将NSCountedSet保留为NSUserDefaults

Question

I need to persist an NSCountedSet of custom objects to NSUserDefaults .

I think the problem is that -(id)objectForKey: for NSUserDefaults has a special consideration in the docs here :

Special Considerations
The returned object is immutable, even if the value you originally set was mutable.

Question

How would I go about persisting an NSCountedSet if (as I am currently assuming?) NSUserDefaults returns a non-mutable NSSet from -(id)objectForKey: when I need to retain the internal count metadata that NSCountedSet contains?

Answer 1

You can't write an NSCountedSet to user defaults. Only arrays, dictionaries, strings, numbers, date and data.

You could create a dictionary matching the NSCountedSet, with the set elements as keys and the counts converted to NSNumber as values. And of course when you read the dictionary, convert it to a counted set. Just a few lines of code.

Alternatively, convert to an array with values duplicated depending on their count. If the counted set contains "Hello" with a count of 3, add it to the array three times.