Making Substitutions During Message Encoding

Like its abstract superclass, NSCoder, NSPortCoder makes use of substitution methods that allow an object to encode itself as an instance of another class or to replace another object for itself. An object may need to offer a different replacement when being encoded specifically by an NSPortCoder object, however, so instead of the generic classForCoder and replacementObjectForCoder: methods, NSPortCoder invokes classForPortCoder and replacementObjectForPortCoder:. Their default implementations in NSObject fall back to the generic methods, providing reasonable default behavior. (NSPortCoder does not use a special substitution method for decoding; it simply uses awakeAfterUsingCoder: as NSCoder does.)

The generic classForCoder method is most useful for mapping private subclass hierarchies through a public superclass, which (for example) aids the stability of archives when subclasses are private or subject to change. Since processes communicating at run time typically use the same version of a class library, this mapping is often not needed in distributed objects communication. classForPortCoder allows an object to bypass or override the generic classForCoder behavior, sending its real class (or simply a different one from the generic case) to the communicating process or thread. If you implement a group of classes that use the generic classForCoder method, you should also consider using classForPortCoder to handle the special case of encoding with the distributed objects system.

The generic replacementObjectForCoder: method offers a standard way to substitute a different instance at encoding time. replacementObjectForPortCoder: specifically allows for the substitution of proxies over a distributed objects connection. The receiver of a replacementObjectForPortCoder: message can ask the NSPortCoder instance whether it should be encoded bycopy or not, and return itself or a proxy as appropriate. NSObject's implementation always returns a proxy, so subclasses that allow bycopy encoding should override replacementObjectForPortCoder: to perform at least as this sample does:

- (id)replacementObjectForPortCoder:(NSPortCoder *)encoder
    if ([encoder isBycopy]) return self;
    return [super replacementObjectForPortCoder:encoder];

If the NSPortCoder object returns YES when sent an isBycopy message, this example method returns self, which results in the receiver being sent an encodeWithCoder: message. If the NSPortCoder object returns NO, this method invokes the superclass’s implementation, which typically returns an instance of NSDistantObject.