工具栏上的Barbutton项的标题在IOS 6上可见，但在IOS 7上不可见

我正在使用下面的代码作为工具栏，它显示的是IOS 6的标题，而不是IOS 7的标题。

UIBarButtonItem *settingButton = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"multimedia/icon_settings.png"] style:UIBarButtonItemStylePlain                                                                  target:self                                                                 action:@selector(pressSettings:)];
UIBarButtonItem *refreshButton = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"multimedia/icon_refresh.png"]                                                                style:UIBarButtonItemStylePlain target:self                                                                 action:@selector(pressRefresh:)];
UIBarButtonItem *helpButton = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"multimedia/icon_help.png"]                                                                  style:UIBarButtonItemStylePlain target:self action:@selector(pressHelp:)];
UIBarButtonItem *flexSpace = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];

/* Set the title of the button */
[settingButton setTitle:@"Settings"];
[refreshButton setTitle:@"Refresh"];
[helpButton setTitle:@"Help"];
[feedbackButton setTitle:@"Feedback"];

NSArray *toolbarButtons = @[settingButton, flexSpace, helpButton, flexSpace, feedbackButton, flexSpace, refreshButton];

[self setToolbarItems:toolbarButtons];

这段代码可以在IOS 6上正常工作，但是在IOS 7中仅显示条形按钮图像，而没有标题。