NSObject中methodSignatureForSelector、forwardInvocation两个消息方法

在obj-c中我们可以向一个实例发送消息，相当于c/c++ java中的方法调用，只不过在这儿是说发送消息，实例收到消息后会进行一些处理。比如我们想调用一个方法，便向这个实例发送一个消息，实例收到消息后，如果能respondsToSelector，那么就会调用相应的方法。如果不能respond一般情况下会crash。今天要的，就是不让它crash。

首先说一下向一个实例发送一个消息后，系统是处理的流程：

1. 发送消息如：[self startwork]

2. 系统会check是否能response这个消息

3. 如果能response则调用相应方法，不能则抛出异常

在第二步中，系统是如何check实例是否能response消息呢？如果实例本身就有相应的response,那么就会相应之，如果没有系统就会发出methodSignatureForSelector消息，寻问它这个消息是否有效？有效就返回对应的方法地址之类的，无效则返回nil。如果是nil,Runtime则会发出-doesNotRecognizeSelector:消息，程序这时也就挂掉了. 如果不是nil接着发送forwardInvocation消息。

所以我们在重写methodSignatureForSelector的时候就人工让其返回有效实例。

我们定义了这样一个类

@interface TargetProxy : NSProxy {  
    id realObject1;  
    id realObject2;  
}  
   
- (id)initWithTarget1:(id)t1 target2:(id)t2;  
   
@end

实现：

@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

现在我们还用这个类，注意向它发送的消息：

  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.");  
    } else {  
        NSLog(@"Appending failed, got: '%@'", proxy);  
    }

运行的结果是：

count should be 1, it is: 1

Appending successful.

TargetProxy声明中是没有appendString与addObject消息的，在这儿却可以正常发送，不crash，原因就是发送消息的时候，如果原本类没有这个消息响应的时候，转向询问methodSignatureForSelector，接着在forwardInvocation将消息重定向。

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NSObject中methodSignatureForSelector、forwardInvocation两个消息方法

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