NSUserDefaults returns swift array [String] instead of Set<String>

Question

I'm storing a set of strings (Set<String>) into a NSUserDefaults.standardUserDefaults() and when retrieving back the object, it comes back as an array of strings instead of a set.

This block works, as I'm recreating the array as a NSSet

if let products = NSUserDefaults.standardUserDefaults().objectForKey("products") as? [String] {

   // recreate the array into a NSSet / Set
   if let productSet = NSSet(array: products) as? Set<NSObject> {
     // now it's a set 
   }

}

However, it's not possible to get the object directly as a swift Set:

if let products = NSUserDefaults.standardUserDefaults().objectForKey("products") as? Set<String> {
   // will not cast into Set<String>
}

I assume NSUserDefaults coverts set into an array internally? Is there a way to receive the items as a set and not an array?

According to the documentation, a user defaults object must be an instance or a combination of NSData, NSString, NSNumber, NSDate, NSArray, or NSDictionary. How did you store a Set? — Martin R
– Martin R, Commented Oct 27, 2015 at 18:56
Convert anything in NSData and on retrieval type cast it to whatever you want when originally converted — Rajan Maheshwari
– Rajan Maheshwari, Commented Oct 27, 2015 at 18:58
NSUserDefaults.standardUserDefaults().setObject(Set(["a", "b"]), forKey: "products") throws a runtime exception. Without seeing how you set the defaults it is impossible to give a (helpful) answer. — Martin R
– Martin R, Commented Oct 27, 2015 at 18:59
I'm getting the data as a JSON object and store it with NSUserDefaults.standardUserDefaults().setValuesForKeysWithDictionary(response) — aporat
– aporat, Commented Oct 27, 2015 at 19:01
JSON objects have arrays, dictionaries, strings, numbers, but no sets. — Martin R
– Martin R, Commented Oct 27, 2015 at 19:02

deadbeef · Accepted Answer · 2015-10-27 19:33:57Z

9

Short answer : No

NSUserDefaults cannot store sets. It's documented that way and it's because of the limitations of the format used to store the data on disk.

~~If saving a set works for you and automatically converts it to an array, you're actually lucky as I don't think this is a documented behavior and it should just throw an error.~~
EDIT : It doesn't work and you should not try it.

The best practice is to convert it to an array before saving and convert it back to a set after retrieving. You could also write a category on NSUserDefaults that does that automatically. Here is an example with objective-C :

//
//  NSUserDefaults+SetAdditions.h
//

#import <Foundation/Foundation.h>

@interface NSUserDefaults (SetAdditions)

- (NSSet *)setForKey:(NSString *)defaultName;
- (void)setSet:(NSSet *)set forKey:(NSString *)defaultName;

@end


//
//  NSUserDefaults+SetAdditions.m
//

#import "NSUserDefaults+SetAdditions.h"

@implementation NSUserDefaults (SetAdditions)

- (NSSet *)setForKey:(NSString *)defaultName
{
    NSArray *array = [self arrayForKey:defaultName];
    if (array) {
        return [NSSet setWithArray:array];
    } else {
        return nil;
    }
}

- (void)setSet:(NSSet *)set forKey:(NSString *)defaultName
{
    if (set) {
        [self setObject:[set allObjects] forKey:defaultName];
    } else {
        [self setObject:nil forKey:defaultName];
    }
}

@end

edited Oct 27, 2015 at 19:33

answered Oct 27, 2015 at 19:07

deadbeef

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2 Comments

Martin R Over a year ago

As stated above, saving a set does not work and does not automatically convert to an array.

deadbeef Over a year ago

@MartinR Edited my answer. That's what I thought too.

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