Do not retrieve twice the same message

lib/helpers/conversations_helper.dart

@@ -10,6 +10,7 @@ import 'package:comunic/models/conversation.dart';
 import 'package:comunic/models/conversation_message.dart';
 import 'package:comunic/models/new_conversation_message.dart';
 import 'package:comunic/utils/account_utils.dart';
+import 'package:meta/meta.dart';
 
 /// Conversation helper
 ///
@@ -137,7 +138,8 @@ class ConversationsHelper {
   ///
   /// Set [lastMessageID] to 0 to specify that we do not have any message of the
   /// conversation yet or another value else
-  Future<ConversationMessagesList> downloadNewMessagesSingle(int conversationID,
+  Future<ConversationMessagesList> _downloadNewMessagesSingle(
+      int conversationID,
       {int lastMessageID = 0}) async {
     // Execute the request on the server
     final response = await APIRequest(
@@ -168,6 +170,17 @@ class ConversationsHelper {
     return list;
   }
 
+  /// Get new messages for a given conversation
+  ///
+  /// If [lastMessageID] is set to 0 then we retrieve the last messages of
+  /// the conversation.
+  /// Otherwise [lastMessageID] contains the ID of the last known message
+  Future<ConversationMessagesList> getNewMessages(
+      {@required int conversationID, int lastMessageID = 0}) async {
+    return await _downloadNewMessagesSingle(conversationID,
+        lastMessageID: lastMessageID);
+  }
+
   /// Send a new message to the server
   Future<SendMessageResult> sendMessage(NewConversationMessage message) async {
     final request = APIRequest(

lib/lists/conversation_messages_list.dart

@@ -32,4 +32,12 @@ class ConversationMessagesList extends ListBase<ConversationMessage> {
 
     return users;
   }
+
+  /// Get the ID of the last message present in this list
+  int get lastMessageID {
+    int lastMessageID = 0;
+    for (ConversationMessage message in this)
+      if (message.id > lastMessageID) lastMessageID = message.id;
+    return lastMessageID;
+  }
 }

lib/ui/screens/conversation_screen.dart

@@ -58,8 +58,9 @@ class _ConversationScreenState extends State<ConversationScreen> {
   /// Load a list of messages
   Future<void> _loadMessages() async {
     //First, get the messages
-    final messages = await _conversationsHelper
+    final messages = await _conversationsHelper.getNewMessages(
-        .downloadNewMessagesSingle(widget.conversationID);
+        conversationID: widget.conversationID,
+        lastMessageID: _messages == null ? 0 : _messages.lastMessageID);
 
     if (messages == null) return _errorLoading();
 
@@ -81,7 +82,7 @@ class _ConversationScreenState extends State<ConversationScreen> {
 
       //Reverse the order of the messages
       _messages.sort();
-      final reverse =_messages.reversed;
+      final reverse = _messages.reversed;
       _messages = ConversationMessagesList();
       _messages.addAll(reverse);
     });
@@ -164,7 +165,8 @@ class _ConversationScreenState extends State<ConversationScreen> {
   /// Check if a message is the first message of a user or not
   bool _isFirstMessage(int index) {
     return index == _messages.length - 1 ||
-        (index < _messages.length - 1 && _messages[index + 1].userID != _messages[index].userID);
+        (index < _messages.length - 1 &&
+            _messages[index + 1].userID != _messages[index].userID);
   }
 
   /// Error handling
@@ -176,7 +178,7 @@ class _ConversationScreenState extends State<ConversationScreen> {
   Widget _buildMessagesList() {
     return Expanded(
       child: ListView.builder(
-        reverse: true,
+          reverse: true,
           itemCount: _messages.length,
           itemBuilder: (c, i) {
             return ConversationMessageTile(