让 UISearchBar 支持空搜索

让 UISearchBar 支持空搜索，当没有输入的时候，search 按钮一样可以点击- (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;}