谈谈NSProxy

NSProxy 是作为其他类或者到目前尚且不存在类的替身类的一个抽象父类。规定了一些需要具体实现的API。
一个proxy接收到的消息被转发到目标对象，或者触发proxy对象来加载目标对象，当然也可能转变自身来称为目标对象。NSProxy的子类可以用来做透明的消息分发或者懒加载。
NSProxy实现了类的基本方法，包括NSObject Protocol规定的。作为一个抽象类它没有初始化方法，收到它无法回复的消息时会抛出异常。一个具体实现的子类必须提供一个初始化方法， 并且重写forwardInvocation:和methodSignatureForSelector:来处理他自身没有实现的消息。熟悉iOS 消息转发机制的朋友对这两个方法应该不陌生，它们处于方法转发机制的第三阶段，用于实现方法签名和对象消息调用。
看来这个类天生就是代理类。

• 我们可以看看官网示例实现的多继承：

/*
     File: main.m
 Abstract: This example shows how to do Objective C message forwarding in Foundation.
  Version: 1.0
 
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 */
 
#import <Foundation/Foundation.h>
#include <stdio.h>
 
 
@interface TargetProxy : NSProxy {
    id realObject1;
    id realObject2;
}
 
- (id)initWithTarget1:(id)t1 target2:(id)t2;
 
@end
 
int main(int argc, const char *argv[]) {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    
    // Create an empty mutable string, which will be one of the
    // real objects for the proxy.
    NSMutableString *string = [[NSMutableString alloc] init];
 
    // Create an empty mutable array, which will be the other
    // real object for the proxy.
    NSMutableArray *array = [[NSMutableArray alloc] init];
 
    // Create a proxy to wrap the real objects.  This is rather
    // artificial for the purposes of this example -- you'd rarely
    // have a single proxy covering two objects.  But it is possible.
    id proxy = [[TargetProxy alloc] initWithTarget1:string target2:array];
 
    // Note that we can't use appendFormat:, because vararg methods
    // cannot be forwarded!
    [proxy appendString:@"This "];
    [proxy appendString:@"is "];
    [proxy addObject:string];
    [proxy appendString:@"a "];
    [proxy appendString:@"test!"];
 
    NSLog(@"count should be 1, it is: %d", [proxy count]);
    
    if ([[proxy objectAtIndex:0] isEqualToString:@"This is a test!"]) {
        NSLog(@"Appending successful.", proxy);
    } else {
        NSLog(@"Appending failed, got: '%@'", proxy);
    }
 
    NSLog(@"Example finished without errors.");
    [pool release];
    return 0;
}
 
 
@implementation TargetProxy
 
- (id)initWithTarget1:(id)t1 target2:(id)t2 {
    realObject1 = [t1 retain];
    realObject2 = [t2 retain];
    return self;
}
 
- (void)dealloc {
    [realObject1 release];
    [realObject2 release];
    [super dealloc];
}
 
// The compiler knows the types at the call site but unfortunately doesn't
// leave them around for us to use, so we must poke around and find the types
// so that the invocation can be initialized from the stack frame.
 
// Here, we ask the two real objects, realObject1 first, for their method
// signatures, since we'll be forwarding the message to one or the other
// of them in -forwardInvocation:.  If realObject1 returns a non-nil
// method signature, we use that, so in effect it has priority.
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *sig;
    sig = [realObject1 methodSignatureForSelector:aSelector];
    if (sig) return sig;
    sig = [realObject2 methodSignatureForSelector:aSelector];
    return sig;
}
 
// Invoke the invocation on whichever real object had a signature for it.
- (void)forwardInvocation:(NSInvocation *)invocation {
    id target = [realObject1 methodSignatureForSelector:[invocation selector]] ? realObject1 : realObject2;
    [invocation invokeWithTarget:target];
}
 
// Override some of NSProxy's implementations to forward them...
- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([realObject1 respondsToSelector:aSelector]) return YES;
    if ([realObject2 respondsToSelector:aSelector]) return YES;
    return NO;
}
@end

• NSProxy既然可以实现消息转发，我们当然也可以用消息转发机制来截胡。NSProxy实现的是第三阶段的消息转发，如YYWeakProxy.

- (id)forwardingTargetForSelector:(SEL)selector {
    return _target;
}
- (void)forwardInvocation:(NSInvocation *)invocation {
    void *null = NULL;
    [invocation setReturnValue:&null];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector {
    return [NSObject instanceMethodSignatureForSelector:@selector(init)];
}

- (id)forwardingTargetForSelector:(SEL)selector已经把消息转给了新的消息接收者，后面的转发阶段不再进行，但是NSProxy自身要求的重写，里面实现了对NSObject的init方法签名。