Strange behavoir when decoding an NSArray via NSSecureCoding

Question

i spent all afternoon banging my head against the wall trying to figure out why decoding of this class was failing. the class has a property that is an NSArray of Foo objects. Foo conforms to NSSecureCoding, and i have successfully encoded and decoded that class by itself. i was getting an error in initWithCoder: that said failed to decode class Foo. through some experimentation, i discovered that i needed to add [Foo class] to initWithCoder: in order for it to work. maybe this will help someone else who's having the same problem. my question is, why is this necessary? i found no suggestion that this is necessary in apple's documentation.

#import "Foo.h"

@interface MyClass : NSObject <NSSecureCoding>
@property (nonatomic) NSArray *bunchOfFoos;
@end

@implementation MyClass

static NSString *kKeyFoo = @"kKeyFoo";

+ (BOOL) supportsSecureCoding
{
    return YES;
}

- (void) encodeWithCoder:(NSCoder *)encoder
{
    [encoder encodeObject:self.bunchOfFoos forKey:kKeyFoo];
}

- (id) initWithCoder:(NSCoder *)decoder
{
    if (self = [super init])
    {
        [Foo class]; // Without this, decoding fails
        _bunchOfFoos = [decoder decodeObjectOfClass:[NSArray class] forKey:kKeyFoo];
    }
    return self;
}

@end

Answer 1

i think i may have figured this out. without the line [Foo class], there is no reference to the class Foo in this file. because of this, i believe the compiler is optimizing the Foo class out, and then the Foo objects within the array cannot be decoded. having [Foo class] in there prevents this.

Answer 2

For those who are still struggling with this: @Ben H's solution didn't solve my problem. And I keep having the following error message:

Terminating app due to uncaught exception 'NSInvalidUnarchiveOperationException', reason: >'value for key 'NS.objects' was of unexpected class 'ClassA'. Allowed classes are '{(
NSArray
)}'.'

And finally, I realized that for custom classes. You have to use the following function instead decodeObjectOfClasses:

- (id)decodeObjectOfClasses:(NSSet *)classes forKey:(NSString *)key

And you to pass a NSSet of all possible classes in the NSArray to the function above! I am not sure why @Ben H could solve the issue by simply adding a [Foo class] outside of the function. Maybe it is a compiler issue. But anyway, if his solution doesn't work, try this one as well.

Answer 3

I've just encountered similar issue and that was weird and extremely time consuming. I wanted to test my class to be NSSecureCoded correctly with Specta/Expecta. So I've implemented everything as needed specifying class when decoded. At the end of my trials I got weirdest exception:

value for key 'key' was of unexpected class 'MyClass'. Allowed classes are '{(
    MyClass
)}'.

Test looked something like that:

MyClass *myClassInstance = ...
NSMutableData *data = [NSMutableData data];
NSKeyedArchiver *secureEncoder = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
[secureEncoder setRequiresSecureCoding:YES]; // just to ensure things

NSString *key = @"key";
[secureEncoder encodeObject:myClassInstance forKey:key];
[secureEncoder finishEncoding];

NSKeyedUnarchiver *secureDecoder = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
[secureDecoder setRequiresSecureCoding:YES];
MyClass *decodedInstance = [secureDecoder decodeObjectOfClass:[MyClass class] forKey:key]; // exception here
[secureDecoder finishDecoding];

...expect...

While plain NSCoding (requiresSecureCoding = NO) test succeeded, NSSecureCoding tests kept failing. After vast range of trials I found solution for that, just a single line:

[secureDecoder setClass:[MyClass class] forClassName:NSStringFromClass([MyClass class])];

After that all my tests succeeded, objects were created as expected.

I'm not sure why did that happened, my guess would be that class is not visible as Ben H suggested and it uses something like NSClassFromString(@"MyClass"). The above code worked fine in AppDelegate. MyClass was from development pods I'm developing.

Answer 4

Yuchen's answer is/was on the right track but the important thing to know is that the NSSet parameter needs to include the class for the collection in addition to the custom class, like so:

_bunchOfFoos = [decoder decodeObjectOfClasses:[NSSet setWithObjects:[NSArray class], [Foo class], nil] forKey:kKeyFoo];

At least that's what seems to be working for me at this point...