Override onSaveInstanceState Method

Add public member variable mResult into CalculatorImpl
Add private member savedRes into MainActivity
Update calc constructor
Getting Saved res from savedInstanceState

app/src/main/kotlin/com/simplemobiletools/calculator/activities/MainActivity.kt

@@ -3,6 +3,7 @@ package com.simplemobiletools.calculator.activities
 import android.content.Context
 import android.content.Intent
 import android.os.Bundle
+import android.util.Log
 import android.view.View
 import android.view.WindowManager
 import androidx.core.content.res.ResourcesCompat
@@ -30,6 +31,12 @@ class MainActivity : SimpleActivity(), Calculator {
     private var decimalSeparator = DOT
     private var groupingSeparator = COMMA
 
+    //============================================================
+
+    private var savedRes: String = "999"
+
+    //============================================================
+
     private lateinit var calc: CalculatorImpl
 
     override fun onCreate(savedInstanceState: Bundle?) {
@@ -39,7 +46,17 @@ class MainActivity : SimpleActivity(), Calculator {
         setupOptionsMenu()
         refreshMenuItems()
 
-        calc = CalculatorImpl(this, applicationContext)
+        //============================================================
+
+        if(savedInstanceState != null) {
+            Log.v("MainActivity", "LOG TEST");
+            savedRes = savedInstanceState?.getCharSequence("res", "123") as String
+
+        }
+        Log.v("MainActivity", "LOG NO IF TEST");
+        calc = CalculatorImpl(this, applicationContext, savedRes)
+
+        //============================================================
 
         btn_plus.setOnClickOperation(PLUS)
         btn_minus.setOnClickOperation(MINUS)
@@ -240,4 +257,14 @@ class MainActivity : SimpleActivity(), Calculator {
             calc.handleOperation(operation)
         }
     }
+
+    //============================================================
+
+    // ****** ADD ONSAVEINSTANCESTATE FUNCTION ******
+    override fun onSaveInstanceState(bundle: Bundle) {
+        super.onSaveInstanceState(bundle)
+        bundle.putString("res", calc.mResult)
+    }
+
+    //============================================================
 }

app/src/main/kotlin/com/simplemobiletools/calculator/helpers/CalculatorImpl.kt

@@ -11,11 +11,19 @@ import java.math.BigDecimal
 class CalculatorImpl(
     calculator: Calculator,
     private val context: Context,
+    //============================================================
+    var res: String,
+    //============================================================
     private var decimalSeparator: String = DOT,
     private var groupingSeparator: String = COMMA
 ) {
     private var callback: Calculator? = calculator
 
+    //============================================================
+    // Trying Fix it
+    public var mResult = res
+    //============================================================
+
     private var baseValue = 0.0
     private var secondValue = 0.0
     private var inputDisplayedFormula = "0"
@@ -30,7 +38,10 @@ class CalculatorImpl(
     )
 
     init {
-        showNewResult("0")
+        //============================================================
+        //showNewResult("0")
+        showNewResult(mResult)
+        //============================================================
     }
 
     private fun addDigit(number: Int) {
@@ -271,6 +282,12 @@ class CalculatorImpl(
                     return
                 }
 
+                //============================================================
+
+                mResult = result.format()
+
+                //============================================================
+
                 showNewResult(result.format())
                 val newFormula = "${baseValue.format()}$sign${secondValue.format()}"
                 HistoryHelper(context).insertOrUpdateHistoryEntry(