Question

I have two classes which look like this in barebones form:

@interface GraphNode {
    NSMutableArray *edges;
}

@implementation GraphNode

-(id)initWithCoder:(NSCoder *)coder {
    if (self = [super init]) {
        edges = [coder decodeObjectForKey:@"edges"];
    }
    return self;
}

-(void)encodeWithCoder:(NSCoder*)coder {
    [coder encodeObject:edges forKey:@"edges"];
}

@end

@interface GraphEdges {
  NSMutableArray *edges;
}

@implementation GraphEdges

-(id)initWithCoder:(NSCoder *)coder {
    if (self = [super init]) {
        edges = [coder decodeObjectForKey:@"edges"];
    }
    return self;
}

-(void)encodeWithCoder:(NSCoder*)coder {
    [coder encodeObject:edges forKey:@"edges"];
}

@end

You’ll notice they seem to express the same behaviour – encoding/decoding an NSMutableArray whose contents are objects of a type GraphEdge which implements NSCoding. The difference between the classes in practise is that GraphEdges creates edges in its edges property, while GraphNode adds to its own edges only references to already-existing GraphEdge instances in GraphEdges.edges .

My issue is that when decoding GraphNode and GraphEdges objects the object graph is not maintained since each class creates its own versions of the original GraphEdge objects stored in the edges property for each class.

I tried to do an encodeConditionalObject on the whole NSMutableArray *edges in GraphNode while keeping the encode in GraphEdges unconditional, but unsurprisingly this simply fails to encode GraphNode.edges as the condition is placed on the array itself, not its objects.

Is the solution to iterate through the arrays in both cases, and perform conditional encoding on each edges member for GraphNode but unconditional encoding for each member of edges in GraphEdges?

Any advice is appreciated, many thanks.

Answer 1

I think the problem here is that you’re archiving GraphNode and GraphEdges objects separately. Combine all related GraphNode and GraphEdges objects in some sort of container, maybe ‘Graph,’ so that they all become part of the same object graph. I believe that NSKeyedArchiver will then archive each object only once.