ios - 在预设演示之前丢失对 transitioningDelegate 的引用

Question

在 Xcode beta 2 和 iOS 10 beta 2 之前，我有一个可用的自定义 UIPresentationController。我没有更改任何代码，但演示文稿现在以标准模式演示文稿呈现。

在 UIPresentationController 的 Apple 示例代码中有一条说明:

For presentations which will use a custom presentation controller, it is possible for that presentation controller to also be the transitioningDelegate. This avoids introducing another object or implementing in the source view controller.

transitioningDelegate does not hold a strong reference to its destination object.To prevent presentationController from being released prior to calling -presentViewController:animated:completion: the NS_VALID_UNTIL_END_OF_SCOPE attribute is appended to the declaration.

我在呈现前后检查了呈现的 View Controller 上的 transitioningDelegate。在它之前是我的自定义 UIPresentationController，但在它之后是零。我的猜测是引用正在发布，但我在 Swift 中找不到与 NS_VALID_UNTIL_END_OF_SCOPE 等效的内容。 编辑: 我已经确认 transitioningDelegate 已设置到演示之前，然后在演示时设置为 nil。

我在呈现 View Controller 中的代码:

@IBAction func buttonAction(_ sender: UIButton) {
    let secondViewController = storyboard!.instantiateViewController(withIdentifier: "NewViewController") as! NewViewController
    let presentationController = MyPresentationController(presentedViewController: secondViewController, presenting: self)
    presentationController.initialFrame = button.frame
    secondViewController.transitioningDelegate = presentationController

    // Move map
    let pixelsToMove: CGFloat = mapView.frame.height / 4
    let region = self.mapView.region

    self.mapView.setRegion(region, offsetBy: pixelsToMove, animated: true)

    // Delegate to NewViewController
    secondViewController.mapView = mapView
    mapView.delegate = secondViewController

    print(secondViewController.transitioningDelegate)
    UIView.animate(withDuration: 0.3, animations: {
        let tabBar = self.tabBarController!.tabBar
        tabBar.frame.origin.y += tabBar.frame.height

        self.present(secondViewController, animated: true, completion: nil)
    })
}

还有我在 UIPresentationController 中的代码:

override init(presentedViewController: UIViewController, presenting presentingViewController: UIViewController?) {
    super.init(presentedViewController: presentedViewController, presenting: presentingViewController)
    presentedViewController.modalPresentationStyle = .custom
}

Answer 1

transitioningDelegate 属性是一个弱变量。查看docs here .这意味着当设置 secondViewController.transitioningDelegate = presentationController 时，presentationController 的保留计数不会增加。由于您正在该方法中实例化 presentationController 并且没有其他对象对该对象有强引用，因此它的保留计数将变为 0，并且一旦从该函数返回控制权(紧随其后)，它就会变为 nil print(secondViewController.transitioningDelegate)，因为 UIView.animate(...) 是异步的)。

在整个 View Controller 的呈现过程中，您需要一些东西来保持对 presentationController 的强引用。由于某些东西强烈地保留在引用中，它的保留计数不会低于 1，除非您将该引用专门设置为 nil。一种解决方案是将其保留为当前类的属性或 secondViewController 的属性。