write the formula above the result

app/src/main/java/calculator/simplemobiletools/com/simple_calculator/Calculator.java

@@ -4,4 +4,6 @@ public interface Calculator {
     void setValue(String value);
 
     void setValueDouble(double d);
+
+    void setFormula(String value);
 }

app/src/main/java/calculator/simplemobiletools/com/simple_calculator/CalculatorImpl.java

@@ -8,17 +8,20 @@ public class CalculatorImpl {
     private String lastKey;
     private String lastOperation;
     private Calculator callback;
+    private boolean isFirstOperation;
 
     public CalculatorImpl(Calculator calculatorInterface) {
         callback = calculatorInterface;
         resetValues();
         setValue("0");
+        setFormula("");
     }
 
     public CalculatorImpl(Calculator calculatorInterface, String value) {
         callback = calculatorInterface;
         resetValues();
         displayedValue = value;
+        setFormula("");
     }
 
     private void resetValueIfNeeded() {
@@ -35,6 +38,7 @@ public class CalculatorImpl {
         lastKey = "";
         lastOperation = "";
         displayedValue = "";
+        isFirstOperation = true;
     }
 
     public void setValue(String value) {
@@ -42,6 +46,22 @@ public class CalculatorImpl {
         displayedValue = value;
     }
 
+    private void setFormula(String value) {
+        callback.setFormula(value);
+    }
+
+    private void updateFormula() {
+        final String first = Formatter.doubleToString(baseValue);
+        final String second = Formatter.doubleToString(secondValue);
+        final String sign = getSign(lastOperation);
+
+        if (sign.equals("√")) {
+            setFormula(sign + first);
+        } else if (!sign.isEmpty()) {
+            setFormula(first + sign + second);
+        }
+    }
+
     public void setLastKey(String lastKey) {
         this.lastKey = lastKey;
     }
@@ -77,6 +97,10 @@ public class CalculatorImpl {
     }
 
     public void calculateResult() {
+        if (!isFirstOperation) {
+            updateFormula();
+        }
+
         switch (lastOperation) {
             case Constants.PLUS:
                 updateResult(baseValue + secondValue);
@@ -102,6 +126,7 @@ public class CalculatorImpl {
             default:
                 break;
         }
+        isFirstOperation = false;
     }
 
     private void divideNumbers() {
@@ -162,6 +187,7 @@ public class CalculatorImpl {
     public void handleReset() {
         resetValues();
         setValue("0");
+        setFormula("");
     }
 
     public void handleEquals() {
@@ -190,6 +216,26 @@ public class CalculatorImpl {
         setValue(value);
     }
 
+    private String getSign(String lastOperation) {
+        switch (lastOperation) {
+            case Constants.PLUS:
+                return "+";
+            case Constants.MINUS:
+                return "-";
+            case Constants.MULTIPLY:
+                return "*";
+            case Constants.DIVIDE:
+                return "/";
+            case Constants.MODULO:
+                return "%";
+            case Constants.POWER:
+                return "^";
+            case Constants.ROOT:
+                return "√";
+        }
+        return "";
+    }
+
     public void numpadClicked(int id) {
         if (lastKey.equals(Constants.EQUALS))
             lastOperation = Constants.EQUALS;

app/src/main/java/calculator/simplemobiletools/com/simple_calculator/MainActivity.java

@@ -13,6 +13,7 @@ import butterknife.OnLongClick;
 
 public class MainActivity extends AppCompatActivity implements Calculator {
     @Bind(R.id.result) TextView result;
+    @Bind(R.id.formula) TextView formula;
 
     private CalculatorImpl calc;
 
@@ -29,6 +30,9 @@ public class MainActivity extends AppCompatActivity implements Calculator {
         final Resources res = getResources();
         result.setBackgroundColor(res.getColor(android.R.color.white));
         result.setTextColor(res.getColor(R.color.dark_grey));
+
+        formula.setBackgroundColor(res.getColor(android.R.color.white));
+        formula.setTextColor(res.getColor(R.color.dark_grey));
     }
 
     @OnClick(R.id.btn_plus)
@@ -104,6 +108,11 @@ public class MainActivity extends AppCompatActivity implements Calculator {
         calc.setLastKey(Constants.DIGIT);
     }
 
+    @Override
+    public void setFormula(String value) {
+        formula.setText(value);
+    }
+
     public CalculatorImpl getCalc() {
         return calc;
     }

app/src/main/java/calculator/simplemobiletools/com/simple_calculator/MyWidgetProvider.java

@@ -184,6 +184,12 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
 
     }
 
+    @Override
+    public void setFormula(String value) {
+        remoteViews.setTextViewText(R.id.formula, value);
+        widgetManager.updateAppWidget(widgetIds, remoteViews);
+    }
+
     @Override
     public void onDeleted(Context context, int[] appWidgetIds) {
         super.onDeleted(context, appWidgetIds);

app/src/main/res/layout/activity_main.xml

@@ -9,6 +9,18 @@
     android:orientation="vertical"
     tools:context="calculator.simplemobiletools.com.simple_calculator.MainActivity">
 
+    <TextView
+        android:id="@+id/formula"
+        android:layout_width="match_parent"
+        android:layout_height="match_parent"
+        android:layout_weight="2.1"
+        android:gravity="right|bottom"
+        android:lines="1"
+        android:paddingLeft="@dimen/activity_margin"
+        android:paddingRight="@dimen/activity_margin"
+        android:textColor="@android:color/white"
+        android:textSize="@dimen/formula_text_size"/>
+
     <TextView
         android:id="@+id/result"
         android:layout_width="match_parent"

app/src/main/res/values/dimens.xml

@@ -3,5 +3,6 @@
     <dimen name="result_padding">64dp</dimen>
     <dimen name="config_text_size">18sp</dimen>
     <dimen name="button_text_size">28sp</dimen>
-    <dimen name="display_text_size">36sp</dimen>
+    <dimen name="formula_text_size">20sp</dimen>
+    <dimen name="display_text_size">40sp</dimen>
 </resources>