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Possibly even more interesting than how messages are sent is what happens when an object is sent a message it doesn t implement. If you don t do anything special, the results are similar to Java; the runtime throws an unrecognized selector exception. But before that exception is thrown, the object is given the opportunity to handle the message in some other way. When the message dispatch function finds that an object doesn t implement that
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CHAPTER 6 SENDING MESSAGES
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method, it converts the message into an NSInvocation object. It then passes that NSInvocation to the object s -forwardInvocation: method. The root NSObject s -forwardInvocation: simply sends itself a -doesNotRecognizeSelector: message, which (unless overridden) throws the unrecognized selector exception. A class can override -forwardInvocation: and intercept unimplemented messages. As the name implies, -forwardInvocation: is designed to redirect or forward a message to another object. The StandIn class in Listing 6-12 shows how this is accomplished. The object responds normally to all of the methods it implements or inherits. When sent a message it does not implement, it receives a -forwardInvocation: message. StandIn passes the message, and all of the call s parameters, to its actor object. If actor doesn t implement the message, it will either throw an exception or forward the invocation.
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Listin g 6-12. Forwarding an Unimplemented Message
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@interface StandIn : NSObject { id actor; } @property (assign) id actor; @end @implementation StandIn @synthesize actor; - (void)forwardInvocation:(NSInvocation*)invocation { if (actor==nil) [self doesNotRecognizeSelector:[invocation selector]]; // does not return [invocation invokeWithTarget:actor]; } - (NSMethodSignature*)methodSignatureForSelector:(SEL)sel { NSMethodSignature *signature = [super methodSignatureForSelector:sel]; if (signature==nil) signature = [actor methodSignatureForSelector:sel]; return signature; } @end It s also necessary to override -methodSignatureForSelector:. The message dispatcher first sends the object -methodSignatureForSelector: and uses the returned object to create the invocation argument passed to -forwardInvocation:. Through a little sleight-of-hand, any value returned by the method invoked by -invokeWithTarget: will be returned to the code that originally sent the message. For this to work, it s important to return immediately after sending the -invoke message. The return value will still be in the registers or on the stack, so the caller gets them as if your handler had explicitly returned them.
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CHAPTER 6 SENDING MESSAGES
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You can do lots of interesting things with -forwardInvocation:: Wrap one object in a logger object that intercepts and records the invocation of interesting messages. Implement synthetic messages that are handled by other methods in your class. Imagine creating a generic database record object that catches any property message it receives (i.e., -saleDate, -setSaleDate:) and automatically translates it into a record query. Instead of coding date = [record getDateFieldWithKey:@"SaleDate"], you could simply write date = [record saleDate], without ever writing a -saleDate method. NSManagedObject and CALayer are examples of classes that implement synthetic properties. Create an object that forwards the message to a hierarchy of other objects, like a responder chain. 20 talks about responder chains. The proxy object would search a collection of other objects looking for one that implements the message.
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-forwardInvocation: will not work with variable argument methods because the NSInvocation object won t include a copy of the extra parameters.
Summary
While conceptually similar to Java methods, Objective-C messages are subtly different. Really start thinking in terms of sending messages, rather than calling method. Objective-C s lightweight method dispatching makes it easy to send methods programmatically. Not only can a class send messages dynamically, but it can also respond to them dynamically. The ease and efficiency by which messages can be manipulated is why dynamic messages play such a key role in so many Objective-C solutions.
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CHAPTER 7
Making Friends with nil
Dealing with nil (null) references is an inevitable part of programming. This chapter will explain how nil and NULL pointers are handled in Objective-C, and some of the surprising consequences. Learning to use nil object pointers to your advantage can substantially simplify rather than complicate your design. nil (null) and NULL references are sometimes treated more harshly in Objective-C than they are in Java, but are at other times permitted embraced, even. Java is very consistent in its treatment of null; use of a null reference is universally considered a programming error and throws a java.lang.NullPointerException object at runtime. Runtime exceptions typically terminate the Java application, but can be caught, potentially recovering from the misstep. In Objective-C, the consequences of using a nil or NULL reference are mixed. Accessing the memory at or near address 0 which is what happens if you attempt use a pointer with a nil value causes a memory address violation. Address violations are detected by the hardware and cause a SIGBUS signal to be sent to the process, resulting in its immediate termination. A system CrashReporter daemon usually catches the SIGBUS signal and prepares a crash report document, detailing the state of the process before it was terminated. While it s technically possible for an application to intercept some signals, the ability to recover from a SIGBUS signal is extremely limited.
Note The Objective-C constants nil and NULL are technically interchangeable. By convention, nil (defined by
Objective-C) is used with object pointers and NULL (the traditional C constant) is used for all other pointer types. Thus, you would write MyClass *object = nil and int *iPtr = NULL.
Just as you do in Java, you should avoid direct access of member variables using object pointers (int i = object->iVar) or values via a pointer (int i = *intPtr) without first ensuring the pointer variable contains a valid address. The code in Listing 7-1 demonstrates some typical coding strategies for avoiding NULL pointer references.
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