gistfile1.m

#define MESSAGE_KEY @"message_key"
#define ARGUMENTS_KEY @"arguments_key"
#define RESPONSE_KEY @"response_key"

@implementation ...

@synthesize message=_message, arguments=_arguments, response=_response, client;

- (id)initWithCoder:(NSCoder *)aDecoder {
    self = [super init];
    
    _message = [aDecoder decodeObjectForKey:MESSAGE_KEY];
    _arguments = [aDecoder decodeObjectForKey:ARGUMENTS_KEY];
    _response = [aDecoder decodeObjectForKey:RESPONSE_KEY];
    
    self.client = nil;
    return self;
}

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [aCoder encodeObject:self.message forKey:MESSAGE_KEY];
    [aCoder encodeObject:self.arguments forKey:ARGUMENTS_KEY];
    [aCoder encodeObject:self.response forKey:RESPONSE_KEY];
}

...

@end