How to copy a struct and modify one of its properties at the same time?

Question

If I want to represent my view controller\'s state as a single struct and then implement an undo mechanism, how would I change, say, one property on the struct and, at the s

Answer 1

I really like @Shadow answer, but I had a hard time adapting it to a scenario were the fields of the struct could be nullable, so I decided to use the builder pattern instead. My code looks something like this:

struct A {
    let a: Int?
    let b: Int?

    class Builder {
        private var a: Int?
        private var b: Int?

        init(struct: A) {
            self.a = struct.a
            self.b = struct.b
        }

        func build() -> A {
            return A(a: self.a, b: self.b)
        }

        func withA(_ a: Int?) -> Builder {
            self.a = a
            return self
        }

        func withB(_ b: Int?) -> Builder {
            self.b = b
            return self
        }
    }
}

And then you can use it like:

A.Builder(struct: myA).withA(a).withB(nil).build()

With this my structs are really immutable, and I can quickly create a copy and change one or many of its field to be either nil or another Int