feat: change local data interface implementation to singleton

According to our agreed appshell architecture we will make the local implementations
as singletons to represent a single source persistent layer, like a database.

On top of that, the service should be usable across multiple instances of the service, even if no datasource is provided.

packages/flutter_availability/lib/src/config/availability_options.dart

@@ -3,7 +3,6 @@ import "dart:async";
 import "package:flutter/material.dart";
 import "package:flutter_availability/src/config/availability_translations.dart";
 import "package:flutter_availability/src/service/errors.dart";
-import "package:flutter_availability/src/service/local_data_interface.dart";
 import "package:flutter_availability/src/ui/widgets/defaults/default_base_screen.dart";
 import "package:flutter_availability/src/ui/widgets/defaults/default_buttons.dart";
 import "package:flutter_availability/src/ui/widgets/defaults/default_confirmation_dialog.dart";

packages/flutter_availability_data_interface/lib/flutter_availability_data_interface.dart

@@ -2,6 +2,7 @@
 library flutter_availability_data_interface;
 
 export "src/data_interface.dart";
+export "src/local_repository/local_data_interface.dart";
 export "src/models/availability.dart";
 export "src/models/templates.dart";
 export "src/service/availability_service.dart";

packages/flutter_availability_data_interface/lib/src/local_repository/initial_data.dart

@@ -1,4 +1,3 @@
-import "package:flutter/material.dart";
 import "package:flutter_availability_data_interface/flutter_availability_data_interface.dart";
 
 /// Returns the default local availabilities for a user
@@ -36,18 +35,18 @@ List<AvailabilityModel> getDefaultLocalAvailabilitiesForUser() {
 List<AvailabilityTemplateModel> getDefaultLocalTemplatesForUser() {
   var currentMonth = DateTime(DateTime.now().year, DateTime.now().month);
   return [
-    for (var template in <(String, String, Color)>[
-      ("1", "Morning", Colors.blue),
-      ("2", "Afternoon", Colors.green),
-      ("3", "Evening", Colors.red),
-      ("4", "Night", Colors.purple),
-      ("5", "All day", Colors.orange),
+    for (var template in <(String, String, int)>[
+      ("1", "Morning", 0xFF0000FF),
+      ("2", "Afternoon", 0xFF00FF00),
+      ("3", "Evening", 0xFFFF0000),
+      ("4", "Night", 0xFFFF00FF),
+      ("5", "All day", 0xFF9999),
     ]) ...[
       AvailabilityTemplateModel(
         id: template.$1,
         userId: "",
         name: template.$2,
-        color: template.$3.value,
+        color: template.$3,
         templateType: AvailabilityTemplateType.day,
         templateData: DayTemplateData(
           startTime: currentMonth,

packages/flutter_availability_data_interface/lib/src/local_repository/local_data_interface.dart

@@ -1,17 +1,26 @@
 import "dart:async";
 
-import "package:flutter_availability/src/service/initial_data.dart";
 import "package:flutter_availability_data_interface/flutter_availability_data_interface.dart";
+import "package:flutter_availability_data_interface/src/local_repository/initial_data.dart";
+import "package:rxdart/rxdart.dart";
 
 /// A local implementation of the [AvailabilityDataInterface] that stores data
 /// in memory.
 class LocalAvailabilityDataInterface implements AvailabilityDataInterface {
+  /// Creates a representation of the local data interface.
   ///
-  LocalAvailabilityDataInterface() {
+  /// A second call of the constructor will return the same value as earlier
+  /// calls according to the singleton principle
+  factory LocalAvailabilityDataInterface() =>
+      _localDataInterface ??= LocalAvailabilityDataInterface._();
+
+  LocalAvailabilityDataInterface._() {
     _notifyAvailabilityChanges();
     _notifyTemplateChanges();
   }
 
+  static LocalAvailabilityDataInterface? _localDataInterface;
+
   final Map<String, List<AvailabilityModel>> _userAvailabilities = {
     "": getDefaultLocalAvailabilitiesForUser(),
   };
@@ -21,9 +30,9 @@ class LocalAvailabilityDataInterface implements AvailabilityDataInterface {
   };
 
   final StreamController<Map<String, List<AvailabilityModel>>>
-      _availabilityController = StreamController.broadcast();
+      _availabilityController = BehaviorSubject();
   final StreamController<Map<String, List<AvailabilityTemplateModel>>>
-      _templateController = StreamController.broadcast();
+      _templateController = BehaviorSubject();
 
   void _notifyAvailabilityChanges() {
     _availabilityController.add(_userAvailabilities);

packages/flutter_availability_data_interface/lib/src/service/availability_service.dart

@@ -1,4 +1,5 @@
 import "package:flutter_availability_data_interface/src/data_interface.dart";
+import "package:flutter_availability_data_interface/src/local_repository/local_data_interface.dart";
 import "package:flutter_availability_data_interface/src/models/availability.dart";
 import "package:flutter_availability_data_interface/src/models/templates.dart";
 import "package:flutter_availability_data_interface/src/utils/public.dart";
@@ -7,10 +8,10 @@ import "package:rxdart/rxdart.dart";
 ///
 class AvailabilityService {
   ///
-  const AvailabilityService({
+  AvailabilityService({
     required this.userId,
-    required this.dataInterface,
-  });
+    AvailabilityDataInterface? dataInterface,
+  }) : dataInterface = dataInterface ?? LocalAvailabilityDataInterface();
 
   /// The user id for which the availabilities are managed
   final String userId;