6. Advanced Topics

6.1 Forwarding

When an object is unable to respond to a message, the Objective-C runtime sends a forwardInvocation: message to the object, and the object is then able to use the information provided to handle the message in some way, a common mechanism being to forward the message to another object known as a delegate, so that the other object can deal with it.

- (void) forwardInvocation: (NSInvocation*)objectInvoke
{
  if ([forwardObject respondsToSelector: [objectInvoke selector]])
    return [objectInvoke invokeWithTarget: forwardObject];
  else
    return [self doesNotRecognizeSelector: [objectInvoke selector]];
}

• objectInvoke is another object (of the NSInvocation class) which has all the information about the original message sent, including its selector and its arguments.

• self refers to the object or receiver.

Note. this is a powerful method for creating software patterns for multiple inheritance, journaling, and dispatching messages to dynamically loaded code.

6.1.1 Forwarding and Multiple Inheritance

6.1.2 Surrogate Objects

• provides a local address for a remote object

• queues messages

• forwards messages to typically remote receivers

• copies and retrieves arguments over the connection.

6.1.3 Forwarding vs Inheritance

(See Section 1.6.6 respondsToSelector.)

Note. respondsToSelector does not trace the forwarding chain, and can therefore erroneously report that an object does not respond to a particular message, when it does.

6.2 Exception Handling

6.3 Copying, Comparing, Hashing Objects

• deep copy vs swallow copy

• simple copy vs mutable copy

• isEqual:, hash

6.4 Dictionaries, Arrays, Containers

6.5 Coding

• Possibilities offered by Coding

• Type Encoding

6.6 Property Lists

• What are property lists

6.7 Bundles

6.8 UserDefaults

6.9 Threading

General discussion about threading with gnustep base, what is thread-safe, what is not, how to start new threads, NSThread briefly introduced with examples.

[Nicola: important: talk about NSConnection enableMultipleThreads]].