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The root NSObject class implements a family of -performSelector: methods. You can use them to send any message to an object, exactly as if your code had invoked the method directly. The code in Listing 66 shows the -className message being sent to an arbitrary object using a direct message, and then again programmatically using the -performSelector: method.
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Listin g 6-6. Sending a Message with -performSelector:
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id anything = ; NSString *name; name = [anything className]; SEL variableMessage = @selector(className); name = [anything performSelector:variableMessage]; The two invocations of -className in Listing 6-6 are functionally identical. The second is slightly slower because it sends two messages: performSelector: is sent to the object, which then immediately sends itself the className message. The value returned by the className method is passed back to the caller. Sending a message using -performSelector: is quick, easy, and lightweight. The principle drawback of -performSelector: is that it only accepts none, one, or two object identifiers as parameters and always returns an object identifier as a return value. By casting, you can coerce the compiler to pass or return any pointer value; all pointers are the same size and format. Listing 6-7 shows how to pass an integer pointer instead of an object pointer. The pointer to (address of) the integer is cast to an object
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CHAPTER 6 SENDING MESSAGES
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identifier so that it agrees with the parameter type of the message. This just mollifies the compiler; no actual value conversion takes place.
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Listin g 6-7. Passing Other Kinds of Pointers to -performSelector:
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@implementation ShiftyMethods - (void)incrementAnInteger:(int*)valuePtr { *valuePtr += 1; } @end ShiftyMethods *shifty = [ShiftyMethods new]; int i = 1; [shifty performSelector:@selector(incrementAnInteger:) withObject:(id)&i]; If the methods being called expect more parameters, or more exotic parameters such as long long integers, floating point values, or structures, you will have to use one of the other techniques in this chapter. Table 6-1 lists the basic -performSelector: methods implemented by NSObject. All of these methods are compatible with methods returning some kind of pointer or void. When used with a void method, ignore the value returned by -performSelector:. Table 6-1. Immediate Invocation Methods
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-(id)performSelector:(SEL)message
Des cripti on
Sends the message specified by the selector to the receiver. The message includes no parameter and returns the object identifier returned by the method. Sends the message to the receiver. The message includes a single pointer as the first parameter, and returns the returned pointer to the caller. Sends the message to the receiver. The message includes two pointer parameters, returning the returned pointer to the caller.
-(id)performSelector:(SEL)message withObject:(id)firstParam
-(id)performSelector:(SEL)message withObject:(id)firstParam withObject:(id)secondParam
Deferred Messages
A message can also be queued and sent to an object at some future time. The method might be invoked on a different thread, after some time has elapsed, or just whenever. To understand how messages are deferred, you need to know a little about run loops. A run loop is an event queue that executes in its own thread. Every Cocoa application has at least one run loop. The first run loop started is christened the main run loop or main thread. All user
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CHAPTER 6 SENDING MESSAGES
interaction (mouse clicks, keyboard commands, display updates, animation, and so on) is fed to the main run loop and executes in the main thread. You can create additional threads in your process. Starting a run loop takes over a thread and turns it into an event driven thread. A run loop s life is spent pulling an event from its event queue, processing it, and immediately looping around to process the next waiting event. If there are no pending events, the thread suspends. You can read more about creating threads and run loops in 15. One type of event that can be pushed onto a run loop s queue is a method invocation. It is little more than a reference to an object and the message and parameters the object will receive. When an object message event appears at the top of the event queue, the run loop sends the message to the object. The root NSObject class implements several methods for queuing a message on a run loop for later execution. The principal ones are shown in Table 6-2. Table 6-2. Deferred Invocation Methods
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