Added missing interfaces and implementations for backing stores

lib/kiota_abstractions.dart

@@ -46,5 +46,9 @@ part 'src/serialization/serialization_writer_proxy_factory.dart';
 part 'src/store/backed_model.dart';
 part 'src/store/backing_store.dart';
 part 'src/store/backing_store_factory.dart';
+part 'src/store/backing_store_factory_singleton.dart';
+part 'src/store/backing_store_parse_node_factory.dart';
+part 'src/store/backing_store_serialization_writer_proxy_factory.dart';
 part 'src/store/backing_store_subscription_callback.dart';
 part 'src/store/in_memory_backing_store.dart';
+part 'src/store/in_memory_backing_store_factory.dart';

lib/src/store/backing_store_factory_singleton.dart

@@ -0,0 +1,9 @@
+part of '../../kiota_abstractions.dart';
+
+/// This class is used to register the backing store factory.
+class BackingStoreFactorySingleton {
+  static final BackingStoreFactory _instance = InMemoryBackingStoreFactory();
+
+  /// The backing store factory singleton instance.
+  static BackingStoreFactory get instance => _instance;
+}

lib/src/store/backing_store_parse_node_factory.dart

@@ -0,0 +1,28 @@
+part of '../../kiota_abstractions.dart';
+
+/// Proxy implementation of [ParseNodeFactory] that allows for the
+/// [BackingStore] that automatically sets the state of the [BackingStore]
+/// when deserializing.
+class BackingStoreParseNodeFactory extends ParseNodeProxyFactory {
+  /// Creates a new instance of the [BackingStoreParseNodeFactory] class.
+  BackingStoreParseNodeFactory({
+    required super.concrete,
+  }) : super(
+          onBefore: (parsable) {
+            if (parsable is BackedModel) {
+              final model = parsable as BackedModel;
+              if (model.backingStore != null) {
+                model.backingStore!.initializationCompleted = false;
+              }
+            }
+          },
+          onAfter: (parsable) {
+            if (parsable is BackedModel) {
+              final model = parsable as BackedModel;
+              if (model.backingStore != null) {
+                model.backingStore!.initializationCompleted = true;
+              }
+            }
+          },
+        );
+}

lib/src/store/backing_store_serialization_writer_proxy_factory.dart

@@ -0,0 +1,42 @@
+part of '../../kiota_abstractions.dart';
+
+/// Proxy implementation of [SerializationWriterFactory] for the [BackingStore]
+/// that automatically sets the state of the backing store when serializing.
+class BackingStoreSerializationWriterProxyFactory
+    extends SerializationWriterProxyFactory {
+  /// Creates a new instance of [BackingStoreSerializationWriterProxyFactory]
+  /// with the provided concrete factory.
+  BackingStoreSerializationWriterProxyFactory({
+    required super.concrete,
+  }) : super(
+          onBefore: (p) {
+            if (p is BackedModel) {
+              final model = p as BackedModel;
+              if (model.backingStore != null) {
+                model.backingStore!.returnOnlyChangedValues = true;
+              }
+            }
+          },
+          onAfter: (p) {
+            if (p is BackedModel) {
+              final model = p as BackedModel;
+              if (model.backingStore != null) {
+                model.backingStore!.returnOnlyChangedValues = false;
+                model.backingStore!.initializationCompleted = true;
+              }
+            }
+          },
+          onStart: (p, writer) {
+            if (p is BackedModel) {
+              final model = p as BackedModel;
+              if (model.backingStore != null) {
+                model.backingStore!
+                    .iterateKeysForValuesChangedToNull()
+                    .forEach((element) {
+                  writer.writeNullValue(element);
+                });
+              }
+            }
+          },
+        );
+}

lib/src/store/in_memory_backing_store_factory.dart

@@ -0,0 +1,7 @@
+part of '../../kiota_abstractions.dart';
+
+/// This class is used to create instances of [InMemoryBackingStore].
+class InMemoryBackingStoreFactory implements BackingStoreFactory {
+  @override
+  BackingStore createBackingStore() => InMemoryBackingStore();
+}