Merge pull request #479 from levibostian/objective-c-refactor

Objective-C refactor

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+---
+
+language: Objective-C
+contributors:
+    - ["Eugene Yagrushkin", "www.about.me/yagrushkin"]
+    - ["Yannick Loriot", "https://github.com/YannickL"]
+filename: LearnObjectiveC.m
+
+---
+
+Objective-C is the main programming language used by Apple for the OS X and iOS operating systems and their respective frameworks, Cocoa and Cocoa Touch.
+It is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. 
+
+```cpp
+// Single-line comments start with //
+
+/*
+Multi-line comments look like this.
+*/
+
+// Imports the Foundation headers with #import
+#import <Foundation/Foundation.h>
+#import "MyClass.h"
+
+// Your program's entry point is a function called
+// main with an integer return type.
+int main (int argc, const char * argv[])
+{
+    // Create an autorelease pool to manage the memory into the program
+    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
+    // If using automatic reference counting (ARC), use @autoreleasepool instead:
+    @autoreleasepool {
+
+    // Use NSLog to print lines to the console
+    NSLog(@"Hello World!"); // Print the string "Hello World!"
+ 
+    ///////////////////////////////////////
+    // Types & Variables
+    ///////////////////////////////////////
+    
+    // Primitive declarations
+    int myPrimitive1  = 1;
+    long myPrimitive2 = 234554664565;
+    
+    // Object declarations
+    // Put the * in front of the variable names for strongly-typed object declarations
+    MyClass *myObject1 = nil;  // Strong typing
+    id       myObject2 = nil;  // Weak typing
+    // %@ is an object
+    // 'description' is a convention to display the value of the Objects
+    NSLog(@"%@ and %@", myObject1, [myObject2 description]); // Print "(null) and (null)"
+    
+    // String
+    NSString *worldString = @"World";
+    NSLog(@"Hello %@!", worldString); // prints => "Hello World!" 
+    // NSMutableString is a mutable version of the NSString object.
+    NSMutableString *mutableString = [NSMutableString stringWithString:@"Hello"];
+    [mutableString appendString:@" World!"];
+    NSLog(@"%@", mutableString); // prints => "Hello World!"
+    
+    // Character literals
+    NSNumber *theLetterZNumber = @'Z';
+    char theLetterZ            = [theLetterZNumber charValue]; // or 'Z'
+    NSLog(@"%c", theLetterZ);
+
+    // Integral literals
+    NSNumber *fortyTwoNumber = @42;
+    int fortyTwo             = [fortyTwoNumber intValue]; // or 42
+    NSLog(@"%i", fortyTwo);
+    
+    NSNumber *fortyTwoUnsignedNumber = @42U;
+    unsigned int fortyTwoUnsigned    = [fortyTwoUnsignedNumber unsignedIntValue]; // or 42
+    NSLog(@"%u", fortyTwoUnsigned);
+    
+    NSNumber *fortyTwoShortNumber = [NSNumber numberWithShort:42];
+    short fortyTwoShort           = [fortyTwoShortNumber shortValue]; // or 42
+    NSLog(@"%hi", fortyTwoShort);
+
+    NSNumber *fortyTwoShortNumber   = [NSNumber numberWithShort:41];
+    unsigned short fortyTwoUnsigned = [fortyTwoShortNumber unsignedShortValue]; // or 41
+    NSLog(@"%hu", fortyTwoUnsigned);
+    
+    NSNumber *fortyTwoLongNumber = @42L;
+    long fortyTwoLong            = [fortyTwoLongNumber longValue]; // or 42
+    NSLog(@"%li", fortyTwoLong);
+
+    NSNumber *fortyTwoLongNumber     = @53L;
+    unsigned long fiftyThreeUnsigned = [fortyTwoLongNumber unsignedLongValue]; // or 53
+    NSLog(@"%lu", fiftyThreeUnsigned);
+
+    // Floating point literals
+    NSNumber *piFloatNumber = @3.141592654F;
+    float piFloat           = [piFloatNumber floatValue]; // or 3.141592654f
+    NSLog(@"%f", piFloat); // prints => 3.141592654
+    NSLog(@"%5.2f", piFloat); // prints => " 3.14"
+    
+    NSNumber *piDoubleNumber = @3.1415926535;
+    double piDouble          = [piDoubleNumber doubleValue]; // or 3.1415926535
+    NSLog(@"%f", piDouble);
+    NSLog(@"%4.2f", piDouble); // prints => "3.14"
+
+    // NSDecimalNumber is a fixed-point class that's more precise then float or double
+    NSDecimalNumber *oneDecNum = [NSDecimalNumber decimalNumberWithString:@"10.99"];
+    NSDecimalNumber *twoDecNum = [NSDecimalNumber decimalNumberWithString:@"5.002"];
+    // NSDecimalNumber isn't able to use standard +, -, *, / operators so it provides its own:
+    [oneDecNum decimalNumberByAdding:twoDecNum]; 
+    [oneDecNum decimalNumberBySubtracting:twoDecNum];
+    [oneDecNum decimalNumberByMultiplyingBy:twoDecNum];
+    [oneDecNum decimalNumberByDividingBy:twoDecNum];
+    NSLog(@"%@", oneDecNum); // prints => 10.99 as NSDecimalNumber is immutable. 
+
+    // BOOL literals
+    NSNumber *yesNumber = @YES;
+    NSNumber *noNumber  = @NO;
+    // or
+    BOOL yesBool = YES;
+    BOOL noBool  = NO;
+    NSLog(@"%i", yesBool); // prints => 1
+
+    // Array object
+    NSArray *anArray      = @[@1, @2, @3, @4];
+    NSNumber *thirdNumber = anArray[2];
+    NSLog(@"Third number = %@", thirdNumber); // Print "Third number = 3"
+    // NSMutableArray is mutable version of NSArray allowing to change items in array
+    // and extend or shrink array object. Convenient, but not as efficient as NSArray.
+    NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:2];
+    [mutableArray addObject:@"Hello"];
+    [mutableArray addObject:@"World"];
+    [mutableArray removeObjectAtIndex:0];
+    NSLog(@"%@", [mutableArray objectAtIndex:0]); // prints => "World"
+
+    // Dictionary object
+    NSDictionary *aDictionary = @{ @"key1" : @"value1", @"key2" : @"value2" };
+    NSObject *valueObject     = aDictionary[@"A Key"];
+    NSLog(@"Object = %@", valueObject); // Print "Object = (null)"
+    // NSMutableDictionary also available as a mutable dictionary object.
+    NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionaryWithCapacity:2];
+    [mutableDictionary setObject:@"value1" forKey:@"key1"];
+    [mutableDictionary setObject:@"value2" forKey:@"key2"];
+    [mutableDictionary removeObjectForKey:@"key1"];
+
+    // Set object
+    NSSet *set = [NSSet setWithObjects:@"Hello", @"Hello", @"World", nil];
+    NSLog(@"%@", set); // prints => {(Hello, World)} (may be in different order)
+    // NSMutableSet also available as a mutable set object. 
+    NSMutableSet *mutableSet = [NSMutableSet setWithCapacity:2];
+    [mutableSet addObject:@"Hello"];
+    [mutableSet addObject:@"Hello"];
+    NSLog(@"%@", mutableSet); // prints => {(Hello)}
+
+    // Set object
+    NSSet *set = [NSSet setWithObjects:@"Hello", @"Hello", @"World", nil];
+    NSLog(@"%@", set); // prints => {(Hello, World)}
+
+    ///////////////////////////////////////
+    // Operators
+    ///////////////////////////////////////
+    
+    // The operators works like in the C language
+    // For example:
+    2 + 5; // => 7
+    4.2f + 5.1f; // => 9.3f
+    3 == 2; // => 0 (NO)
+    3 != 2; // => 1 (YES)
+    1 && 1; // => 1 (Logical and)
+    0 || 1; // => 1 (Logical or)
+    ~0x0F; // => 0xF0 (bitwise negation)
+    0x0F & 0xF0; // => 0x00 (bitwise AND)
+    0x01 << 1; // => 0x02 (bitwise left shift (by 1))
+
+    ///////////////////////////////////////
+    // Control Structures
+    ///////////////////////////////////////
+
+    // If-Else statement
+    if (NO)
+    {
+        NSLog(@"I am never run");
+    } else if (0)
+    {
+        NSLog(@"I am also never run");
+    } else
+    {
+        NSLog(@"I print");
+    }
+
+    // Switch statement
+    switch (2)
+    {
+        case 0:
+        {
+            NSLog(@"I am never run");
+        } break;
+        case 1:
+        {
+            NSLog(@"I am also never run");
+        } break;
+        default:
+        {
+            NSLog(@"I print");
+        } break;
+    }
+    
+    // While loops statements
+    int ii = 0;
+    while (ii < 4)
+    {
+        NSLog(@"%d,", ii++); // ii++ increments ii in-place, after using its value.
+    } // => prints "0," 
+      //           "1,"
+      //           "2,"
+      //           "3,"
+
+    // For loops statements
+    int jj;
+    for (jj=0; jj < 4; jj++)
+    {
+        NSLog(@"%d,", jj);
+    } // => prints "0," 
+      //           "1,"
+      //           "2,"
+      //           "3,"
+     
+    // Foreach statements             
+    NSArray *values = @[@0, @1, @2, @3];
+    for (NSNumber *value in values)
+    {
+        NSLog(@"%@,", value);
+    } // => prints "0," 
+      //           "1,"
+      //           "2,"
+      //           "3,"
+
+    // Object for loop statement. Can be used with any Objective-C object type.
+    for (id item in values) { 
+        NSLog(@"%@,", item); 
+    } // => prints "0," 
+      //           "1,"
+      //           "2,"
+      //           "3,"
+
+    // Try-Catch-Finally statements
+    @try
+    {
+        // Your statements here
+        @throw [NSException exceptionWithName:@"FileNotFoundException"
+                            reason:@"File Not Found on System" userInfo:nil];
+    } @catch (NSException * e)
+    {
+        NSLog(@"Exception: %@", e);
+    } @finally
+    {
+        NSLog(@"Finally");
+    } // => prints "Exception: File Not Found on System"
+      //           "Finally"
+ 
+    ///////////////////////////////////////
+    // Objects
+    ///////////////////////////////////////
+    
+    // Create an object instance by allocating memory and initializing it.
+    // An object is not fully functional until both steps have been completed.
+    MyClass *myObject = [[MyClass alloc] init];
+        
+    // The Objective-C model of object-oriented programming is based on message
+    // passing to object instances. 
+    // In Objective-C one does not simply call a method; one sends a message.
+    [myObject instanceMethodWithParameter:@"Steve Jobs"];
+
+    // Clean up the memory you used into your program
+    [pool drain];
+
+    // End of @autoreleasepool.
+    }
+    
+    // End the program
+    return 0;
+}
+
+///////////////////////////////////////
+// Classes And Functions
+///////////////////////////////////////
+
+// Declare your class in a header file (MyClass.h):
+// Class declaration syntax:
+// @interface ClassName : ParentClassName <ImplementedProtocols>
+// {
+//    type name; <= variable declarations;
+// }
+// @property type name; <= property declarations.
+// -/+ (type) Method declarations; <= Method declarations. 
+// @end
+@interface MyClass : NSObject <MyProtocol> // NSObject is Objective-C's base object class.
+{
+    // Instance variable declarations (can exist in either interface or implementation file).
+    int count; // Protected access by default. 
+    @private id data; // Private access. (More convenient to declare in implementation file).
+    NSString *name; 
+}
+// Convenient notation for public access variables to auto generate a setter method. 
+// By default, setter method name is 'set' followed by @property variable name.
+@property int count; // Setter name = 'setCount'
+@property (copy) NSString *name; // (copy) => Copy the object during assignment.
+@property (readonly) id data;    // (readonly) => Declare only a getter method.
+// You can customize the getter and setter names instead of using default 'set' name:
+@property (getter=lengthGet, setter=lengthSet:) int length;
+ 
+// Methods
++/- (return type)methodSignature:(Parameter Type *)parameterName;
+
+// + for class method
++ (NSString *)classMethod;
+
+// - for instance method
+- (NSString *)instanceMethodWithParameter:(NSString *)string;
+- (NSNumber *)methodAParameterAsString:(NSString*)string andAParameterAsNumber:(NSNumber *)number;
+
+@end // States the end of the interface. 
+
+
+// To access public variables from the implementation file, @property generates a setter method
+// automatically. Method name is 'set' followed by @property variable name:
+MyClass *myClass = [[MyClass alloc] init]; // create MyClass object instance.
+[myClass setCount:10]; 
+NSLog(@"%@", [myClass count]); // prints => 10
+// Or using the custom getter and setter method defined in @interface:
+[myClass lengthSet:32];
+NSLog(@"%i", [myClass lengthGet]); // prints => 32
+// For convenience, you may use dot notation to set and access object instance variables:
+myClass.count = 45;
+NSLog(@"%i", myClass.count); // prints => 45
+
+
+// Implement the methods in an implementation (MyClass.m) file:
+@implementation MyClass {
+    long distance; // Private access instance variable.
+}
+
+// To access public variable from the interface file, use '_' followed by variable name:
+_count = 5; // References "int count" from MyClass interface. 
+NSLog(@"%d", _count); // prints => 5
+// Access variables defined in implementation file:
+distance = 18; // References "long distance" from MyClass implementation.
+NSLog(@"%li", distance); // prints => 18
+
+// Call when the object is releasing
+- (void)dealloc
+{
+}
+
+// Constructors are a way of creating classes
+// This is a default constructor which is called when the object is creating
+- (id)init
+{
+    if ((self = [super init]))
+    {
+        self.count = 1;
+    }
+    return self;
+}
+
++ (NSString *)classMethod
+{
+    return [[self alloc] init];
+}
+
+- (NSString *)instanceMethodWithParameter:(NSString *)string
+{
+    return @"New string";
+}
+
+- (NSNumber *)methodAParameterAsString:(NSString*)string andAParameterAsNumber:(NSNumber *)number
+{
+    return @42;
+}
+
+// Methods declared into MyProtocol
+- (void)myProtocolMethod
+{
+    // statements
+}
+
+@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;
+@end
+
+
+///////////////////////////////////////
+// Memory Management
+///////////////////////////////////////
+/* 
+For each object used in an application, memory must be allocated for that object. When the application
+is done using that object, memory must be deallocated to ensure application efficiency. 
+Objective-C does not use garbage collection and instead uses reference counting. As long as 
+there is at least one reference to an object (also called "owning" an object), then the object
+will be available to use (known as "ownership"). 
+
+When an instance owns an object, its reference counter is increments by one. When the
+object is released, the reference counter decrements by one. When reference count is zero,
+the object is removed from memory. 
+
+With all object interactions, follow the pattern of: 
+(1) create the object, (2) use the object, (3) then free the object from memory. 
+*/
+
+MyClass *classVar = [MyClass alloc]; // alloc sets classVar's reference count to one. Returns pointer to object.
+[classVar release]; // Decrements classVar's reference count.  
+// retain claims ownership of existing object instance and increments reference count. Returns pointer to object.
+MyClass *newVar = [classVar retain]; // If classVar is released, object is still in memory because newVar is owner.
+[classVar autorelease]; // Removes ownership of object at end of @autoreleasepool block. Returns pointer to object.
+
+// @property can use retain or assign as well for small convenient definitions. 
+@property (retain) MyClass *instance; // Release old value and retain a new one (strong reference).
+@property (assign) NSSet *set; // Pointer to new value without retaining/releasing old (weak reference).
+
+// Because memory management can be a pain, Xcode 4.2 and iOS 4 introduced Automatic Reference Counting (ARC).
+// ARC is a compiler feature that inserts retain, release, and autorelease automatically for you, so when using ARC, 
+// you must not use retain, relase, or autorelease.
+MyClass *arcMyClass = [[MyClass alloc] init]; // Without ARC, you will need to call: [arcMyClass release] after
+// you're done using arcMyClass. But with ARC, there is no need. It will insert this release statement for you. 
+
+// As for the "assign" and "retain" @property attributes, with ARC you use "weak" and "strong". 
+@property (weak) MyClass *weakVar; // weak does not take ownership of object. If original instance's reference count
+// is set to zero, weakVar will automatically receive value of nil to avoid application crashing.
+@property (strong) MyClass *strongVar; // strong takes ownership of object. Ensures object will stay in memory to use.
+
+// For regular variables (not @property declared variables), use the following:
+__strong NSString *strongString; // Default. Variable is retained in memory until it leaves it's scope. 
+__weak NSSet *weakSet; // Weak reference to existing object. When existing object is released, weakSet is set to nil.
+__unsafe_unretained NSArray *unsafeArray; // Like __weak but unsafeArray not set to nil when existing object is released.
+
+```
+## Further Reading
+
+[Wikipedia Objective-C](http://en.wikipedia.org/wiki/Objective-C)
+
+[Programming with Objective-C. Apple PDF book](https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/ProgrammingWithObjectiveC.pdf)
+
+[iOS For High School Students: Getting Started](http://www.raywenderlich.com/5600/ios-for-high-school-students-getting-started)