在ios下的一个tableview下我添加了下面代码：

 //创建搜索栏和搜索显示控制器
    _searchBar = [[UISearchBar  alloc] initWithFrame:CGRectMake(0, 0, BOUNDSSIZE.width, 44)];
    _searchBar.autocorrectionType = UITextAutocorrectionTypeNo;
    _searchBar.autocapitalizationType = UITextAutocapitalizationTypeNone;
    _searchBar.keyboardType = UIKeyboardTypeAlphabet;
    _searchBar.placeholder = [NSString stringWithCString:"搜索（请输入项目关键字）                        "  encoding: NSUTF8StringEncoding];

    self.tableView.tableHeaderView = _searchBar;

    _searchDc=[[UISearchDisplayController alloc] initWithSearchBar:_searchBar contentsController:self];
    _searchDc.searchResultsDataSource=self;
    _searchDc.searchResultsDelegate=self;

就添加了一个searchBar在tableview上面，可是我对着tableview或者那个searchBar点了几次之后它会突然消失，怎么回事