自定义searchBar和searchDisplay 关联

- (void)viewDidLoad
{
    [super viewDidLoad];
    UISearchBar *searchBar = [[UISearchBar alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width
                                                                           , 44)];
    searchBar.placeholder = @"搜索";    
    // 添加 searchbar 到 headerview
    self.tableView.tableHeaderView = searchBar;    
    // 用 searchbar 初始化 SearchDisplayController    // 并把 searchDisplayController 和当前 controller 关联起来
    searchDisplayController = [[UISearchDisplayController alloc] initWithSearchBar:searchBar contentsController:self];    
    // searchResultsDataSource 就是 UITableViewDataSource
    searchDisplayController.searchResultsDataSource = self;    // searchResultsDelegate 就是 UITableViewDelegate
    searchDisplayController.searchResultsDelegate = self;
}