使超类的初始化程序返回特定的子类？

bogen 发表于 Dev

这本书

我正在Swift中制作一个单元框架，该框架具有针对不同单位（例如质量和体积）的度量超类和子类。一个功能是允许框架正确猜测它是用哪个单元创建的，并返回正确的类。示例代码：

class Measurement {
   var unitString : String
   init(unknownUnit: String) {
       // checks if the unit is either Volume or Mass, and returns an instance of that class
   }
}

class Volume : Measurement {
    init(unitString: String) {

    }
}

class Mass : Measurement {
    init(unitString: String) {

    }
}

let mass = Mass("kg")                   // class: Mass
let volume = Volume("ml")               // class: Volume
let shouldBeVolume = Measurement("ml")  // class: Volume
let shouldBeMass = Measurement("kg")    // class: Mass

继承的类在初始化时是否可以创建特定子类的对象？

库名为Indus Valley，并在GitHub上开源

格里姆森

它通过父类了解其子类（非常差的反模式！）来进行继承的快速和松散的操作，但这可以工作...

class Measurement {
    var unitString : String

    class func factory(unknownUnit: String) -> Measurement {
        if unknownUnit == "kg" {
            return Mass(myUnit: unknownUnit)
        } else { // Random default, or make func return Measurement? to trap
            return Volume(myUnit: unknownUnit)
        }
    }

    init(myUnit: String) {
        // checks if the unit is either Volume or Mass, and returns an instance of that class
        self.unitString = myUnit
    }
}

class Volume : Measurement {
}

class Mass : Measurement {
}

let mass = Mass(myUnit: "kg")                   // class: Mass
let volume = Volume(myUnit: "ml")               // class: Volume
let shouldntBeVolume = Measurement(myUnit: "ml")  // class: Measurement
let shouldntBeMass = Measurement(myUnit: "kg")    // class: Measurement
let isVolume = Measurement.factory("ml")  // class: Volume
let shouldBeMass = Measurement.factory("kg")    // class: Mass