In order to understand how UISearchDisplayController works i’ve written the following code in viewDidLoad method:
UISearchBar * searchBar = [[[UISearchBar alloc] initWithFrame:CGRectMake(0.0f, 0.0f, 320.0f, 40.0f)] autorelease];
searchBar.delegate = self;
searchBar.showsCancelButton = YES;
_searchController = [[UISearchDisplayController alloc] initWithSearchBar:searchBar contentsController:self];
_searchController.delegate = self;
_searchController.searchResultsDataSource = self;
_searchController.searchResultsDelegate = self;
[_searchController setActive:YES animated:YES];
[_searchController.searchBar becomeFirstResponder];
But when i run my code the UISearchBar doesn’t being shown as you can see in the image.

You create UISearchBarcontroller its good but not added in uiviewcontroller. So You add UISearchBarcontroller in uiviewcontroller.
coding:
[self.view addsubview:_searchController];