How does NSProxy "transform itself into another object"?

Question

The NSProxy Class Reference says this:

Typically, a message to a proxy is forwarded to the real object or causes the proxy to load (or transform itself into) the real object.

How exactly would the "transform itself into the real object" work?

To make things a little more specific, suppose class Foo has a method newFooWithString: that takes a string and returns a new instance of Foo. Would it be possible to setup an NSProxy that sits around, and if a pleaseBecomeAFooUsingString: @"bar" message is received, transforms itself into [Foo newFooWithString: @"bar"], occupying the same memory, without messing with other references to itself that may exist?

Larkspur

Answer 1

If you have a pointer to the same NSProxy instance all over the code and will "transform" it, it will change all over the code. There is no way to differentiate a caller of a method for an object, so you will not be able to alternate targets for forwarding of methods invocation in your code automatically. Common transformable proxy will looks in following way:

_{MyTrickyProxy.h}

#import <Foundation/Foundation.h>

@interface MyTrickyProxy : NSProxy {
    NSObject *object;
}

- (id)transformToObject:(NSObject *)anObject;

@end

_{MyTrickyProxy.m}

#import "MyTrickyProxy.h"

@implementation MyTrickyProxy

- (void)dealloc 
{
    [object release];
    object = nil;

    [super dealloc];
}

- (NSString *)description 
{
    return [object description];
}

//Stupid transform implementation just by assigning a passed in object as transformation target. You can write your factory here and use passed in object as id for object that need ot be created.
- (id)transformToObject:(NSObject *)anObject 
{
    if(object != anObject) {
        [object release];
    }
    object = [anObject retain];

    return object;
}

- (void)forwardInvocation:(NSInvocation *)invocation 
{
    if (object != nil) {
        [invocation setTarget:object];    
        [invocation invoke];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel 
{
    NSMethodSignature *result;
    if (object != nil) {
        result = [object methodSignatureForSelector:sel];
    } else {
        //Will throw an exception as default implementation
        result = [super methodSignatureForSelector:sel];
    }

    return result;
}

@end

So what you requested is some sort of code-magic, but NSProxy is a simple forwarder of a messages, there is no any magic at all, so your goal is not achievable in a way as you described.

Honeysucker

why to use NSProxy? forwardInvocation is available in NSObject also. – Malpighiaceous

Answer 2

You could create a subclass of of NSProxy that changes which object it forwards it methods to based of what every criteria you want. So you object will always point to NSProxy but you pleaseBecomeAFooUsingString: will change the the object it forwards to as a Foo.

Roye