Add posts with surveys support

lib/models/post.dart

@@ -3,6 +3,7 @@ import 'package:comunic/enums/post_visibility_level.dart';
 import 'package:comunic/enums/user_access_levels.dart';
 import 'package:comunic/lists/comments_list.dart';
 import 'package:comunic/models/like_element.dart';
+import 'package:comunic/models/survey.dart';
 import 'package:meta/meta.dart';
 
 /// Single post information
@@ -31,6 +32,7 @@ class Post implements LikeElement {
   bool userLike;
   final UserAccessLevels access;
   final CommentsList comments;
+  final Survey survey;
 
   Post(
       {@required this.id,
@@ -53,7 +55,8 @@ class Post implements LikeElement {
       @required this.likes,
       @required this.userLike,
       @required this.access,
-      @required this.comments})
+      @required this.comments,
+      @required this.survey})
       : assert(id != null),
         assert(userID != null),
         assert(userPageID != 0 || groupID != 0),
@@ -62,6 +65,7 @@ class Post implements LikeElement {
         assert(visibilityLevel != null),
         assert(kind != null),
         assert(kind != PostKind.COUNTDOWN || timeEnd != null),
+        assert(kind != PostKind.SURVEY || survey != null),
         assert(likes != null),
         assert(userLike != null),
         assert(access != null);

lib/models/survey.dart

@@ -0,0 +1,52 @@
+import 'package:comunic/models/survey_choice.dart';
+import 'package:meta/meta.dart';
+
+/// Survey information
+///
+/// @author Pierre HUBERT
+
+class Survey {
+  final int id;
+  final int userID;
+  final int postID;
+  final int creationTime;
+  final String question;
+  int userChoice;
+  final Set<SurveyChoice> choices;
+
+  Survey({
+    @required this.id,
+    @required this.userID,
+    @required this.postID,
+    @required this.creationTime,
+    @required this.question,
+    @required this.userChoice,
+    @required this.choices,
+  })  : assert(id != null),
+        assert(userID != null),
+        assert(postID != null),
+        assert(creationTime != null),
+        assert(question != null),
+        assert(userChoice != null),
+        assert(choices != null),
+        assert(choices.length > 0);
+
+  bool get hasResponded => this.userChoice != null && this.userChoice > 0;
+
+  SurveyChoice get userResponse {
+    if (!hasResponded) return null;
+
+    return choices.firstWhere((e) => e.id == userChoice);
+  }
+
+  void cancelUserResponse() {
+    if (hasResponded) userResponse.responses--;
+    userChoice = 0;
+  }
+
+  void setUserResponse(SurveyChoice choice) {
+    cancelUserResponse();
+    userChoice = choice.id;
+    choice.responses++;
+  }
+}

lib/models/survey_choice.dart

@@ -0,0 +1,19 @@
+import 'package:meta/meta.dart';
+
+/// Single survey choice
+///
+/// @author Pierre HUBERT
+
+class SurveyChoice {
+  final int id;
+  final String name;
+  int responses;
+
+  SurveyChoice({
+    @required this.id,
+    @required this.name,
+    @required this.responses,
+  })  : assert(id != null),
+        assert(name != null),
+        assert(responses != null);
+}