我想把一个按钮居中在我的navigationBar上,这个按钮当前被定义为rightBarButtonItem:
UIBarButtonItem *audioBtn = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemPlay target:self action:@selector(toggleAudioPlayback:)];
self.navigationItem.rightBarButtonItem = audioBtn;
[audioBtn release];