当用作UINavigationBar标题视图时未调用UISearchBar委托?

我有一个UITableViewController,我已将其指定为UISearchBarDelegate。到目前为止,我已经通过编程方式将UISearchBar添加到表的headerView中,没有出现任何问题。

我开始用尽屏幕空间,所以我决定取消我的普通UINavigationController标题(文本),并添加以下代码,将我的搜索栏从表移动到UINavigationBar:

// (Called in viewDidLoad)
// Programmatically make UISearchBar
UISearchBar *tmpSearchBar = [[UISearchBar alloc] initWithFrame:CGRectMake(0,0,320,45)];
tmpSearchBar.delegate = self;
tmpSearchBar.showsCancelButton = YES;
tmpSearchBar.autocorrectionType = UITextAutocorrectionTypeNo;
tmpSearchBar.autocapitalizationType = UITextAutocapitalizationTypeNone;
[self set_searchBar:tmpSearchBar];
[tmpSearchBar release];
self.navigationItem.titleView = [self _searchBar];

此代码按预期工作-我的UINavigationBar现在是UISearchBar。

/** Only show the cancel button when the keyboard is displayed */
- (void) searchBarDidBeginEditing:(UISearchBar*) lclSearchBar
{
  lclSearchBar.showsCancelButton = YES;
}

…不再被召唤。我已经中断,并确认UISearchBar的委托确实是self,即视图控制器。奇怪的是,这个委托方法的调用仍然很好:

/** Run the search and resign the keyboard */
- (void) searchBarSearchButtonClicked:(UISearchBar *)lclSearchBar
{
  _deepSearchRan = NO;
  [self runSearchForString:[[self _searchBar] text] isSlowSearch:NO];
  [lclSearchBar resignFirstResponder];
}

你知道为什么UINavigationBar会拒绝我的代表电话吗??我错过了什么?