How do I use NSSecureCoding to guarantee the content of collection classes?

Question

I have an object of class SGBContainer which has an array named objects that contains objects of class SGBObject. Currently, they each implement NSCoding but not NSSecureCoding. The -initWithCoder: for SGBContainer looks like this:

- (id)initWithCoder:(NSCoder *)aCoder
{
    self = [self init];
    if (self)
    {
        _objects = [aCoder decodeObjectForKey:@"objects"];
    }
}

I want to switch to using NSSecureCoding, and from what I can tell, it would mean changing the above to this:

- (id)initWithCoder:(NSCoder *)aCoder
{
    self = [self init];
    if (self)
    {
        _objects = [aCoder decodeObjectOfClass:[NSArray class] forKey:@"objects"];
    }
}

...which isn't much of an improvement, as the contents of the array will be instantiated whatever their class. How do I make sure that the array only contains objects of class SGBObject without instantiating them?

Answer 1

Although it's not at all clear from the documentation (and just sounds like an oddly ungrammatical method name), this is what -decodeObjectOfClasses:forKey: does. You'd do something like the following:

NSSet *classes = [NSSet setWithObjects:[NSArray class], [SGBObject class], nil];
_objects = [aCoder decodeObjectOfClasses:classes forKey:@"objects"];

(Credit where it's due: See NSSecureCoding trouble with collections of custom class)