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git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/base/trunk@12373 72102866-910b-0410-8b05-ffd578937521
130 lines
3.5 KiB
Text
130 lines
3.5 KiB
Text
@paragraphindent 0
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@node Advanced Topics
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@chapter Advanced Topics
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@cindex advanced topics
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@section Forwarding
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@cindex forwarding
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When an object is unable to respond to a message, the Objective-C
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runtime sends a @code{forwardInvocation:} message to the object, and
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the object is then able to use the information provided to handle the
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message in some way, a common mechanism being to forward the message
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to another object known as a @b{delegate}, so that the other object
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can deal with it.
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@example
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- (void) forwardInvocation: (NSInvocation*)objectInvoke
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@{
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if ([forwardObject respondsToSelector: [objectInvoke selector]])
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return [objectInvoke invokeWithTarget: forwardObject];
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else
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return [self doesNotRecognizeSelector: [objectInvoke selector]];
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@}
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@end example
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@itemize @bullet
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@item
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@code{objectInvoke} is another object (of the @code{NSInvocation} class)
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which has all the information about the original message sent, including
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its @b{selector} and its arguments.
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@item
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@code{self} refers to the object or receiver.
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@end itemize
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@b{Note. this is a powerful method for creating software patterns for
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multiple inheritance, journaling, and dispatching messages to
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dynamically loaded code.}
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@subsection Forwarding and Multiple Inheritance
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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.
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@subsection Surrogate Objects
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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:
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@itemize @bullet
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@item
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provides a local address for a remote object
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@item
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queues messages
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@item
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forwards messages to typically remote receivers
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@item
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copies and retrieves arguments over the connection.
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@end itemize
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@subsection Forwarding vs Inheritance
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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.
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(See Section 1.6.6 @code{respondsToSelector.})
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@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.}
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@comment Making Forwarding Transparent
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@section Exception Handling
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@section Copying, Comparing, Hashing Objects
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@itemize @bullet
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@item
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deep copy vs swallow copy
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@item
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simple copy vs mutable copy
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@item
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isEqual:, hash
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@end itemize
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@section Dictionaries, Arrays, Containers
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@section Coding
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@itemize @bullet
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@item
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Possibilities offered by Coding
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@item
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Type Encoding
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@end itemize
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@section Property Lists
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@itemize @bullet
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@item
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What are property lists
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@end itemize
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@section Bundles
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@section UserDefaults
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@section Threading
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General discussion about threading with gnustep base, what is
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thread-safe, what is not, how to start new threads, NSThread briefly
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introduced with examples.
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[Nicola: important: talk about NSConnection enableMultipleThreads]].
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