UISearchBarのscopeBar(検索バーの選択ボタン)を使うとき。

   UISearchBar *_searchBar = [[UISearchBar alloc]init];  

   _searchBar.frame = CGRectMake(0, 0, self.tableView.bounds.size.width,0);  

   _searchBar.delegate = self;  

   _searchBar.tintColor = [UIColor darkGrayColor];  

   _searchBar.translucent = YES;  

   _searchBar.placeholder = @"Search Twitter";  

   NSArray *scopeButtonTitles = [[NSArray alloc] initWithObjects:@"All Tweets",@"User", nil];  

   _searchBar.scopeButtonTitles = scopeButtonTitles;  

   _searchBar.showsScopeBar = YES;  

   [_searchBar sizeToFit];  

   self.tableView.tableHeaderView = _searchBar;  

showsScopeBarをYesにした後、
[_searchBar sizeToFit]; すると、
スコープバーが現れるんですね...

refer the link below:
UISearchBar Tricks – Showing The Cancel And The Scope Bar Only When Editing
http://www.alexandre-gomes.com/?p=418