Selections are saved during orientation change in the MainActivity

app/src/main/java/com/paktalin/quickmax/MainActivity.kt

@@ -13,33 +13,52 @@ class MainActivity : AppCompatActivity() {
     private var secToSolve: Int = 4
     private var numDigits: Int = 3
     private lateinit var checkedCard: MaterialCardView
+    private lateinit var cards: List<MaterialCardView>
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         setContentView(R.layout.activity_main)
-        retrieveSharedPrefs()
-        seek_bar.setMinStartValue(secToSolve.toFloat()).apply()
 
-        seek_bar.setOnSeekbarChangeListener { n ->
+        if (savedInstanceState == null) retrieveSharedPrefs()
+        else restoreInstanceState(savedInstanceState)
+
+        checkedCard.isChecked = true
+
+        seek_bar.apply {
+            setMinStartValue(secToSolve.toFloat()).apply()
+            setOnSeekbarChangeListener { n ->
                 seek_bar_value.text = resources.getString(R.string.time_to_solve, n.toString())
                 secToSolve = n.toInt()
             }
+        }
 
-        card_2_digits.setOnClickListener(cardOnClickListener)
-        card_3_digits.setOnClickListener(cardOnClickListener)
-        card_4_digits.setOnClickListener(cardOnClickListener)
+        cards = listOf(card_2_digits, card_3_digits, card_4_digits)
+        cards.forEach { card -> card.setOnClickListener(cardOnClickListener) }
 
         btn_play.setOnClickListener(playClickListener)
     }
 
+    private fun restoreInstanceState(savedInstanceState: Bundle) {
+        checkedCard = findViewById(savedInstanceState.getInt("checked_num_id"))
+        secToSolve = savedInstanceState.getInt("sec_to_solve")
+        numDigits = savedInstanceState.getInt("num_digits")
+    }
+
+    override fun onSaveInstanceState(outState: Bundle) {
+        super.onSaveInstanceState(outState)
+        outState.putInt("num_digits", numDigits)
+        outState.putInt("sec_to_solve", secToSolve)
+        outState.putInt("checked_num_id", checkedCard.id)
+    }
+
     private val cardOnClickListener = View.OnClickListener { card ->
         val cardM = card as MaterialCardView
         if (!cardM.isChecked) {
             checkedCard = cardM
             checkedCard.isChecked = true
             numDigits = numDigitsFromCard(checkedCard)
-            listOf(card_2_digits, card_3_digits, card_4_digits).forEach { c ->
-                if (c.id != cardM.id)
+            cards.forEach { c ->
+                if (c.id != checkedCard.id)
                     c.isChecked = false
             }
         }
@@ -64,11 +83,13 @@ class MainActivity : AppCompatActivity() {
     private fun retrieveSharedPrefs() {
         val prefs = getSharedPreferences("my_prefs", Context.MODE_PRIVATE)
         secToSolve = prefs.getInt("sec_to_solve", 4)
-        checkedCard = findViewById(prefs.getInt("checked_num_id",
+        checkedCard = findViewById(
+            prefs.getInt(
+                "checked_num_id",
                 R.id.card_3_digits
-        ))
+            )
+        )
         numDigits = numDigitsFromCard(checkedCard)
-        checkedCard.isChecked = true
     }
 
     private fun numDigitsFromCard(card: MaterialCardView): Int {