NSKeyedUnarchiver cannot decode class clusters

main.m

#import <Foundation/Foundation.h>



@interface Node : NSObject <NSCoding>

@property NSString *name;
@property NSMutableArray<Node *> *targets;

@end

@implementation Node

- (instancetype)initWithCoder:(NSCoder *)decoder {
    self = [super init];
    self.name = [decoder decodeObjectForKey:@"name"];
    self.targets = [decoder decodeObjectForKey:@"targets"];
    return self;
}

- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:self.name forKey:@"name"];
    [encoder encodeObject:self.targets forKey:@"targets"];
}

@end



int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        
        Node *A = [Node new];
        Node *B = [Node new];
        A.name = @"A";
        B.name = @"B";
        
        // This object graph contains multiple references to `targets` mutable array, which in turn holds the same
        // objects again. This creates cyclic reference chain, which should be OK in archive.
        NSMutableArray<Node *> *targets = [NSMutableArray arrayWithArray:@[A, B]];
        A.targets = targets;
        B.targets = targets;
        
        // References to the `targets` array will NOT be properly preserved. The references will point to
        // uninitialized array instance, which was returned from [NSMutableArray alloc], but since
        // [NSMutableArray initWithCoder:] replaced this with another instance, the references were not updated.
        NSArray<Node *> *decodedTargets = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:targets]];
        
        NSLog(@"Before: %@",       targets[0].targets[0]);
        NSLog(@"After: %@", decodedTargets[0].targets[0]); // This crashes:
        // *** -[NSMutableArray objectAtIndex:]: method sent to an uninitialized mutable array object
        
        // Conclusion:
        // It is not safe for -initWithCoder: to return anything else than the original receiver. If this occurs in
        // cyclic reference chain, the other end of this chain will point to original receiver of -initWithCoder:
        // no matter what object that method returned. Same rule applies to -awakeAfterUsingCoder: method.
        
        // I think this problem has no other solution, since I consider it impossible to update all existing
        // references of an object to a new object. Class clusters are the place, where change need to happen.
        // The address of final object must be know to NSCoder before the contents of NSArray are decoded.
        
    }
    return 0;
}