private async void OnSelectedChannel()
 {
     if (SelectedChannel != null)
     {
         SelectedChannel.HasUnreadedMessages = false;
         SelectedChannel.ChannelFontWeight   = FontWeights.Normal;
         IEnumerable <Message> messages;
         if (SelectedChannel is PrivateGroup)
         {
             messages = await rocketChatRest.GetPrivateGroupHistory(SelectedChannel.Id);
         }
         else
         {
             messages = await rocketChatRest.GetChannelHistory(SelectedChannel.Id);
         }
         foreach (var message in messages)
         {
             message.User = Users.FirstOrDefault(x => x.Id == message.User.Id);
             if (message.Attachments != null && message.Attachments.Length > 0)
             {
                 foreach (var attachment in message.Attachments)
                 {
                     attachment.ImagePreview = await rocketChatRest.GetImage(attachment.ImageUrl);
                 }
             }
         }
         Messages = new ObservableCollection <Message>(messages);
     }
 }