMenu("버튼 변경 >>")
        sMenu.add(0, 4, 0, "버튼 45도 회전")
        sMenu.add(0, 5, 0, "버튼 2배 확대")

        return true
    }

    override fun onOptionsItemSelected(item: MenuItem) : Boolean {
        when (item.itemId) {
            1-> {
                baseLayout.setBackgroundColor(Color.RED)
                return true
            }
            2 -> {
                baseLayout.setBackgroundColor(Color.GREEN)
                return true
            }
            3 -> {
                baseLayout.setBackgroundColor(Color.BLUE)
                return true
            }
            4  -> {
                button1.rotation = 45f
                return true
            }
            5 -> {
                button1.scaleX = 2f
                return true
            }
        }
        return false
    }
}

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*****      themes.xml  **********
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<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Base.Theme.MenuOptionmenu" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <item name="colorPrimary">#4255B3</item>
        <item name="colorPrimaryDark">#FF9800</item>
    </style>

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