无法使用目标c更改uitoolbar的高度

我已成功添加一个UIToolBar,代码如下:

-(void) setUpBottom{

    UIBarButtonItem *flexible = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:self action:nil];
    UIBarButtonItem *feature = [[UIBarButtonItem alloc]initWithImage:[UIImage imageNamed:@"features.png"] style:UIBarButtonItemStylePlain target:self action:@selector(selectFeature:)];

    UIBarButtonItem *fav = [[UIBarButtonItem alloc]initWithImage:[UIImage imageNamed:@"favorite-outline.png"] style:UIBarButtonItemStylePlain target:self action:@selector(selectFavorite:)];

    //following line was intended to change the height of the tool bar, but it doesn't work.
    [[self navigationController].toolbar setFrame:CGRectMake(0, [[UIScreen mainScreen] bounds].size.height - 55, [[UIScreen mainScreen] bounds].size.width, 55)];

    self.toolbarItems = [NSArray arrayWithObjects:flexible,feature, flexible,fav,flexible,nil];
    [[self navigationController] setToolbarHidden:NO];
}

• [self navigationController].toolbarItems 项目将不会显示

• [self navigationController]

所以我的问题是,我如何修改boolbar的高度,以及 self.toolbarItems 和 [自导航控制器]。工具栏项目 ? 谢谢