如何提供UIPageViewControllerDataSource的默认实现？

问题描述

我假设此问题的答案通常会解决Objective-C协议的问题，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。我必须实施这些胡言乱语才能让事情运转起来。你能告诉我为什么吗？是否有代码较少的解决方案可用？

// 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)
    }
}

推荐答案

当您遇到这样的问题时，如果您想知道SWIFT语言本身的限制，将其简化为问题的更简单版本会有所帮助。

首先，让我们问一下：是否可以扩展协议采用协议，将该协议要求的默认实现注入到最终采用类中？是的，它是合法的；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呢？它还注入了一个额外的需求(实例变量)。这合法吗？是的，是这样的。此代码也是合法的：

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

那么斯威夫特不喜欢的是什么呢？我们在这里没有做什么，你的代码做了什么？事实是，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。因为我们所需的协议方法的实现依赖于协议扩展，所以从Objective-C的观点来看，我们不能以满足编译器的要求的方式采用该协议。我们必须在类中实现需求，Objective-C可以看到我们的实现(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，我得出结论，您的解决方案是最好的。

您所做的实际上更像这样，我们在Adadter类上使用一个扩展来注入"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()
    }
}

这是可行，因为最后extension是客观类可以看到的东西：对象-C类的扩展实际上是一个范畴，目标-C可以理解。

这篇关于如何提供UIPageViewControllerDataSource的默认实现？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！