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diff --git a/objective-c.html.markdown b/objective-c.html.markdown new file mode 100644 index 00000000..772e72ca --- /dev/null +++ b/objective-c.html.markdown @@ -0,0 +1,749 @@ +--- + +language: Objective-C +contributors: + - ["Eugene Yagrushkin", "www.about.me/yagrushkin"] + - ["Yannick Loriot", "https://github.com/YannickL"] + - ["Levi Bostian", "https://github.com/levibostian"] +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 +// Use <> to import global files (in general frameworks) +// Use "" to import local files (from project) +#import <Foundation/Foundation.h> +#import "MyClass.h" + +// If you enable modules for iOS >= 7.0 or OS X >= 10.9 projects in +// Xcode 5 you can import frameworks like that: +@import Foundation; + +// 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) // use: @catch (id exceptionName) to catch all objects. + { + NSLog(@"Exception: %@", e); + } @finally + { + NSLog(@"Finally. Time to clean up."); + } // => prints "Exception: File Not Found on System" + // "Finally. Time to clean up." + + // NSError objects are useful for function arguments to populate on user mistakes. + NSError *error = [NSError errorWithDomain:@"Invalid email." code:4 userInfo:nil]; + + /////////////////////////////////////// + // 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 propInt; // Setter method name = 'setPropInt' +@property (copy) id copyId; // (copy) => Copy the object during assignment +// (readonly) => Cannot set value outside @interface +@property (readonly) NSString *roString; // Use @synthesize in @implementation to create accessor +// 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 methods: ++ (NSString *)classMethod; ++ (MyClass *)myClassFromHeight:(NSNumber *)defaultHeight; + +// - for instance methods: +- (NSString *)instanceMethodWithParameter:(NSString *)string; +- (NSNumber *)methodAParameterAsString:(NSString*)string andAParameterAsNumber:(NSNumber *)number; + +// Constructor methods with arguments: +- (id)initWithDistance:(int)defaultDistance; +// Objective-C method names are very descriptive. Always name methods according to their arguments + +@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 + +// Call class methods: +NSString *classMethodString = [MyClass classMethod]; +MyClass *classFromName = [MyClass myClassFromName:@"Hello"]; + +// Call instance methods: +MyClass *myClass = [[MyClass alloc] init]; // Create MyClass object instance +NSString *stringFromInstanceMethod = [myClass instanceMethodWithParameter:@"Hello"]; + +// Selectors +// Way to dynamically represent methods. Used to call methods of a class, pass methods +// through functions to tell other classes they should call it, and to save methods +// as a variable +// SEL is the data type. @selector() returns a selector from method name provided +// methodAParameterAsString:andAParameterAsNumber: is method name for method in MyClass +SEL selectorVar = @selector(methodAParameterAsString:andAParameterAsNumber:); +if ([myClass respondsToSelector:selectorVar]) { // Checks if class contains method + // Must put all method arguments into one object to send to performSelector function + NSArray *arguments = [NSArray arrayWithObjects:@"Hello", @4, nil]; + [myClass performSelector:selectorVar withObject:arguments]; // Calls the method +} else { + // NSStringFromSelector() returns a NSString of the method name of a given selector + NSLog(@"MyClass does not have method: %@", NSStringFromSelector(selectedVar)); +} + +// Implement the methods in an implementation (MyClass.m) file: +@implementation MyClass { + long distance; // Private access instance variable + NSNumber height; +} + +// To access a public variable from the interface file, use '_' followed by variable name: +_count = 5; // References "int count" from MyClass interface +// Access variables defined in implementation file: +distance = 18; // References "long distance" from MyClass implementation +// To use @property variable in implementation, use @synthesize to create accessor variable: +@synthesize roString = _roString; // _roString available now in @implementation + +// Called before calling any class methods or instantiating any objects ++ (void)initialize +{ + if (self == [MyClass class]) { + distance = 0; + } +} + +// Counterpart to initialize method. Called when an object's reference count is zero +- (void)dealloc +{ + [height release]; // If not using ARC, make sure to release class variable objects + [super dealloc]; // and call parent class dealloc +} + +// Constructors are a way of creating instances of a class +// This is a default constructor which is called when the object is initialized. +- (id)init +{ + if ((self = [super init])) // 'super' used to access methods from parent class + { + self.count = 1; // 'self' used for object to call itself + } + return self; +} +// Can create constructors that contain arguments: +- (id)initWithDistance:(int)defaultDistance +{ + distance = defaultDistance; + return self; +} + ++ (NSString *)classMethod +{ + return [[self alloc] init]; +} + ++ (MyClass *)myClassFromHeight:(NSNumber *)defaultHeight +{ + height = defaultHeight; + return [[self alloc] init]; +} + +- (NSString *)instanceMethodWithParameter:(NSString *)string +{ + return @"New string"; +} + +- (NSNumber *)methodAParameterAsString:(NSString*)string andAParameterAsNumber:(NSNumber *)number +{ + return @42; +} + +// 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; +} +[self secretPrivateMethod]; // Calls private method + +// Methods declared into MyProtocol +- (void)myProtocolMethod +{ + // statements +} + +@end // States the end of the implementation + +/////////////////////////////////////// +// 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 + + +/////////////////////////////////////// +// Blocks +/////////////////////////////////////// +// Blocks are statements of code, just like a function, that are able to be used as data. +// Below is a simple block with an integer argument that returns the argument plus 4. +int (^addUp)(int n); // Declare a variable to store the block. +void (^noParameterBlockVar)(void); // Example variable declaration of block with no arguments. +// Blocks have access to variables in the same scope. But the variables are readonly and the +// value passed to the block is the value of the variable when the block is created. +int outsideVar = 17; // If we edit outsideVar after declaring addUp, outsideVar is STILL 17. +__block long mutableVar = 3; // __block makes variables writable to blocks, unlike outsideVar. +addUp = ^(int n) { // Remove (int n) to have a block that doesn't take in any parameters. + NSLog(@"You may have as many lines in a block as you would like."); + NSSet *blockSet; // Also, you can declare local variables. + mutableVar = 32; // Assigning new value to __block variable. + return n + outsideVar; // Return statements are optional. +} +int addUp = add(10 + 16); // Calls block code with arguments. +// Blocks are often used as arguments to functions to be called later, or for callbacks. +@implementation BlockExample : NSObject + + - (void)runBlock:(void (^)(NSString))block { + NSLog(@"Block argument returns nothing and takes in a NSString object."); + block(@"Argument given to block to execute."); // Calling block. + } + + @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) |