在Swift中将一个默认属性值传递给另一个

Question

Imagine I want to do something like the following. I have a custom subclass of UIView, with several properties. One of those properties I want to initialise from another prior property.

class MyView :UIView {

    var driver = HumanDriver()

    var car = Car(driver: driver)
    var plane = Plane(driver: driver)
    var train = Train(driver: driver)
    var spaceship = Spaceship(driver: driver)

    override init() {
        super.init()
    }
    required init(coder aDecoder: NSCoder) {
        super.init(coder:aDecoder)
    }
    override init(frame: CGRect) {
        super.init(frame: frame)
    }
}

This code will present an error: 'MyView.Type' does not have a member named 'driver' . So one obvious attempt at fixing this would be to define the defaults in the initialiser. This is fine if there is only ONE initialiser. But what if we have several? Well I can try to collect the common code in a single setup function, as below:

class MyView :UIView {

    var driver = HumanDriver()

    var car
    var plane
    var train
    var spaceship

    override init() {
        setupMyView()
        super.init()
    }
    required init(coder aDecoder: NSCoder) {
        setupMyView()
        super.init(coder:aDecoder)
    }
    override init(frame: CGRect) {
        setupMyView()
        super.init(frame: frame)
    }

    func setupMyView() {
        car = Car(driver: driver)
        plane = Plane(driver: driver)
        train = Train(driver: driver)
        spaceship = Spaceship(driver: driver)
    }

}

But this also gives rise to an error, this time: 'self' used before super.init call . This is understandable since we are using self before init has been called.

Is there any nice solution to this? Obviously I could just used implicitly unwrapped optionals, but it seems like there should be a better way!

Answer 1

How about overriding the driver setter and set the properties in there? Something like this:

class MyView :UIView {

    var driver:HumanDriver = HumanDriver() {
        didSet{
            car = Car(driver: driver)
            plane = Plane(driver: driver)
            train = Train(driver: driver)
            spaceship = Spaceship(driver: driver)
        }
    }

    var car: Car?
    var plane: Plane?
    var train: Train?
    var spaceship: Spaceship?
}

In this way you'll keep tidy your initializers. Btw you'll have probably to add at least the required initializer but that's something else. I think this is one of the cleanest solution you might want to end up with.

Hope this helps.

Answer 2

The answer here: https://stackoverflow.com/a/26772103/1025041 was really what I was looking for, although it really is a lot of boilerplate for what is trying to be achieved! Lets hope Apple make this easier in future.

For reference, this is how the method would apply to my example:

extension UIView {
    enum InitMethod {
        case Default
        case Coder(NSCoder)
        case Frame(CGRect)
    }
}

class MyView: UIView {

    var driver = HumanDriver()

    var car
    var plane
    var train
    var spaceship

    override convenience init() {
        self.init(.Default)
    }

    override convenience init(frame: CGRect) {
        self.init(.Frame(frame))
    }

    required convenience init(coder aDecoder: NSCoder) {
        self.init(.Coder(aDecoder))
    }

    private init(_ initMethod: InitMethod) {

        car = Car(driver: driver)
        plane = Plane(driver: driver)
        train = Train(driver: driver)
        spaceship = Spaceship(driver: driver)

        switch initMethod {
            case .Default: super.init()
            case let .Coder(coder): super.init(coder: coder)
            case let .Frame(frame): super.init(frame: frame)
        }
    }
}