Cache groups informations

lib/helpers/groups_helper.dart

@@ -32,13 +32,11 @@ const _APIGroupsPostsCreationLevelsMap = {
   "members": GroupPostCreationLevel.MEMBERS
 };
 
+final _groupsListCache = GroupsList();
+
 class GroupsHelper {
-  /// Get a list of groups from the server
-  Future<GroupsList> getList(Set<int> groups) async {
-
-    if(groups.length == 0)
-      return GroupsList();
-
+  /// Download a list of groups information from the server
+  Future<GroupsList> _downloadList(Set<int> groups) async {
     final response = await APIRequest(
       uri: "groups/get_multiple_info",
       needLogin: true,
@@ -56,6 +54,32 @@ class GroupsHelper {
     return list;
   }
 
+  /// Get a list of groups from the server
+  Future<GroupsList> getList(Set<int> groups, {bool force = false}) async {
+    final list = GroupsList();
+
+    // Check which groups information to download
+    final toDownload = Set<int>();
+    groups.forEach((groupID) {
+      if (_groupsListCache.containsKey(groupID))
+        list[groupID] = _groupsListCache[groupID];
+      else
+        toDownload.add(groupID);
+    });
+
+    // Download required groups information
+    if (toDownload.length > 0) {
+      final downloaded = await _downloadList(toDownload);
+
+      if (downloaded == null) return null;
+
+      list.addAll(downloaded);
+      _groupsListCache.addAll(downloaded);
+    }
+
+    return list;
+  }
+
   /// Turn an API entry into a group object
   Group _getGroupFromAPI(Map<String, dynamic> map) {
     return Group(