Category / Extensions / Protocol /Informal Protocol

nil      a null object pointer (id)0

Nil     a null class pointer   (Class)0

NO     a boolean false value  (BOOL)0

YES    a boolena true value  (BOOL)1

1. Category

   1.1 the declaration of a category interface looks very much like a class interface declarartion

        //"ClassName+CategoryName.h"

        #import "ClassName.h"

        

        @interface ClassName (CategoryName)

           //method declaration

        @end 

        --------------------------------

        //"ClassName+CategoryName.m"

        #import "ClassName+CategoryName.h"

        

        @implementation ClassName (CategoryName)

         //method definitions

        @end

2.  Extensions

   2.1  extensions is just like inhrite

3. Protocal 

   3.1 protocol use for declaring interface for others to implement

   3.2 delcaring a protocol (Formal protocols)

         @protocol MyXMLSupport

             //method declarations

            -(NSXIMLElemtn *)XMLRepresentation;

            -initFromXMLRepresentation:(NSXMLElement *)XMLElement;

         @end

         ----------------------------------

         @protocol MyXMLSupport

            @optional

              -(void)showText;

            @required

            //...

         @end

    3.4 informal protocols

        informal protocols are typically declared as categories of the NSObject class.

        @interface NSObject (MyXMLSupport)

           //method declaretion

        @end

   3.5 use the protocols

        @interface ClassName:ItsSuperClass <protocol list>

        @end