前端：

1、更新界面图标
2、修改消息缓存逻辑
3、新增清空未读消息数
后端：
1、修复不同时区导致的时间显示问题
2、新增清空未读消息数接口

backend/IM_API/Application/EventHandlers/ConversationEventHandler.cs

@@ -46,8 +46,10 @@ namespace IM_API.Application.EventHandlers
                 }
                 userAConversation.LastMessage = @event.MessageContent;
                 userAConversation.LastReadMessageId = @event.MessageId;
+                userAConversation.LastMessageTime = @event.MessageCreated;
                 userBConversation.LastMessage = @event.MessageContent;
                 userBConversation.UnreadCount += 1;
+                userBConversation.LastMessageTime = @event.MessageCreated;
                 _context.UpdateRange(userAConversation,userBConversation);
                 await _context.SaveChangesAsync();
             }

backend/IM_API/Application/EventHandlers/SignalREventHandler.cs

@@ -23,7 +23,17 @@ namespace IM_API.Application.EventHandlers
         {
             if(@event.ChatType == Models.ChatType.PRIVATE)
             {
-                MessageBaseDto messageBaseDto = _mapper.Map<MessageBaseDto>(_mapper.Map<Message>(@event));
+                MessageBaseDto messageBaseDto = new MessageBaseDto
+                {
+                    MsgId = @event.MessageId.ToString(),
+                    ChatType = @event.ChatType.ToString(),
+                    Content = @event.MessageContent,
+                    GroupMemberId = null,
+                    ReceiverId = @event.MsgRecipientId,
+                    SenderId = @event.MsgSenderId,
+                    TimeStamp = @event.MessageCreated,
+                    Type = @event.MessageMsgType.ToString()
+                };
                 await _hub.Clients.Users(@event.MsgRecipientId.ToString()).SendAsync("ReceiveMessage", messageBaseDto);
             }
         }