检查对象是否是特定类并符合协议

Question

不确定这是否是一种奇怪的方法，但在 Swift 3 中，我想检查一个对象是否为 UIViewController 并符合我的协议 Transitionable。我有：

guard let toViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to),
    let fromViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.from),
    let toTransitionable = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to) as? Transitionable,
    let fromTransitionable = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.from) as? Transitionable
else {
    transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
    return
}

但我希望我能得到和反对既是UIViewController 又是Transitionable 的往返。 我试过let toViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to) as? Transitionable as? UIViewController，但这只是一个UIViewController

我也试过了：

extension Transitionable where Self: UIViewController {
    var viewController: UIViewController { return self }
}

但我得到错误：'Transitionable' is not a subtype of 'UIViewController' 当我打电话给toTransitionable.viewController 例如

transitionContext.containerView.addSubview(toTransitionable.viewController.view)

我知道我们可以在 Swift 4 中使用 as? (UIViewController & Transitionable)，但这个项目将在 Swift 3 中进行

Answer 1

我不确定这是否有帮助，但请尝试：

if let toViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to) as? UIViewController{
    if let _ = toViewController as? Transitionable{
       print("conforms to protocol")
    }
}

if let fromViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.from) as? UIViewController{
    if let _ = fromViewController as? Transitionable{
       print("conforms to protocol")
    }
}

或者，如果您只希望在一个变量中同时使用两者

您可以创建一个扩展 UIViewController 的新类并实现协议 Transitionable。

类似：

class YourNewViewControllerClass : UIViewController,Transitionable{
    //implements your  Transitionable methods
} 

之后：

if let viewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.from) as? YourNewViewControllerClass{
       //success
}