条件绑定的初始化程序必须具有可选类型，而不是“UIView”

Question

我最近下载了该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“初始值设定条件结合必须有可选的类型，而不是‘UIView的’

误差线存在的：

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) 
     } 
    ) 
} 

Answer 1

这是因为containerView属性不是一个可选类型，你可以在这里看到（这是从文档（命令+ LMB）采取UIViewControllerContextTransitioing）：

... 
public protocol UIViewControllerContextTransitioning : NSObjectProtocol { 


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

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

您可以在guard声明之后放置错误的行。