搜索支持空搜索

• (void)searchBarTextDidBeginEditing:(UISearchBar *) searchBar
  {
  UITextField *searchBarTextField = nil;
  NSArray *views = ([[[UIDevice currentDevice] systemVersion] floatValue] < 7.0) ? searchBar.subviews : [[searchBar.subviews objectAtIndex:0] subviews];
  for (UIView *subview in views)
  {
  if ([subview isKindOfClass:[UITextField class]])
  {
  searchBarTextField = (UITextField *)subview;
  break;
  }
  }
  searchBarTextField.enablesReturnKeyAutomatically = NO;
  }