Selectors in Objective-C

Question

First  I m not sure I really understand what a selector is    From my understanding  it s the name of a method  and you can assign it to a class of type  SEL  and then run methods such as respondToSelector to see if the receiver implements that method    Can someone offer up a better explanation   Secondly  to this point  I have the following code     NSString  thing     Hello  this is Craig    SEL sel    selector lowercaseString    NSString  lower      thing respondsToSelector sel       YES      NO    NSLog    Responds to lowercaseString       lower   if   thing respondsToSelector sel      lower      YES       NSLog   lowercaseString is        thing lowercaseString      However  even though thing is clearly a kind of NSString  and should respond to lowercaseString  I cannot get the  respondsToSelector  conditional to return  YES

User · Answer

Selectors are an efficient way to reference methods directly in compiled code - the compiler is what actually assigns the value to a SEL.

Other have already covered the second part of your q, the ':' at the end matches a different signature than what you're looking for (in this case that signature doesn't exist).

User · Answer

As per apple docs  https   developer apple com library archive documentation General Conceptual DevPedia-CocoaCore Selector html     A selector is the name used to select a method to execute for an object  or the unique identifier that replaces the name when the source code is compiled  A selector by itself doesn   t do anything  It simply identifies a method  The only thing that makes the selector method name different from a plain string is that the compiler makes sure that selectors are unique  What makes a selector useful is that  in conjunction with the runtime  it acts like a dynamic function pointer that  for a given name  automatically points to the implementation of a method appropriate for whichever class it   s used with  Suppose you had a selector for the method run  and classes Dog  Athlete  and ComputerSimulation  each of which implemented a method run   The selector could be used with an instance of each of the classes to invoke its run method   even though the implementation might be different for each    Example   lldb  breakpoint --set selector viewDidLoad  This will set a breakpoint on all viewDidLoad implementations in your app  So selector is kind of a global identifier for a method

User · Answer

In this case  the name of the selector is wrong  The colon here is part of the method signature  it means that the method takes one argument  I believe that you want  SEL sel    selector lowercaseString

User · Answer

From my understanding of the Apple documentation  a selector represents the name of the method that  you want to call  The nice thing about selectors is you can use them in cases where the exact method to be called varies   As a simple example  you can do something like   SEL selec  if  a    b    selec    selector method1    else   selec    selector method2      self performSelector selec

User · Answer

NSString s method is lowercaseString  0 arguments   not lowercaseString   1 argument

User · Answer

That s because you want  selector lowercaseString   not  selector lowercaseString    There s a subtle difference  the second one implies a parameter  note the colon at the end   but -  NSString lowercaseString  does not take a parameter

User · Answer

You have to be very careful about the method names   In this case  the method name is just  quot lowercaseString quot   not  quot lowercaseString  quot   note the absence of the colon    That s why you re getting NO returned  because NSString objects respond to the lowercaseString message but not the lowercaseString  message  How do you know when to add a colon   You add a colon to the message name if you would add a colon when calling it  which happens if it takes one argument   If it takes zero arguments  as is the case with lowercaseString   then there is no colon   If it takes more than one argument  you have to add the extra argument names along with their colons  as in compare options range locale   You can also look at the documentation and note the presence or absence of a trailing colon

User · Answer

Don t think of the colon as part of the function name  think of it as a separator  if you don t have anything to separate  no value to go with the function  then you don t need it   I m not sure why but all this OO stuff seems to be foreign to Apple developers   I would strongly suggest grabbing Visual Studio Express and playing around with that too   Not because one is better than the other  just it s a good way to look at the design issues and ways of thinking   Like  introspection   reflection   before functions properties   static -   instance level   It s always good to look at a problem in different ways and programming is the ultimate puzzle

[objective-c] Selectors in Objective-C?

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