On Sep 4, 2016, at 22:50 , Gerriet M. Denkmann <[email protected]> wrote:
>
> I really want “onlyKnownBySubclass” to be a constant (i.e. let instead of var)
There’s an easy way if you can declare both classes in the same file. Then, you
can just declare the instance variable like this:
> private(set) var onlyKnownBySubclass: Int
(That’s Swift 2. For Swift 3, use “fileprivate” instead of “private”.) In this
case, “onlyKnownBySubclass” looks like a “let” property outside that one source
file.
If you can’t do that, you can do it with a closure, assuming the places of
definition aren’t more complicated than in your code. Something like this:
> class SuperClass
> {
> let knownBySuperclass: Int
> let onlyKnownBySubclass: Int
>
> init(some: Int, calc: (known: Int) -> Int)
> {
> knownBySuperclass = some * 2
> onlyKnownBySubclass = calc (known: knownBySuperclass)
> }
>
> final func someFunction() -> Void
> {
> print("Should never be zero: \(onlyKnownBySubclass)")
> }
> }
>
> final class SubClass: SuperClass
> {
> init(some: Int)
> {
> super.init(some: some) { $0 + 5 }
> }
> }
>
> let a = SubClass(some:11)
> a.someFunction() // prints: “Should never be zero: 27”
This closure “{ $0 + 5 }” is shorthand for the longer form:
{
valuePassed in
return valuePassed + 5
}
The idea is that the superclass contributes values by parameters to the closure
(just one, knownBySuperclass, in this case), and the subclass contributes the
rest of the expression (which could make use of other subclass properties,
since this closure occurs after the property initialization phase of the
subclass init.
More globally, this sort of thing is not terribly idiomatic for Swift, because
you’re trying to hide things that could get exposed other ways, for example, by
“hostile” subclassing. The Swift-ier way would be to use a protocol instead of
(or in addition to, but preferably instead of) the superclass. The protocol
would “force” the subclass to define its own “onlyKnownBySubclass” locally.
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