Re: Category or Protocol? (objc newbie)

FROM : Rick Kitts
DATE : Tue Apr 19 17:33:50 2005

Ah. This is pretty sexy. So categories are 0 runtime cost constructs? 
Your example also seems to raise the question of how should one deal 
with the delegate.  It seems that I could either create a default/do 
nothing implementation or I could send a respondsTo: message to the 
delegate. I'd presume that the former is faster at runtime, whereas the 
second seems to better convey what's actually happening (note: For what 
I'm doing there *is* a reasonable default behavior and this is what 
raises this issue for me).

Any suggestions?

BTW I think what I'm looking for, ideally, is a sort of best practices 
forum or something. Objc is trivial to learn but, like most languages, 
difficult to skillfully wield. An Effective Objective-C book would be 
really useful.

Thanks again,
---Rick

On Apr 18, 2005, at 9:28 PM, Andrew White wrote:

>
> Rick Kitts wrote:

>> Hello. I'm working on a class that uses a delegate. I was looking at 
>> how Apple does this sort of thing and ran into NSXMLParser. It takes 
>> a delegate, of course, and the delegate interface is declared as 
>> follows:
>> // The parser's delegate is informed of events through the methods in 
>> the NSXMLParserDelegateEventAdditions category.
>> @interface NSObject (NSXMLParserDelegateEventAdditions)
>> To be honest, I'm not sure that this means. Does 
>> NSXMLParserDelegateEventAdditions
>> define a type that I can subclass?

>
> What the declaration means is that the 
> NSXMLParserDelegateEventAdditions category adds the listed methods to 
> the NSObject class.  Since everything inherits from NSObject, these 
> methods are available to all objects in the system.  Typically, a 
> category will also define default implementations so that objects that 
> are unaware of the category don't have problems if called 
> unintentionally.
>
>

>> So, I guess what I'm asking is can anyone either a) explain what this 
>> stuff is trying to do or b) point me to some docs or a book that can 
>> help me understand this sort of thing? I have read the objc manual 
>> (PDF) from Apples site and I didn't see this sort of thing covered.

>
> Since these methods are available to all NSObjects (and, by 
> inheritance, whatever object you want to be the delegate), all you 
> need to do is set your object to be the parser object's delegate and 
> then implement (override) the methods specified by the category.  You 
> don't need to declare anything special, because the category 
> declaration has already done that for you.
>
>
> * example:
>
> Declarations and definitions:
>
> @interface NSObject (MyExample)
>
> - (BOOL) dummyExampleMethod;
>
> @end
>
> @implementation NSObject (MyExample)
>
> - (BOOL) dummyExampleMethod
> {
>     return NO;
> }
>
> @end
>
>
> @interface MyObject : NSObject
> {
> }
> @end
>
> @implementation MyObject
>
> - (BOOL) dummyExampleMethod
> {
>     return YES;
> }
>
> @end
>
>
>
> Code snippet:
>     NSString * s = @"String";
>     MyObject * m = [[MyObject alloc] init];
>
> [s dummyExampleMethod] will return NO.
> [m dummyExampleMethod] will return YES.
>
> Note that neither the original NSObject nor NSString knew anything 
> about dummyExampleMethod - we've added it on later.
>
> -- Andrew White
>
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