Swift computed property to return copy of underlying array

Question

I have a model class written in Objective-C that I'm converting to Swift. It contains an NSMutableArray internally, but the method signature for the getter, as well as the actual return value, are NSArray. When called, it creates an immutable copy to return.

Essentially, I want callers to be able to iterate/inspect the container, but not modify it. I have this test snippet:

class Container {
    internal var myItems = [String]()

    func sayHello() {
        "I have: \(myItems)"
    }
}

let cont = Container()

cont.myItems.append("Neat") // ["Neat"]

cont.sayHello() // This causes sayHello() to print: "I have: [Neat]"

var isThisACopy = cont.myItems

isThisACopy.append("Huh") // ["Neat", "Huh"]

cont.sayHello() // This ALSO causes sayHello() to print: "I have: [Neat]"

I've been trying to find a way to override the getter for myItems so that it returns an immutable copy, but can't seem to determine how.

Attempt #1

This produces a compiler error: Function produces expected type '_ArrayBuffer<(String)>'; did you mean to call it with '()'?

internal var myItems = [String]() {
    var copy = [String]()
    for item in ... { // What to use in the ...?
        copy.append(item)
    }
    return copy
}

Attempt #2

This also produces a compiler error, because I'm (understandably) redefining the generated getter Invalid redeclaration of 'myItems()':

internal func myItems() -> [String] {
    var copy = [String]()
    for item in myItems {
        copy.append(item)
    }
    return copy
}

Answer 1

Try this:

class Container {
    private var _myItems: [String] = ["hello"]
    internal var myItems: [String] {
        return _myItems
    }
}

let cont = Container()
cont.myItems.append("Neat") //not allowed

It uses a private stored property and a computed property that returns an immutable copy. It's not possible for a stored property to use custom getters.

Answer 2

A better way to expose mutable properties as immutable:

class Container {
    private (set) internal var myItems: [String]
}