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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