Add UITextField to title view of navigation item

As my comment suggested, try the following, which seems to me is what you're after.

UITextField *textField = [[UITextField alloc]initWithFrame:CGRectMake(0, 0, self.navigationController.navigationBar.frame.size.width, 21.0)];

self.navigationItem.titleView = textField;

[textField becomeFirstResponder];