如何为 UIPageViewControllerDataSource 提供默认实现？

Question

我认为这个问题的答案将解决 Objective-C 协议的一般问题，但这是我遇到的第一个此类问题。

我希望在实现 UIPageViewControllerDataSourceWithConnections 时使用这些方法。

import UIKit

protocol UIPageViewControllerDataSourceWithConnections: UIPageViewControllerDataSource {
    var connectedViewControllers: [UIViewController] {get}
}

extension UIPageViewControllerDataSourceWithConnections {
    func pageViewController(pageViewController: UIPageViewController,
        viewControllerBeforeViewController viewController: UIViewController
    ) -> UIViewController? {return connectedViewController(
        current: viewController,
        adjustIndex: -
    )}

    func pageViewController(pageViewController: UIPageViewController,
        viewControllerAfterViewController viewController: UIViewController
    ) -> UIViewController? {return connectedViewController(
        current: viewController,
        adjustIndex: +
    )}

    private func connectedViewController(
        current viewController: UIViewController,
        adjustIndex: (Int, Int) -> Int
    ) -> UIViewController? {
        let requestedIndex = adjustIndex(connectedViewControllers.indexOf(viewController)!, 1)
        return connectedViewControllers.indices.contains(requestedIndex) ?
            connectedViewControllers[requestedIndex] : nil
    }

    func presentationCountForPageViewController(pageViewController: UIPageViewController)
    -> Int {return connectedViewControllers.count}

    func presentationIndexForPageViewController(pageViewController: UIPageViewController)
    -> Int {
        return connectedViewControllers.indexOf(pageViewController.viewControllers!.first!)!
    }
}

但是，这不会编译。我必须实施这种废话才能使事情正常进行。你能告诉我为什么吗？是否有代码更轻的解决方案可用？

// connectedViewControllers is defined elsewhere in InstructionsPageViewController.
extension InstructionsPageViewController: UIPageViewControllerDataSourceWithConnections {

    // (self as UIPageViewControllerDataSourceWithConnections) doesn't work.
    // Workaround: use a different method name in the protocol

    func pageViewController(pageViewController: UIPageViewController,
        viewControllerBeforeViewController viewController: UIViewController
    ) -> UIViewController? {
        return ????pageViewController(pageViewController,
            viewControllerBeforeViewController: viewController
        )
    }

    func pageViewController(pageViewController: UIPageViewController,
        viewControllerAfterViewController viewController: UIViewController
    ) -> UIViewController? {
        return ????pageViewController(pageViewController,
            viewControllerAfterViewController: viewController
        )
    }


    // (self as UIPageViewControllerDataSourceWithConnections)
    // works for the optional methods.

    func presentationCountForPageViewController(pageViewController: UIPageViewController)
    -> Int {
        return (self as UIPageViewControllerDataSourceWithConnections)
            .presentationCountForPageViewController(pageViewController)
    }

    func presentationIndexForPageViewController(pageViewController: UIPageViewController)
    -> Int {
        return (self as UIPageViewControllerDataSourceWithConnections)
            .presentationIndexForPageViewController(pageViewController)
    }
}

Answer 1

当您遇到此类问题时，您想知道 Swift 语言本身的局限性，这有助于将其简化为问题的更简单版本。

首先，让我们问一下：是否可以扩展协议采用协议，作为将该协议要求的默认实现注入最终采用类的一种方式？是的;此代码是合法的：

protocol Speaker {
    func speak()
}
protocol DefaultSpeaker : Speaker {
}
extension DefaultSpeaker {
    func speak() {
        print("howdy")
    }
}
class Adopter : DefaultSpeaker {

}

好的，那么您的代码还能做什么？好吧，它还注入了一个额外的需求（实例变量）。那合法吗？是的。这段代码也是合法的：

protocol Speaker {
    func speak()
}
protocol DefaultSpeaker : Speaker {
    var whatToSay : String {get}
}
extension DefaultSpeaker {
    func speak() {
        print(self.whatToSay)
    }
}
class Adopter : DefaultSpeaker {
    var whatToSay = "howdy"
}

那么 Swift 不喜欢什么？我们在这里没有做什么，你的代码做了什么？事实上原始协议是@objc。如果我们将protocol Speaker 更改为@objc protocol Speaker（并进行所有其他必要的更改），代码将停止编译：

@objc protocol Speaker {
    func speak()
}
@objc protocol DefaultSpeaker : Speaker {
    var whatToSay : String {get}
}
extension DefaultSpeaker {
    func speak() {
        print(self.whatToSay)
    }
}
class Adopter : NSObject, DefaultSpeaker { // ERROR
    var whatToSay = "howdy"
}

我猜这是因为 Objective-C 对协议扩展一无所知。由于我们对所需协议方法的实现依赖于协议扩展，因此我们不能以满足编译器的方式采用协议，即从 Objective-C 的角度来看，该要求已得到满足。我们必须在课堂上实现需求，Objective-C 可以在其中看到我们的实现（这正是您的解决方案所做的）：

@objc protocol Speaker {
    func speak()
}
@objc protocol DefaultSpeaker : Speaker {
    var whatToSay : String {get}
}
extension DefaultSpeaker {
    func speak2() {
        print(self.whatToSay)
    }
}
class Adopter : NSObject, DefaultSpeaker {
    var whatToSay = "howdy"
    func speak() {
        self.speak2()
    }
}

因此，我得出的结论是，您的解决方案已经足够好了。

您所做的实际上更像是这样，我们在采用者类上使用扩展来注入“钩子”方法：

@objc protocol Speaker {
    func speak()
}
@objc protocol DefaultSpeaker : Speaker {
    var whatToSay : String {get}
}
extension DefaultSpeaker {
    func speak2() {
        print(self.whatToSay)
    }
}
class Adopter : NSObject {
}
extension Adopter : DefaultSpeaker {
    var whatToSay : String { return "howdy" }
    func speak() {
        self.speak2()
    }
}

之所以有效，是因为最后一个 extension 是 Objective-C可以看到的：Objective-C 类的扩展实际上是一个类别，Objective-C 可以理解。