Cannot assign to property: <Enum:case> is not settable - swift

Question

for example we have simple enum
public enum CXActionSheetToolBarButtonItem {
case cancel
case done
case now
private static var titles: [CXActionSheetToolBarButtonItem: String] = [
.cancel: "Cancel",
.done: "Done",
.now: "Now",
]
public var title: String {
get { return CXActionSheetToolBarButtonItem.titles[self] ?? String(describing: self) }
// what am I want to do
set(value) { CXActionSheetToolBarButtonItem.titles[self] = value }
}
// what am I forced to do
public static func setTitle(_ title: String, for item: CXActionSheetToolBarButtonItem) {
CXActionSheetToolBarButtonItem.titles[item] = title
}
}
why I don't can set new title like this
CXActionSheetToolBarButtonItem.cancel.title = "asd"
compiler responded error - Cannot assign to property: 'cancel' is not settable, but I can set title with function
CXActionSheetToolBarButtonItem.setTitle("asd", for: .cancel)
what should I change for worked my var as settable? .cancel.title = "asd"

Answer 1

Using an enum for this seems inappropriate, but I'll address the porblem at face value. You need to mark your setter as nonmutating, so that it can be called on non-var instances of your enum:
public enum CXActionSheetToolBarButtonItem {
// ...
public var title: String {
get { return CXActionSheetToolBarButtonItem.titles[self] ?? String(describing: self) }
nonmutating set(value) { CXActionSheetToolBarButtonItem.titles[self] = value }
}
}
CXActionSheetToolBarButtonItem.cancel.title = "foo" // Works... but why would you want this?!

Answer 2

I think the main reason why you can't do that is because the Swift compiler sees enum cases as immutable by default (similar to an immutable struct declared with let), and here you are trying to mutate it.
A good way to see this is if you try to add a mutating function to this enum
mutating public func setTitle2(_ newValue: String) {
CXActionSheetToolBarButtonItem.titles[self] = newValue
}
You will then receive the error message
error: cannot use mutating member on immutable value: 'cancel' returns immutable value
How to work around this
One way to have a similar behavior is by changing this enum into a set of static variables (which is more consistent with what you are trying to achieve).
public struct CXActionSheetToolBarButtonItem {
var title: String
static var cancel = CXActionSheetToolBarButtonItem(title: "Cancel")
static var done = CXActionSheetToolBarButtonItem(title: "Done")
static var now = CXActionSheetToolBarButtonItem(title: "Now")
}
Now you can use the following
CXActionSheetToolBarButtonItem.cancel.title = "asd"
Also, you still have the ability to use the dot-syntax
let buttonItem: CXActionSheetToolBarButtonItem = .cancel // it works
Hope that helps!
TD;DR: Because enum is an immutable object.

Answer 3

First of all: Whatever you're trying to do here – it's certainly not a good approach and pretty dangerous. As Craig pointed out in his comment, you're messing around with instance properties and static properties. You can have multiple instances of an enum – but when you want to change the title of a particular instance, you also change the title for all other instances. That's unexpected behavior and you should really think of another solution.
That being said, your code actually does work – with a little modification: Instead of
CXActionSheetToolBarButtonItem.cancel.title = "asd"
you can write
var item = CXActionSheetToolBarButtonItem.cancel
item.title = "asd"
This will compile.
The reason behind it is that an enum is a value type. Everytime you create an instance of an enum type, e.g. var item = CXActionSheetToolBarButtonItem.cancel, the enum's value is copied into the new variable item. You can choose if you want that value to be mutable or not by using either var or let. That's how Swift enums are intended to be used.
A single enum case like CXActionSheetToolBarButtonItem.cancel is immutable by definition.
CXActionSheetToolBarButtonItem.cancel.title = "asd"
wouldn't have any meaning because there is no instance to which the title can be assigned. The enum case .cancel is not bound to a variable.