Swift3 条件绑定的初始值设定项必须具有可选类型，而不是；UIView“；

我最近下载了这个项目，并将其转换为最新的swift语法。我不明白为什么我继续得到错误“条件绑定的初始值设定项必须具有可选类型，而不是“UIView”

第行发生错误：

let containerView = transitionContext.containerView

以下是完整的代码：

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
    guard
        let fromVC = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.from),
        let toVC = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to),
        let containerView = transitionContext.containerView
        else {
            return

    }

    containerView.insertSubview(toVC.view, belowSubview: fromVC.view)

    let screenBounds = UIScreen.main.bounds
    let bottomLeftCorner = CGPoint(x: 0, y: screenBounds.height)
    let finalFrame = CGRect(origin: bottomLeftCorner, size: screenBounds.size)

    UIView.animate(
        withDuration: transitionDuration(using: transitionContext),
        animations: {
            fromVC.view.frame = finalFrame
        },
        completion: { _ in
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }
    )
}

---

这是因为

containerView

属性不是可选类型。正如您在这里看到的（取自

UIViewControllerContextTransitioning

上的文档（command+LMB））：

---

您可以将错误行放在

guard

语句之后。

这是因为

containerView

属性不是可选类型。如您所见（取自

UIViewControllerContextTransitioning

上的文档（command+LMB））：

---

您可以将错误的行放在

guard

语句之后。

只需将

让containerView=transitionContext.containerView

移出guard语句，并将其放在

containerView.insertSubview（toVC.view，belowSubview:fromVC.view）上方即可

只需将containerView=transitionContext.containerView从guard语句中移出，并将其放在

containerView.insertSubview（toVC.view，belowSubview:fromVC.view）上方即可

...
public protocol UIViewControllerContextTransitioning : NSObjectProtocol {


    // The view in which the animated transition should take place.

    @available(iOS 2.0, *)
    public var containerView: UIView { get }
...