去除<<导航栏搜索框>>边界

-(void)setupnav {
   // 创建搜索框
    UISearchBar *searchBar = [[UISearchBar alloc]initWithFrame:CGRectMake(5, 5, 190, 35)];
    
    // 去掉搜索框的边界线
    [searchBar setBackgroundImage:[[UIImage alloc]init] ];

    searchBar.placeholder = @"商品搜索";
    
    UIView * centerView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 200, 44)];
    
    [centerView addSubview:searchBar];
   
    self.navigationItem.titleView = centerView;
    
    }