How to init my subclass with an instance of its superclass?

Question

In my app I read calendar events of type EKEvent, and I've made an extension with a lot of computed vars so I can easily get the duration, number of man-hours etc. for each event in the calendar. But in large scale, the performance is bad - so I want to use lazy vars instead, to cache all my extra data.

Therefore, I want to make a subclass of EKEvent - called CustomEvent, which adds the lazy vars, but my problem is that the EKEventStore always returns EKEvents, and I need to convert that to instances of my CustomEvent subclass, in order to be able to access the lazy vars etc.

A simple typecast is not enough, and I've tried in a playground, to see what could work, but got nothing useful. I need a special constructor for CustomRectangle, which can initialize a CustomRectangle from a NativeRectangle. An alternative solution is to make a wrapper class that holds the original object as a property, but that wouldn't be my favorite solution, since I'd then have to map all methods and properties

class NativeRectangle: NSObject {
    var width: Int
    var height: Int

    init(width: Int, height: Int) {
        self.width = width
        self.height = height
        super.init()
    }
}

class CustomRectangle: NativeRectangle {
    var area: Int { return width * height}
}

let rect = NativeRectangle(width: 100, height: 20)

let customRect = CustomRectangle(rect) // This fails, i need a constructor

print(customRect.area)

Answer 1

There is no way in Swift (and in general in most Object Oriented languages) to use an existing instance of a base class object when creating a child class instance.

From a general programming stand-point you have the two options in this situation:

1. Use composition: Make the CustomRectangle contain a NativeRectangle and forward all methods to it that you need.

2. Use a map to link NativeRectangles to additional information. In Objective C and Swift you can you objc_AssociationPolicy to have such an internal map most easily. See https://stackoverflow.com/a/43056053/278842

Btw. There is no way that you will see any speed-up from "caching" a simple computation as width * height.