iOS - UINavigationController 并添加左栏按钮？

Question

下面的方法是 UINavigationController 的委托(delegate)方法。我想成为决定是否将左侧项目添加到每个页面的人。下面的代码不起作用，我做错了什么吗？

我不想通过 ViewController 本身执行此操作。我希望 NavigationCopntroller 负责这项任务。

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    UIBarButtonItem *menuItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemOrganize 
                                                                                  target:self 
                                                                                  action:@selector(menuItemSelected:)];
    [self.navigationItem setLeftBarButtonItem:menuItem];

    // I also tried 
    // [viewController.navigationItem setLeftBarButtonItem:menuItem];
}

Answer 1

我认为问题是您试图访问 viewController 上的 navigationItem 属性，但它不存在，因为在 willShowViewController 方法中 viewController 是还没有在 navigationController 堆栈中，尝试使用 didShowViewController 委托(delegate)方法

像这样:

- (void)navigationController:(UINavigationController *)navigationController didShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    UIBarButtonItem *menuItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemOrganize 
                                                                                  target:self 
                                                                                  action:@selector(menuItemSelected:)];
    [self.navigationItem setLeftBarButtonItem:menuItem];

    [viewController.navigationItem setLeftBarButtonItem:menuItem];