swift2 中协议扩展和默认值的限制 [英] Limits of Protocol Extensions and defaults in swift2

问题描述

所以我在玩协议扩展时遇到了一个有趣"的问题.

So I was playing around with protocol extensions and i ran into an "interesting" problem.

我想写一个 Meters 和 Kilometers 单位类型来测试一些东西.作为一个有基类并且两个子类都覆盖基类的类很容易做到这一点，而只是覆盖一个简单的值

I wanted to write a Meters and Kilometers units type for testing some things out. Its VERY easy to do this as a class where there is a base class and both sub classes override the base, while just overriding a simple value

//Conversion factor between types
enum DISTANCE_UNIT_TYPE : Double {

    case METER = 1.0;
    case KILOMETER = 0.001;
}

protocol DistanceUnit {

    var unitType : DISTANCE_UNIT_TYPE {get}
    var value : Double { get set }
    var baseValue : Double { get set }
}


struct Kilometers : DistanceUnit {

    var unitType = DISTANCE_UNIT_TYPE.KILOMETER
    var value : Double
    var baseValue : Double

    init(_ v : Double) {
        value = v
        baseValue = v * unitType.rawValue
    }
}


struct Meters : DistanceUnit {

    var unitType = DISTANCE_UNIT_TYPE.METER
    var value : Double
    var baseValue : Double

    init(_ v : Double) {
        value = v
        baseValue = v * unitType.rawValue
    }
}

如你所见，我有很多重复的代码(特别是初始化程序)

So as you can see i have lots of duplicate code (specifically the initializers)

我试图做一个协议扩展来设置默认初始化器

I tried to make a protocol extension to set the default initializer

extension DistanceUnit {

    init(_ v : Double) {
        value = v
        baseValue = v * unitType.rawValue
    }
}

但我收到一个错误，变量'self'在初始化之前通过引用传递

有什么办法可以让它工作，还是我只需要输入大量重复的代码?也许使用 unsafe 之类的?

Is there any way to get this to work or do i just have to type lots of duplicate code? Maybe using unsafe or something?

推荐答案

我认为 fqdn 是正确的，我们将无法像我们希望的那样在协议扩展中使用自定义初始化，但只有时间会证明一切.

I assume that fqdn is right and we won't be able to use custom inits inside protocols extension as we would like to, but only time will tell.

但仍有一些解决方法:

struct Meters : DistanceUnit {

    var unitType = DISTANCE_UNIT_TYPE.METER
    var value : Double
    var baseValue : Double

    init() { // this one is needed as designated initializer for your protocol extension
        value = 0
        baseValue = 0
    }
}

protocol DistanceUnit {

    var unitType : DISTANCE_UNIT_TYPE {get}
    var value : Double { get set }
    var baseValue : Double { get set }
    init() // this is new and you will NEED to implement this in your structure or class
}

extension DistanceUnit {

    init(_ v : Double) {
       self.init()
       value = v
       baseValue = v * unitType.rawValue
    }

    // you can now implement a lot more different default inits and they should work fine here :)
    // here is a quick example

    init(_ s : String) {
       self.init(Double(s.characters.count))
    }
}

希望能帮到你.几天前，我在构建带有协议扩展的自定义通用单例生成器时学到了这个技巧(参见此处).

Hope that will help you. I learned this trick a few days ago when i was building a custom generic singleton generator with protocol extensions (see here).

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