UISearchController 的用法［点击搜索框，自动到顶部］

//在ViewDidLoad里面如下代码

  self.searchViewController = [[UISearchController alloc]initWithSearchResultsController:nil];
    self.searchViewController.active = NO;
    self.searchViewController.dimsBackgroundDuringPresentation = NO;
    self.searchViewController.hidesNavigationBarDuringPresentation = YES;
    [self.searchViewController.searchBar sizeToFit];
    self.searchViewController.searchBar.barTintColor = kALittleGray;
    //设置显示搜索结果的控制器
    self.searchViewController.searchResultsUpdater = self; //协议(UISearchResultsUpdating)
    self.searchViewController.delegate = self;

    self.tableView.tableHeaderView = self.searchViewController.searchBar;
    self.searchViewController.searchBar.keyboardType = UIKeyboardAppearanceDefault;
    self.searchViewController.searchBar.placeholder = @"请输入城市关键字";