使用NSProxy实现消息转发机制，模拟多重继承

Objective-C不支持多重继承，但是我们可以使用NSProxy的消息转发机制，来转发可由其它类的对象处理的任务，达成同样的目的。

Xcode的Documentation中有示例（搜索ForwardInvocation，在Sample Code类别中），如下：

/*

     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

使用NSProxy实现代理模式

// MyProxy.h

#import<Foundation/Foundation.h>

@interface MyProxy :NSProxy {

    NSObject *object;

}

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

@end

// MyProxy.m

#import"MyProxy.h"

@implementation MyProxy

- (void)dealloc

{

    [objectrelease];

    object = nil;

    [superdealloc];

}

- (void)fun

{

   // Do someting virtual

    //先做一些代理工作，然后创建一个后台线程，在后台线程中再调用真正的[object fun];

}

// 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) {

        [objectrelease];

    }

    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 = [objectmethodSignatureForSelector:sel];

    } else {

       // Will throw an exception as default implementation

        result = [supermethodSignatureForSelector:sel];

    }

    return result;

}

@end

// RealSubject.h

#import<Foundation/Foundation.h>

@implementation RealSubject : NSObject

- (void)fun;

@end

// RealSubject.m

#import"RealSubject.h"

@implementation RealSubject

- (void)fun

{

    //这个方法需要代理进行惰性调用

   // Do something real

}

- (void)otherFun

{

    //这个方法不需要代理做任何处理，可直接被调用

   // Do something real

}

@end

// main.m

int main(int argc,char *argv[]) 

{

   NSAutoreleasePool * pool = [[NSAutoreleasePoolalloc]init];

    MyProxy *myProxy = [MyProxy alloc];

    RealSubject *realSub = [[RealSubject alloc] init];

    [myProxytransformToObject:realSub];

    [myProxyfun];//直接调用myProxy的fun，执行代理工作

    [myProxyotherFun];//依次调用myProxy的methodSignatureForSelector和forwardInvocation转发给realSub，realSub调用otherFun

    [realSubject release];

    [myProxyrelease];

    [pool release];

    return 0;

}

注意，调用MyProxy中未定义的方法otherFun会出现'MyProxy' may not respond to 'fun'的警告，可通过使用私有范畴或通过performSelector:withObject:来避免，如果有更好的方法，请告知。