xcode - self.navigationController.topViewController 作为 HomeViewController

Question

我在将 objective-c 转换为 swift 时遇到问题。

对于我的火车，我转换了 MMPaper swift ，但在属性中:

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(粗体问题):

libswiftCore.dylib`swift_dynamicCastClassUnconditional:
...
0x10cf7da1e:  leaq   0x36b3d(%rip), %rax       ; "Swift dynamic cast failed"
0x10cf7da25:  movq   %rax, 0xb4a2c(%rip)       ; gCRAnnotations + 8
0x10cf7da2c:  int3   
**0x10cf7da2d:  movq   %rdi, %rax**
.....

你有想法吗？

Answer 1

通过使用 as 运算符，您是在告诉 Swift 您完全确定向下转换为 HomeViewController 会成功。但是，明明presentingVC.nextViewControllerAtPoint(point)可以返回nil，而不能成功将nil向下转型为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的类型的可选值。

以下应该有效:

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来使代码更简洁；当 Swift 看到 as?HomeViewController 时，它发现 presentingVC 的类型必须是 HomeViewController?。很抱歉，如果这很明显。)