如何在指定的初始化程序中调用协议扩展初始化程序？答案

【问题标题】：How to call protocol extensions initializers in designated initializers?如何在指定的初始化程序中调用协议扩展初始化程序？
【发布时间】：2016-02-21 19:36:41
【问题描述】：

我正在尝试将协议扩展初始化程序注入现有类的指定初始化程序。如果不覆盖类中指定的初始化程序，然后在其中调用协议扩展初始化程序，我认为没有办法解决它。

以下是我正在尝试的，特别是 UIViewController 类：

class FirstViewController: UIViewController, MyProtocol {

    var locationManager: CLLocationManager?
    var lastRendered: NSDate?

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        // TODO: How to call MyProtocol initializer?
        // (self as MyProtocol).init(aDecoder) // Didn't compile
    }

}

protocol MyProtocol: CLLocationManagerDelegate {

    var locationManager: CLLocationManager? { get set }
    var lastRendered: NSDate? { get set }

    init?(coder aDecoder: NSCoder)
}

extension MyProtocol where Self: UIViewController {

    // Possible to inject this into initialization process?
    init?(coder aDecoder: NSCoder) {
        self.init(coder: aDecoder)
        setupLocationManager()
    }

    func setupLocationManager() {
        locationManager = CLLocationManager()
        locationManager?.delegate = self
        locationManager?.desiredAccuracy = kCLLocationAccuracyThreeKilometers
        locationManager?.distanceFilter = 1000.0
        locationManager?.startUpdatingLocation()
    }

    func locationManager(manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        // TODO
    }

    func locationManager(manager: CLLocationManager, didChangeAuthorizationStatus status: CLAuthorizationStatus) {
        // TODO
    }

    func locationManager(manager: CLLocationManager, didFailWithError error: NSError) {
        // TODO
    }
}

有没有办法利用协议扩展初始化器，以便在框架现有的初始化过程中自动调用它？

【问题讨论】：

标签： swift class swift2 designated-initializer protocol-extension

【解决方案1】：

您不需要调用不同的初始化程序；你已经在初始化了。此外，您不需要将self 转换为 MyProtocol；您已经声明它采用 MyProtocol。另外，您已经将 MyProtocol 的setupLocationManager 注入 FirstViewController，因为您的 FirstViewController 已经采用了 MyProtocol，并且 MyProtocol 上的扩展针对的是 UIViewController，它是 FirstViewController 的超类。

所以，方法已经被注入了；现在直接在你已经运行的初始化程序中调用注入的方法。您的代码的以下精简版本编译得非常好：

class FirstViewController: UIViewController, MyProtocol {
    var locationManager: CLLocationManager?

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.setupLocationManager() // no problem!
    }
}

protocol MyProtocol: CLLocationManagerDelegate {
    // this next line is necessary so that
    // ...setupLocationManager can refer to `self.locationManager`
    var locationManager: CLLocationManager? { get set }
}

extension MyProtocol where Self: UIViewController {
    func setupLocationManager() {
        locationManager = CLLocationManager()
        // ... etc.
    }
    // ... etc.
}

【讨论】：