//=================================================
    //                 自定义SearchBar
    //=================================================
    
    // 1.创建一个UITextField作为背景
    UITextField *searchBar = [[UITextField alloc] init];
    searchBar.width = 420;
    searchBar.height = 30;
    searchBar.font = [UIFont systemFontOfSize:14];
    searchBar.background = [UIImage imageNamed:@"searchbar_textfield_background"];

    // 2.添加左侧的小图标
//    UIImage *searchIcon = [UIImage imageNamed:@"searchbar_textfield_search_icon"];
//    UIImageView *searchIconView = [[UIImageView alloc] initWithImage:searchIcon]; // 使用此种形式创建出来的ImageView是有默认大小的。
    
    UIImageView *searchIconView = [[UIImageView alloc] init];
    searchIconView.image = [UIImage imageNamed:@"searchbar_textfield_search_icon"];
    searchIconView.width = 30;
    searchIconView.height = 30;
    
    searchIconView.contentMode = UIViewContentModeCenter;
    searchBar.leftView = searchIconView;
    searchBar.leftViewMode = UITextFieldViewModeAlways;
    
    self.navigationItem.titleView = searchBar;