NSProxy如何"将自己转化为另一个对象"？

Question

该NSProxy类参考这样说:

通常,将代理的消息转发到真实对象或使代理加载(或转换为自身)真实对象.

"将自身转化为真实对象"究竟会如何运作？

为了使事情更具体一些,假设class Foo有一个方法newFooWithString:,它接受一个字符串并返回一个新的实例Foo.是否有可能设置一个NSProxy坐在周围,如果pleaseBecomeAFooUsingString: @"bar"收到一条消息,将自身转换为[Foo newFooWithString: @"bar"]占用相同的内存,而不会弄乱可能存在的其他对自身的引用？

Answer 1

如果你有一个遍及代码的同一个NSProxy实例的指针并将"转换"它,它将在整个代码中改变.无法区分对象方法的调用方,因此您将无法自动替换目标以在代码中转发方法调用.常见的可转换代理将以下列方式查找:

_{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

所以你要求的是某种代码魔术,但NSProxy是一个简单的消息转发器,根本没有任何魔法,所以你的目标无法按照你所描述的方式实现.