Define a collection service

lib/models/collection.dart

@@ -24,7 +24,7 @@ class Collection {
   );
 
   static Collection emptyCollection() {
-    return Collection(null, null, null, null, null, null, null, null);
+    return Collection(null, null, null, null, null, null, null, List<String>());
   }
 
   Collection copyWith({

lib/services/collections_service.dart

@@ -1,5 +1,42 @@
+import 'dart:io';
+
+import 'package:dio/dio.dart';
 import 'package:logging/logging.dart';
+import 'package:photos/core/configuration.dart';
+import 'package:photos/core/event_bus.dart';
+import 'package:photos/events/user_authenticated_event.dart';
+import 'package:photos/models/collection.dart';
 
 class CollectionsService {
   final _logger = Logger("CollectionsService");
+
+  CollectionsService._privateConstructor() {
+    Bus.instance.on<UserAuthenticatedEvent>().listen((event) {
+      // TODO: sync();
+    });
+  }
+
+  static final CollectionsService instance =
+      CollectionsService._privateConstructor();
+
+  Future<Collection> getFolder(String path) async {
+    return Dio()
+        .get(
+      Configuration.instance.getHttpEndpoint() + "/collections/folder/",
+      queryParameters: {
+        "path": path,
+      },
+      options:
+          Options(headers: {"X-Auth-Token": Configuration.instance.getToken()}),
+    )
+        .then((response) {
+      return Collection.fromMap(response.data);
+    }).catchError((e) {
+      if (e.response.statusCode == HttpStatus.notFound) {
+        return Collection.emptyCollection();
+      } else {
+        throw e;
+      }
+    });
+  }
 }