How to implement hidden Group Chat Button behind UITableView like WhatsApp on iPhone

I am interested to implement hidden button behind UITableView like WhatsApp on iPhone. For the first time user go to Chats TabBar, UISearchBar and GroupChat Button are hidden, when user scroll down the TableView, GroupChat Button will appear.

I try to create that UI via IB but I can't tap the button. Here is the IB View Hierarchy:

UIView  
-->UIButton  
-->UITableView  
   -->UIView (UIView as TableView's header)  
      -->UISearchBar

I set UIView height to 88 and clearColor开发者_如何学Python as BackgroundColor <-- (UIView that contains UISearchBar). Then I set UISearchBar Y position to 44. On - (void)viewDidLoad I do this

[self.tableView setContentOffset:CGPointMake(0.0, 44.0) animated:YES];

What is the best way to achieve that UI behavior?? Any help will be appreciated.

Regards