NSProxy如何“将自身转换成另一个对象”？ [英] How does NSProxy "transform itself into another object"?

问题描述

NSProxy类别参考说：

通常，到代理的消息被转发到真实对象或导致代理加载

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.

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:

_{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

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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