@paragraphindent 0

@node Advanced Topics
@chapter Advanced Topics
@cindex advanced topics

@section Forwarding
@cindex forwarding

When an object is unable to respond to a message, the Objective-C
runtime sends a @code{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 @b{delegate}, so that the other object
can deal with it.

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

@itemize @bullet
@item
@code{objectInvoke} is another object (of the @code{NSInvocation} class)
which has all the information about the original message sent, including
its @b{selector} and its arguments.

@item
 @code{self} refers to the object or receiver.
@end itemize

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

@subsection Forwarding and Multiple Inheritance
Forwarding is a form of multiple inheritance. The diagram below shows a simple scenario where an instance of the @code{Chat} class passes the @b{negotiate} message to an instance of the @code{ChatTwo} class. The forwarding object therefore inherits methods from its own inheritance path and from that of the receiving object.




@subsection Surrogate Objects
Surrogate objects forward messages to objects that can be assumed to be more complex. The @code{forwardInvocation:} method of the surrogate object receives a message that is to be forwarded; it determines whether or not the receiver exists, and if it does not, then it will attempt to create it. A @b{proxy object} is a common example of a surrogate object. A proxy object performs obvious functions that we have already discussed:

@itemize @bullet
@item
provides a local address for a remote object 

@item
queues messages

@item
forwards messages to typically remote receivers

@item
copies and retrieves arguments over the connection. 
@end itemize


@subsection Forwarding vs Inheritance
The main difference between @b{Forwarding} and @b{Inheritance} surfaces in the application of methods like @code{respondsToSelector} and @code{isKindOfClass:}. This is because these methods search the inheritance path, but ignore the forwarding path. 

(See Section 1.6.6 @code{respondsToSelector.})

@b{Note. @code{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.}


@comment Making Forwarding Transparent


@section Exception Handling


@section Copying, Comparing, Hashing Objects



@itemize @bullet 

@item
deep copy vs swallow copy

@item
simple copy vs mutable copy

@item
isEqual:, hash

@end itemize


@section Dictionaries, Arrays, Containers

@section Coding 

@itemize @bullet 

@item 
Possibilities offered by Coding

@item 
Type Encoding

@end itemize


@section Property Lists

@itemize @bullet 
@item
What are property lists
@end itemize


@section Bundles

@section UserDefaults

@section 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]].