Custom class clusters in Swift

Question

This is a relatively common design pattern:

https://stackoverflow.com/a/17015041/743957

It allows you to return a subclass from your init calls.

Answer 1

We can take advantage of a compiler quirk - self is allowed to be assigned in protocol extensions - https://forums.swift.org/t/assigning-to-self-in-protocol-extensions/4942.

Thus, we can have in place something like this:

/// The sole purpose of this protocol is to allow reassigning `self`
fileprivate protocol ClusterClassProtocol { }

extension ClusterClassProtocol {
    init(reassigningSelfTo other: Self) {
        self = other
    }
}

/// This is the base class, the one that gets circulated in the public space
class ClusterClass: ClusterClassProtocol {
    
    convenience init(_ intVal: Int) {
        self.init(reassigningSelfTo: IntChild(intVal))
    }
    
    convenience init(_ stringVal: String) {
        self.init(reassigningSelfTo: StringChild(stringVal))
    }
}

/// Some private subclass part of the same cluster
fileprivate class IntChild: ClusterClass {
    init(_ intVal: Int) { }
}

/// Another private subclass, part of the same cluster
fileprivate class StringChild: ClusterClass {
    init(_ stringVal: String) { }
}

Now, let's give this a try:

print(ClusterClass(10))    // IntChild
print(ClusterClass("abc")) // StringChild

This works the same as in Objective-C, where some classes (e.g. NSString, NSArray, NSDictionary) return different subclasses based on the values given at initialization time.