Kotlin 메뉴(Menu) : 컨텍스트 메뉴(ContextMenu)
- 배경색 변경 및 버튼 변경 앱 만들기
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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="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="배경색 변경" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="버튼 변경" />

</LinearLayout>

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    MainActivity.kt
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class MainActivity : AppCompatActivity() {
    lateinit var button1 : Button
    lateinit var button2 : Button
    lateinit var baseLayout : LinearLayout

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

        title = "배경색 바꾸기(Kotlin 코드)"
        baseLayout = findViewById<LinearLayout>(R.id.baseLayout) as LinearLayout
        button1 = findViewById<View>(R.id.button1) as Button
        registerForContextMenu(button1)

        button2 = findViewById<View>(R.id.button2) as Button
        registerForContextMenu(button2)
    }

    override fun onCreateContextMenu(menu: ContextMenu, v: View,
                                    menuInfo: ContextMenu.ContextMenuInfo?) {

        super.onCreateContextMenu(menu, v, menuInfo)

        if (v === button1) {
            menu.setHeaderTitle("배경색 변경")
            menu.add(0, 1, 0, "배경색 (빨강)")
            menu.add(0, 2, 0, "배경색 (초록)")
            menu.add(0, 3, 0, "배경색 (파랑)")

        }
        if (v === button2) {
            menu.add(0, 4, 0, "버튼 45도 회전")
            menu.add(0, 5, 0, "버튼 2배 확대")
        }
    }

    override fun onContextItemSelected(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 -> {
                button2.rotation = 45f
                return true
            }
            5 -> {
                button2.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.ContextMenuColor" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <item name="colorPrimary">#E91E63</item>
        <item name="colorPrimaryDark">#3F51B5</item>
    </style>

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