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Hello, all.

Just a quick question regarding General/NSProxy: when is it a good idea to subclass it instead of General/NSObject? I know all that stuff about an object standing in for another one, but what advantages does it have over General/NSObject for those situations?

Thanks – General/RobRix

Okay, one (minor) answer I’ve provided for myself is that General/NSProxy is a different root class so categories on General/NSObject do not apply to your proxy objects. This happens to be quite vital in my current situation. But honestly, there’s got to be more to it than that. Anyone? – General/RobRix

Rob, are you familiar with interprocess communications (IPC)? One purpose of an General/NSProxy is to wrap message passing, synchronization and shared memory issues involved with letting an object in one thread/process communicate with an object in another thread/process. A multithreaded or distributed app is a good candidate to utilize an General/NSProxy (check out the General/NSConnection class) – zootbobbalu

Yesss… but this doesn’t (necessarily) involve subclassing General/NSProxy, and it’s pretty much the only documented example of General/NSProxy use that I can find. I’ve used General/NSProxy and General/NSDistantObject in General/DistributedObjects; right now I’m trying to figure out whether a custom class I’m working on should be a subclass of General/NSProxy or if I can just leave it with General/NSObject.

So yeah, I’m familiar with IPC, and it’s great that we’ve got this, I’m just wondering what the heck advantage it gave anybody in the first place over General/NSObject. Why didn’t they just go with General/NSObject for General/DistributedObjects? – General/RobRix

Probably convenience. The reason for a proxy object is for it to be a stand-in for another object (hence the name). I would think it’d be easier to start off with a class that does next to nothing, rather than hacking around everything that General/NSObject brings to the table. The advantage of General/NSProxy over General/NSObject is that General/NSObject does a whole lot of stuff that can get in the way of what the proxy needs to do.

Okay, fair enough. But the advantage for me is…? – General/RobRix, who notes that his trampoline class “just works” whether its superclass is General/NSObject or General/NSProxy

Rob, The advantage for you is an General/NSProxy can tie your shoes for you ;-)

just kidding……

Are you just trying to stir an intellectual discussion on the merits of an General/NSProxy? Like the post before your last post mentioned, I think an General/NSProxy is only intended to be a convenient way to implement a “proxy” for an object. I don’t think an General/NSProxy is an alternative for an General/NSObject because I’m not sure an General/NSProxy can initialize an object.

There is a great book titled Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (the Gang of Four - GOF). This book does a nice job of explaining the value of a “proxy” design pattern (GOF Proxy). The cocoa implementation of a GOF Proxy is only one of the types of proxies GOF talks about. Scott Anguish (Stepwise) does a nice job of explaining the difference between a GOF Proxy and an General/NSProxy in Cocoa Programming - SAMS Publishing. Apple also talks about General/NSProxy in the online book titled The Objective-C Programming Language:

GOF Patterns mirror a bunch of the Cocoa API and the book was written way after Smalltalk and General/NeXTStep came out, so I wonder who inspired who.

Answering your question is kind of hard because I’m not sure there is a direct answer. Is there an advantage you would like to get out of an General/NSProxy, or are you just curious what the general advantages an General/NSProxy has over an General/NSObject? – zootbobbalu

You know, after thinking about it some more I think I understand your question. I created my own “proxy” like object once and I sub-classed General/NSObject not General/NSProxy, so I see why you’re puzzled. In my case I was using simple POSIX services and not relying on anything specific to the Mach microkernel. I guess I have no idea why an General/NSProxy is a better superclass for creating “proxy” objects.

Good Question!! – zootbobbalu

Yes, I am just looking for general advantages. So far the only one I’ve come up with is:

Thanks, – General/RobRix (Robbat)

General/NSProxy returns YES to isProxy, neh? grin

I think the point of General/NSProxy is it has fewer methods in than General/NSObject, so those methods can be redirected to the object being proxied without needing to override them explicitly. Things like inheritsFrom: and conformsTo:, if I have the names correct.

– General/KritTer

Tip: you can quickly solve many problems using General/NSProxy simply by inserting: General/NSLog(General/[NSString stringWithCString:selector]); at the start of - methodSignatureForSelector:selector in your General/NSProxy subclass. Then you can see every message they receive.

Look for selectors that General/NSProxy doesn’t respond to. Hope this helps. – General/MikeAmy

Actually General/NSProxy works different than General/NSObject… so do all of it’s sublasses… They automatically respond to EVERY selector possible… calling their -(void)forwardInvocation:(General/NSInvocation*) method to handle the invocation… the default one checks if you actually respond to the selector and then calls it, but the one in General/NSDistantObject confers with the object it’s connected with in the other process… If the object doesn’t respond it throws the same kind of error you get when calling a method an General/NSObject subclass doesn’t respond to. I built similar functionality (but with limits) in General/FSObject before I knew about General/NSProxy… And after trying to subclass General/NSProxy to accomplish the same thing cleaner I have found that the methods you need to override to take advantage of this (forwardInvocation: and methodSignatureForSelector:) are more difficult to override than they first appear… I would like to hear from somebody who has had success in doing this… General/MooreSan