learnxinyminutes-docs/objective-c.html.markdown

---

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 *fortyOneShortNumber   = [NSNumber numberWithShort:41];
    unsigned short fortyOneUnsigned = [fortyOneShortNumber unsignedShortValue]; // or 41
    NSLog(@"%u", fortyOneUnsigned);
    
    NSNumber *fortyTwoLongNumber = @42L;
    long fortyTwoLong            = [fortyTwoLongNumber longValue]; // or 42
    NSLog(@"%li", fortyTwoLong);

    NSNumber *fiftyThreeLongNumber   = @53L;
    unsigned long fiftyThreeUnsigned = [fiftyThreeLongNumber 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
    // May contain different data types, but must be an Objective-C 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)}

    ///////////////////////////////////////
    // 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 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 method name = 'setCount'
@property (copy) NSString *name; // (copy) => Copy the object during assignment.
@property (readonly) id data;    // (readonly) => Cannot set value outside interface.
// 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(@"%d", [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 initialized. 
- (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' and '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).

// Automatic Reference Counting (ARC)
// 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]; 
// ... code using arcMyClass
// 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)