back button behaviour

res/values-ru/text_strings.xml

@@ -159,4 +159,7 @@
 	<string name="c_calc_show_release_notes_title">Показывать замечания к выпуску</string>
 	<string name="c_calc_show_release_notes_summary">Определяет показывать или нет всплывающее окно с замечаниями к выпуску после обновления новой версии</string>
 
+	<string name="c_calc_use_back_button_as_prev_summary">Устанавливает поведение по нажатию на кнопку Назад</string>
+	<string name="c_calc_use_back_button_as_prev_title">Использовать кнопку назад как назад по истории</string>
+
 </resources>

res/values/default_values.xml

@@ -24,6 +24,9 @@
 	<string name="p_calc_haptic_feedback_key">org.solovyev.android.calculator.CalculatorModel_haptic_feedback</string>
 	<string name="p_calc_haptic_feedback">false</string>
 
+	<string name="p_calc_use_back_button_as_prev_key">org.solovyev.android.calculator.CalculatorActivity_use_back_button_as_prev</string>
+	<string name="p_calc_use_back_button_as_prev">false</string>
+
 	<string name="p_calc_show_release_notes_key">org.solovyev.android.calculator.CalculatorActivity_show_release_notes</string>
 	<string name="p_calc_show_release_notes">true</string>
 

res/values/text_strings.xml

@@ -159,4 +159,7 @@
 	<string name="c_calc_show_release_notes_title">Show release notes</string>
 	<string name="c_calc_show_release_notes_summary">Defines if release notes popup window should appear after update to the new version</string>
 
+	<string name="c_calc_use_back_button_as_prev_summary">Defines the behaviour of the Back button</string>
+	<string name="c_calc_use_back_button_as_prev_title">Use Back button as history prev</string>
+
 </resources>

res/xml/main_preferences.xml

@@ -92,6 +92,12 @@
 				a:title="@string/c_calc_show_release_notes_title"
 				a:defaultValue="true"/>
 
+		<android.preference.CheckBoxPreference
+				a:key="@string/p_calc_use_back_button_as_prev_key"
+				a:summary="@string/c_calc_use_back_button_as_prev_summary"
+				a:title="@string/c_calc_use_back_button_as_prev_title"
+				a:defaultValue="false"/>
+
 <!--	  <org.solovyev.android.view.prefs.FloatRangeSeekBarPreference
 			  a:key="@string/p_drag_duration_key"
 			  a:title="Duration of drag event"

src/main/java/org/solovyev/android/calculator/CalculatorActivity.java

@@ -56,6 +56,10 @@ public class CalculatorActivity extends Activity implements FontSizeAdjuster, Sh
 	public static final String SHOW_RELEASE_NOTES_P_KEY = "org.solovyev.android.calculator.CalculatorActivity_show_release_notes";
 	public static final boolean SHOW_RELEASE_NOTES_P_DEFAULT = true;
 
+	@NotNull
+	public static final String USE_BACK_AS_PREV_P_KEY = "org.solovyev.android.calculator.CalculatorActivity_use_back_button_as_prev";
+	public static final boolean USE_BACK_AS_PREV_DEFAULT = false;
+
 	@NotNull
 	private final Announcer<DragPreferencesChangeListener> dpclRegister = new Announcer<DragPreferencesChangeListener>(DragPreferencesChangeListener.class);
 
@@ -73,6 +77,8 @@ public class CalculatorActivity extends Activity implements FontSizeAdjuster, Sh
 	@Nullable
 	private Vibrator vibrator;
 
+	private boolean useBackAsPrev = USE_BACK_AS_PREV_DEFAULT;
+
 	/**
 	 * Called when the activity is first created.
 	 */
@@ -510,8 +516,10 @@ public class CalculatorActivity extends Activity implements FontSizeAdjuster, Sh
 	@Override
 	public boolean onKeyDown(int keyCode, KeyEvent event) {
 		if (keyCode == KeyEvent.KEYCODE_BACK) {
-			calculatorModel.doHistoryAction(HistoryAction.undo);
+			if (useBackAsPrev) {
-			return true;
+				calculatorModel.doHistoryAction(HistoryAction.undo);
+				return true;
+			}
 		}
 		return super.onKeyDown(keyCode, event);
 	}
@@ -625,6 +633,10 @@ public class CalculatorActivity extends Activity implements FontSizeAdjuster, Sh
 			this.calculatorModel.evaluate();
 		}
 
+		if ( USE_BACK_AS_PREV_P_KEY.equals(key) ) {
+			useBackAsPrev = preferences.getBoolean(USE_BACK_AS_PREV_P_KEY, USE_BACK_AS_PREV_DEFAULT);
+		}
+
 		if ( CalculatorEngine.MULTIPLICATION_SIGN_P_KEY.equals(key) ) {
 			initMultiplicationButton();
 		}