ios – 为什么在presentmodalviewcontroller调用时,navigationItem.titleView会左对齐？

我正在使用UILabel作为导航栏的titleView(我正在制作简单的应用内网页浏览器).它工作正常,除了当我呈现模态视图控制器时,titleView从导航栏的中心移动到最左边(在后面按钮下面).我在3.0及以上测试过.这是相关代码：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Title view label
    CGRect labelFrame = CGRectMake(0.0,0.0,120.0,36.0); 
    UILabel *label = [[[UILabel alloc] initWithFrame:labelFrame] autorelease];
    label.font = [UIFont boldSystemFontOfSize:14];
    label.numberOfLines = 2;
    label.backgroundColor = [UIColor clearColor];
    label.textAlignment = UITextAlignmentCenter;
    label.textColor = [UIColor whiteColor];
    label.shadowColor = [UIColor blackColor];
    label.shadowOffset = CGSizeMake(0.0,-1.0);
    label.lineBreakMode = UILineBreakModeMiddleTruncation;
    self.navigationItem.titleView = label;
}

-(void)displayComposerSheet:(Nsstring*)mailto 
{
    MFMailComposeViewController *picker = [[MFMailComposeViewController alloc] init];
    picker.mailComposeDelegate = self;
    [self presentModalViewController:picker animated:YES];
    [picker release];
}

截图：

知道为什么会这样吗？谢谢.

self.navigationItem.titleView = myCustomTitleView;

// Fake right button to align titleView properly.
UIBarButtonItem *rightBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:[[UIView alloc] initWithFrame:CGRectMake(0,50,1)]];
// Width equivalent to system default Done button's (which appears on pushed view in my case).
rightBarButtonItem.enabled = NO;
self.navigationItem.rightBarButtonItem = rightBarButtonItem;