Swift mutable dictionary being treated as immutable

Question

I am trying to implement In App Purchases, and I am tracking which purchases a user has purchased via NSUserDefaults. I have a function that sets the values of each purchase, but when it runs, I get an error saying that I am mutating the dictionary of purchase values even though the dictionary is declared with a var instead of a let and is an NSMutableDictionary. Sometimes it does work, but most of the time it doesn't. I get a few warnings about declaring my variables with let instead of var, but I ignore them to give my variables maximum mutability. Why does this not work?

The error I get is:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object'

Code:

static func setPurchased(purchase:PurchaseID, value:Bool) { let defaults = NSUserDefaults.standardUserDefaults() if (defaults.objectForKey(PURCHASES_KEY) == nil) { initializePurchases() // go set some initial values } if var purchases = (defaults.objectForKey(PURCHASES_KEY) as? NSMutableDictionary) // WARNING: should be declared with let because not mutated { print("setting purchase \(purchase.id()) to \(value)") var key = purchase.id() // WARNING: should be declared with let because not mutated purchases[key] = value // CRASH HERE defaults.setObject(purchases, forKey:PURCHASES_KEY) defaults.synchronize() } }

@ozgur Since I am setting a key value pair, it does have to be of NSMutableDictionary type. Regular old AnyObject type won't let me do that as far as I know. — Adam Evans
– Adam Evans, Commented May 13, 2016 at 23:22
@ozgur Make your comment an answer. It seems to have worked, but I have no idea why. — Adam Evans
– Adam Evans, Commented May 13, 2016 at 23:40

Community · Accepted Answer · 2017-05-23 11:59:50Z

This is not the right way of converting an immutable dictionary into its mutable counterpart.

var already ensures that whatever is returned from defaults.objectForKey(PURCHASES_KEY) will be copied as a mutable type so all you need is specify the type of the mutable object which in our case can safely be Dictionary<String: AnyObject> if you are sure all keys are String type:

if var purchases = defaults.objectForKey(PURCHASES_KEY) as? Dictionary<String: AnyObject> { ... purchases[key] = value }

Please see this SO question for more information about mutability/immutability in collection types.

Collectives™ on Stack Overflow

Swift mutable dictionary being treated as immutable

1 Answer 1

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Linked

Related