Ability to cache currency selection (#5)

lib/data/repository/Repository.dart

@@ -0,0 +1,6 @@
+import '../prefs/CurrencyPair.dart';
+
+abstract class Repository {
+  CurrencyPair getConversionPair();
+  Future<void> setConversionPair(String fromCurrency, String toCurrency);
+}

lib/data/repository/RepositoryImpl.dart

@@ -0,0 +1,20 @@
+import 'package:easy_cc_flutter/data/prefs/CurrencyPair.dart';
+import 'package:easy_cc_flutter/data/prefs/PreferenceProvider.dart';
+
+import '../../locator.dart';
+import 'Repository.dart';
+
+class RepositoryImpl extends Repository {
+  final PreferenceProvider _prefs = locator<PreferenceProvider>();
+
+  @override
+  CurrencyPair getConversionPair() {
+    return _prefs.getConversionPair();
+  }
+
+  @override
+  Future<void> setConversionPair(String fromCurrency, String toCurrency) {
+    return _prefs.saveConversionPair(fromCurrency, toCurrency);
+  }
+
+}