Question

I’m exploring the idea of subclassing NSCoder to read/write a proprietary file format. I’m starting to believe this might also require me to subclass NSArray and NSDictionary to override encodeWithCoder: to ensure they are encoded properly:

- (void)encodeWithCoder:(NSCoder *)encoder {
    if ([encoder isKindOfClass:[CustomCoder class]]) {
        // call [encoder encode...] as I see fit.
    }else
        [super encodeWithCoder:encoder];
}

I’m reluctant to do this because NSCoder and its related protocols NSCoding should be self-sufficient… Requiring subclassing classes following the protocol seems like bad design. Am I missunderstanding or over-thinking something?!?

Elaborating a little:
For example, if the file format stipulates that lists of items are encoded like so:

...      // rest of file
0xA1     // marks the beginning of a list
0x0010   // amount of item in list.  The list contains 16 items in this example.
...      // encoded items follows

I would imagine that to encode an array, it’s encodeWithCoder: would need to look like this:

- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeValueOfObjCType:@encode(short) at:&itemCount];
    for ( /*for each item of the array*/ ) {
        [item encodeWithCoder:encoder];
    }
}

Because I ignore how NSArray’s encodeWithCoder: is implement, I imagine I’d have to override it to use my NSCoder class properly.

Answer 1

I don’t understand why do you think you need to subclass collection classes. How your implementation will differ from default one?

At some point encodeObject: method of your NSCoder subclass will get called. In that method you can recognize NSArray and encode it as your format requires. I mean it’s up to your NSCoder subclass to call encodeWithCoder: of objects it encounters or not to call and encode them somehow else.