Xcode self.navigationController.topViewController作为HomeViewController

我对将objective-c转换为swift有问题

对于我的火车，我改为swift，但在酒店：

目标-c

- (void)interactionBeganAtPoint:(CGPoint)point
{
    // Very basic communication between the transition controller and the top view controller
    // It would be easy to add more control, support pop, push or no-op
    HASmallCollectionViewController *presentingVC = (HASmallCollectionViewController *)[self.navigationController topViewController];

    HASmallCollectionViewController *presentedVC = (HASmallCollectionViewController *)[presentingVC nextViewControllerAtPoint:point];
    if (presentedVC!=nil)
    {
        [self.navigationController pushViewController:presentedVC animated:YES];
    }
    else
    {
        [self.navigationController popViewControllerAnimated:YES];
    }
}

Swift

func interactionBeganAtPoint(point: CGPoint) {
        var presentingVC: HomeViewController! = (self.navigationController.topViewController as HomeViewController)
        var presentedVC: HomeViewController! = (presentingVC.nextViewControllerAtPoint(point) as HomeViewController)

        if (presentedVC != nil) {
            self.navigationController.pushViewController(presentedVC!, animated: true)
        } else {
            self.navigationController.popViewControllerAnimated(true)
        }
    }

结果是（问题以粗体显示）：

---

你有主意了吗？

通过使用

作为

操作员，你告诉Swift你绝对确信你对

HomeViewController

的下推会成功。但是，显然

presentingVC.nextviewcontrollerpoint（point）

可以返回

nil

，并且您无法成功地将

nil

向下转换到

HomeViewController

当向下转换可能失败时，使用

as？

运算符返回您尝试向下转换到的类型的可选值

以下方面应起作用：

func interactionBeganAtPoint(point: CGPoint) {
    let presentingVC = self.navigationController.topViewController as? HomeViewController

    if let presentedVC = presentingVC?.nextViewControllerAtPoint(point) as? HomeViewController {
        self.navigationController.pushViewController(presentedVC, animated: true)
    } else {
        self.navigationController.popViewControllerAnimated(true)
    }
}

---

（我还使用了类型推断使代码更简洁；当Swift将

视为？HomeViewController

时，它发现

presentingVC

的类型必须是

HomeViewController？

。如果这很明显，我很抱歉。）

我尝试了var presentingVC:HomeViewController！presentingVC=self.navController.topViewController作为HomeViewController打印LN（“presentingVC=（presentingVC）”）var presentedVC:HomeViewController！PresentDVC=PresentGVC？下一个可视控制器点（点）为？HomeViewController println（“PresentDVC=（PresentDVC）”）结果：presentingVC=PresentDVC=nil我不明白，为什么PresentDVC是nil！

func interactionBeganAtPoint(point: CGPoint) {
    let presentingVC = self.navigationController.topViewController as? HomeViewController

    if let presentedVC = presentingVC?.nextViewControllerAtPoint(point) as? HomeViewController {
        self.navigationController.pushViewController(presentedVC, animated: true)
    } else {
        self.navigationController.popViewControllerAnimated(true)
    }
}