iOS 11 之后 UiSearchBar 添加到 navigationItem.titleView

 // 在iOS11 之后适配titleView

    UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];

    button.frame =CGRectMake(0, 0, kScreenWidth, 44);

    self.navigationItem.titleView = button;

    _searchBar = [[UISearchBar alloc] initWithBackgroundImage:nil placeholder:@"搜一搜，订单一清二楚" isFollow:YES];

    _searchBar.delegate = self;

    if (@available(iOS 11.0, *)) {

        //  https://www.jianshu.com/p/352f101d6df1

        // 下面的 100  可以根据情况添加

        _searchBar.frame = CGRectMake(0, 0, kScreenWidth - 100, button.height);

    }

    [button addSubview:_searchBar];