1 files changed, 212 insertions, 8 deletions
diff --git a/objective-c.html.markdown b/objective-c.html.markdown
index 6f48c155..0f0165ec 100644
--- a/objective-c.html.markdown
+++ b/objective-c.html.markdown
@@ -430,7 +430,8 @@ distance = 18; // References "long distance" from MyClass implementation
     return @42;
 }
 
-// To create a private method, create the method in the @implementation but not in the @interface
+// Objective-C does not have private method declarations, but you can simulate them. 
+// To simulate a private method, create the method in the @implementation but not in the @interface.
 - (NSNumber *)secretPrivateMethod {
     return @72;
 }
@@ -444,15 +445,218 @@ distance = 18; // References "long distance" from MyClass implementation
 
 @end // States the end of the implementation
 
-/*
- * A protocol declares methods that can be implemented by any class.
- * Protocols are not classes themselves. They simply define an interface
- * that other objects are responsible for implementing.
- */
-@protocol MyProtocol
-    - (void)myProtocolMethod;
+///////////////////////////////////////
+// Categories
+///////////////////////////////////////
+// A category is a group of methods designed to extend a class. They allow you to add new methods
+// to an existing class for organizational purposes. This is not to be mistaken with subclasses. 
+// Subclasses are meant to CHANGE functionality of an object while categories instead ADD 
+// functionality to an object.
+// Categories allow you to:
+// -- Add methods to an existing class for organizational purposes.
+// -- Allow you to extend Objective-C object classes (ex: NSString) to add your own methods.
+// -- Add ability to create protected and private methods to classes. 
+// NOTE: Do not override methods of the base class in a category even though you have the ability 
+// to. Overriding methods may cause compiler errors later between different categories and it 
+// ruins the purpose of categories to only ADD functionality. Subclass instead to override methods.
+
+// Here is a simple Car base class.
+@interface Car : NSObject
+
+@property NSString *make;
+@property NSString *color;
+
+- (void)turnOn;
+- (void)accelerate;
+
+@end
+
+// And the simple Car base class implementation:
+#import "Car.h"
+
+@implementation Car
+
+@synthesize make = _make;
+@synthesize color = _color;
+
+- (void)turnOn {
+    NSLog(@"Car is on.");
+}
+- (void)accelerate {
+    NSLog(@"Accelerating.");
+}
+
+@end
+
+// Now, if we wanted to create a Truck object, we would instead create a subclass of Car as it would
+// be changing the functionality of the Car to behave like a truck. But lets say we want to just add 
+// functionality to this existing Car. A good example would be to clean the car. So we would create 
+// a category to add these cleaning methods:
+// @interface filename: Car+Clean.h (BaseClassName+CategoryName.h)
+#import "Car.h" // Make sure to import base class to extend.
+
+@interface Car (Clean) // The category name is inside () following the name of the base class.
+
+- (void)washWindows; // Names of the new methods we are adding to our Car object.
+- (void)wax;
+
 @end
 
+// @implementation filename: Car+Clean.m (BaseClassName+CategoryName.m)
+#import "Car+Clean.h" // Import the Clean category's @interface file.
+
+@implementation Car (Clean)
+
+- (void)washWindows {
+    NSLog(@"Windows washed.");
+}
+- (void)wax {
+    NSLog(@"Waxed.");
+}
+
+@end 
+
+// Any Car object instance has the ability to use a category. All they need to do is import it:
+#import "Car+Clean.h" // Import as many different categories as you want to use.
+#import "Car.h" // Also need to import base class to use it's original functionality.
+
+int main (int argc, const char * argv[]) {
+    @autoreleasepool {
+        Car *mustang = [[Car alloc] init];
+        mustang.color = @"Red";
+        mustang.make = @"Ford";
+
+        [mustang turnOn]; // Use methods from base Car class.
+        [mustang washWindows]; // Use methods from Car's Clean category.
+    }
+    return 0; 
+}
+
+// Objective-C does not have protected method declarations but you can simulate them.
+// Create a category containing all of the protected methods, then import it ONLY into the
+// @implementation file of a class belonging to the Car class:
+@interface Car (Protected) // Naming category 'Protected' to remember methods are protected.
+
+- (void)lockCar; // Methods listed here may only be created by Car objects.
+
+@end
+//To use protected methods, import the category, then implement the methods:
+#import "Car+Protected.h" // Remember, import in the @implementation file only.
+
+@implementation Car 
+
+- (void)lockCar {
+    NSLog(@"Car locked."); // Instances of Car can't use lockCar because it's not in the @interface.
+}
+
+@end
+
+///////////////////////////////////////
+// Extensions
+///////////////////////////////////////
+// Extensions allow you to override public access property attributes and methods of an @interface.
+// @interface filename: Shape.h
+@interface Shape : NSObject // Base Shape class extension overrides below.
+
+@property (readonly) NSNumber *numOfSides;
+
+- (int)getNumOfSides;
+
+@end
+// You can override numOfSides variable or getNumOfSides method to edit them with an extension:
+// @implementation filename: Shape.m
+#import "Shape.h"
+// Extensions live in the same file as the class @implementation. 
+@interface Shape () // () after base class name declares an extension.  
+
+@property (copy) NSNumber *numOfSides; // Make numOfSides copy instead of readonly.
+-(NSNumber)getNumOfSides; // Make getNumOfSides return a NSNumber instead of an int.
+-(void)privateMethod; // You can also create new private methods inside of extensions.
+
+@end
+// The main @implementation:
+@implementation Shape 
+
+@synthesize numOfSides = _numOfSides;
+
+-(NSNumber)getNumOfSides { // All statements inside of extension must be in the @implementation.
+    return _numOfSides;
+}
+-(void)privateMethod {
+    NSLog(@"Private method created by extension. Shape instances cannot call me.");
+}
+
+@end
+
+///////////////////////////////////////
+// Protocols
+///////////////////////////////////////
+// A protocol declares methods that can be implemented by any class.
+// Protocols are not classes themselves. They simply define an interface
+// that other objects are responsible for implementing.
+ // @protocol filename: "CarUtilities.h"
+@protocol CarUtilities <NSObject> // <NSObject> => Name of another protocol this protocol includes.
+    @property BOOL engineOn; // Adopting class must @synthesize all defined @properties and
+    - (void)turnOnEngine; // all defined methods.
+@end
+// Below is an example class implementing the protocol. 
+#import "CarUtilities.h" // Import the @protocol file.
+
+@interface Car : NSObject <CarUtilities> // Name of protocol goes inside <>
+    // You don't need the @property or method names here for CarUtilities. Only @implementation does.
+- (void)turnOnEngineWithUtilities:(id <CarUtilities>)car; // You can use protocols as data too.
+@end
+// The @implementation needs to implement the @properties and methods for the protocol. 
+@implementation Car : NSObject <CarUtilities>
+
+@synthesize engineOn = _engineOn; // Create a @synthesize statement for the engineOn @property.
+
+- (void)turnOnEngine { // Implement turnOnEngine however you would like. Protocols do not define
+    _engineOn = YES; // how you implement a method, it just requires that you do implement it.
+}
+// You may use a protocol as data as you know what methods and variables it has implemented.
+- (void)turnOnEngineWithCarUtilities:(id <CarUtilities>)objectOfSomeKind { 
+    [objectOfSomeKind engineOn]; // You have access to object variables
+    [objectOfSomeKind turnOnEngine]; // and the methods inside. 
+    [objectOfSomeKind engineOn]; // May or may not be YES. Class implements it however it wants.
+}
+
+@end
+// Instances of Car now have access to the protocol. 
+Car *carInstance = [[Car alloc] init];
+[[carInstance setEngineOn:NO];
+[carInstance turnOnEngine];
+if ([carInstance engineOn]) {
+    NSLog(@"Car engine is on."); // prints => "Car engine is on."
+}
+// Make sure to check if an object of type 'id' implements a protocol before calling protocol methods:
+if ([myClass conformsToProtocol:@protocol(CarUtilities)]) {
+    NSLog(@"This does not run as the MyClass class does not implement the CarUtilities protocol.");
+} else if ([carInstance conformsToProtocol:@protocol(CarUtilities)]) {
+    NSLog(@"This does run as the Car class implements the CarUtilities protocol.");
+}
+// Categories may implement protocols as well: @interface Car (CarCategory) <CarUtilities>
+// You may implement many protocols: @interface Car : NSObject <CarUtilities, CarCleaning>
+// NOTE: If two or more protocols rely on each other, make sure to forward-declare them:
+#import "Brother.h"
+
+@protocol Brother; // Forward-declare statement. Without it, compiler would through error.
+
+@protocol Sister <NSObject>
+
+- (void)beNiceToBrother:(id <Brother>)brother;
+
+@end
+// See the problem is that Sister relies on Brother, and Brother relies on Sister.
+#import "Sister.h"
+
+@protocol Sister; // These lines stop the recursion, resolving the issue. 
+
+@protocol Brother <NSObject>
+ 
+- (void)beNiceToSister:(id <Sister>)sister;
+
+@end
 
 ///////////////////////////////////////
 // Memory Management