Kotlin 인텐트(intent) : 명시적 인텐트(Explicit Intent)
- 기본적인 인텐트로 버튼을 클릭하면  Sub Activity 를 수행하고, 버튼을 클릭하면 다시 뒤돌온다.
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      activity_main.xml
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<?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:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Main Activity"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btnGoSubActivity"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Go to SubActivity"
        android:layout_marginBottom="20dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

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*****      activity_sub.xml **
--- res/layout 에서  ---> New  --> Layout Resource File
--- File : activity_sub  ,  Source set : main
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<?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"
    android:background="#E1DCB0"
    tools:context=".SubActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text=" 여기는 \n Sub Activity \n 입니다."
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btnClose"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Go Back"
        android:layout_marginBottom="20dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

------------------------------------------------------------------------------------
    MainActivity.kt
------------------------------------------------------------------------------------
class MainActivity : AppCompatActivity() {
    private lateinit var btnGoSubActivity: Button

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

        btnGoSubActivity = findViewById(R.id.btnGoSubActivity)

        btnGoSubActivity.setOnClickListener {
            val intent = Intent(this, SubActivity::class.java)
            startActivity(intent)
        }
    }
}

------------------------------------------------------------------------------------
*****    SubActivity.kt **
-- New --> Kotlin Class/File
-- Name : SubActivity
------------------------------------------------------------------------------------
class SubActivity : AppCompatActivity() {
    private lateinit var btnClose: Button

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

        btnClose = findViewById(R.id.btnClose)

        btnClose.setOnClickListener {
            finish()
        }
    }
}

------------------------------------------------------------------------------------
    AndroidManifest.xml
------------------------------------------------------------------------------------
        추가하기
        <activity
            android:name=".SubActivity" >
        </activity>