NSProxy如何“将自身转换成另一个对象”？

问题描述

说：

将自身转换为真实对象是如何工作的？

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

为了使事情更具体，假设 Foo 有一个方法 newFooWithString： ，它接受一个字符串并返回 Foo 的新实例。是否可以设置一个 NSProxy ，如果收到 pleaseBecomeAFooUsingString：@bar ，将自身转换为 [Foo newFooWithString：@bar] ，占用相同的内存，而不会混淆其他可能存在的引用？

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?

推荐答案

如果你有一个指向同一NSProxy实例的代码，并将转换它，它将改变所有的代码。没有办法区分对象的方法的调用者，因此您将无法自动替换目标来转发代码中的方法调用。常见的可变形代理将采用以下方式：

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:

#import <Foundation/Foundation.h>

@interface MyTrickyProxy : NSProxy {
    NSObject *object;
}

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

@end

#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

一种代码魔法，但NSProxy是一个消息的简单转发器，没有任何魔法，所以你的目标是不能实现的方式，如你所述。

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.

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