Objective C for iPhone Development

Objective-C

7/20/2009 go4nishu@yahoo.com

Overview

Objective-C is an object oriented language.follows ANSI C style coding with methods from SmalltalkRelies mostly on libraries written by othersFlexible almost everything is done at runtime.

Dynamic BindingDynamic TypingDynamic Linking

id - The generic object type that canhold a pointer to any type of object. This uses dynamic typing

Isa - The isa variable is used to identify the class to which an object belongs at runtime.nil – represents an invalid object.self - in a method self refers to the receiver of the messageClass – when used as a message returns the class object

Some Special Keywords

Object Messaging

[receiver message]eg. [myRectangle display];[myRectangle setOriginX: 30.0 y: 50.0];

Sending Messages to nilPerson *motherInLaw = [[aPerson spouse] mother];

Polymorphism & Dynamic Binding

The selection of a method implementation happens at runtime. When a message is sent, a runtime messaging routine looks at the receiver and at the method named in the message. It locates the receiver’s implementation of a method matching the name, “calls” the method,

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Dot Syntax

Compact and convenientInvoke accessor methods .Doesn't access instance variable directly

myInstance.value = 10;Is same as[myInstance setValue:10];

Dot Syntax

myInstance.value is same as[myInstance value];

compiler can signal an error when it detects a write to a read-only property using dot syntax

Key-value coding (KVC)

identify properties with string-based keys

valueForKey:setValue: forKey:

Key-value coding (KVC)

@property NSString *stringProperty;

NSString *string = [objectInstance valueForKey:@"stringProperty"];

[objectInstance setValue:[NSString @”constantString”] forKey:@“stringProperty"];

Dynamic Language

Almost everything is done at runtimeUses dynamic typing, linking, and bindingThis allows for greater flexibilityMinimizes RAM and CPU usage

To Import or Include?

C/C++’s #include will insert head.h into the code even if its been added before.Obj-C’s #import checks if head.h has been imported beforehand.

#import head.h

Messages

Almost every object manipulation is done by sending objects a messageTwo words within a set of brackets, the object identifier and the message to send.

Because of dynamic binding, the message and receiver are joined at runtime

[Identifier message ]

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Type Instrospection

[anObject isMemberOfClass:someClass]

[anObject isKindOfClass:someClass]

receiver is an instance of a particular class

Checks whether receiver inherits from or is an instance of a particular class:

Memory Allocation

Objects are created dynamically through the keyword, “alloc”Objects are dynamically deallocated using the words “release” and “autorelease”autorelease dealocates the object once it goes out of scope.NOTE: None of these words are built-in

Ownership

Objects are initially owned by the id that created them.Like C++ pointers, multiple IDs can use the same object.However, like in C++ if one ID releases the object, then any remaining pointers will be referencing invalid memory.A method like “retain” can allow the object to stay if one ID releases it.

Prototyping functions

When declaring or implementing functions for a class, they must begin with a + or -+ indicates a “class method” that can only be used by the class itself. In other words, they’re for private functions.- indicates “instance methods” to be used by the client program (public functions).

Class Declaration (Interface)

#import <Cocoa/Cocoa.h>@interface Node : NSObject {Node *link;int contents; }+(id)new;-(void)setContent:(int)number;-(void)setLink:(Node*)next;-(int)getContent;-(Node*)getLink;@end

node.h

Class Definition (Implementation)#import "node.h”@implementation Node+(id)new{ return [Node alloc];}-(void)setContent:(int)number{contents = number;}-(void)setLink:(Node*)next {[link autorelease];link = [next retain];}-(int)getContent{return contents;}-(Node*)getLink{return link;}@end

node.m

Class Object

One per classResponsible for producing instancesClass methods are for class objectAll class objects are of type class.Class name stands for the class object only as the receiver in a message expression

Class Object

int versionNumber = [Rectangle version];Retrieving class object id

id aClass = [anObject class];id rectClass = [Rectangle class];

class objects can also be more specifically typed to the Class data type:

Class aClass = [anObject class];7/20/2009 go4nishu@yahoo.com

Allocation and Intitalization

myRectangle = [Rectangle alloc];allocates memory for the new object’s instance variables and initializes them all to 0. isa connects new instance to its class.

myRectangle = [[Rectangle alloc] init];Initialize the newly allocated memory to appropriate values

Returned Object

Init method can return object other than newly allocated receiver or even return nil.

Use the value returned by the initialization method, not just that returned by alloc or allocWithZone:

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Returned Object

id anObject = [SomeClass alloc];[anObject init];[anObject someOtherMessage];

Code is dangerous since it ignores return of init.

Safe Initialization: combine allocation and initialization

id anObject = [[SomeClass alloc] init];[anObject someOtherMessage];

Custom Initializer Name must begin with init e.g. initWithFormatReturn type should be idinvoke the superclass’s designated initializerassign self to the value returned by the designated initializerreturn self, unless the initializer fails in which case you return nilif there is a problem during an initialization method, you should call [self release] and return nil.

- (id)init {// Assign self to value returned by super's designated initializer// Designated initializer for NSObject is initif (self = [super init]) {creationDate = [[NSDate alloc] init];}return self;}7/20/2009 go4nishu@yahoo.com

Designated Initializer

Method that guarantees inherited instance variables are initialized (by sending a message to super to perform an inherited method).

Designated initializers are chained to each other through messages to super.

Other initialization methods are chained to designated initializers through messages to self.

Designated Initializer- (id)initWithName:(NSString *)string{if ( self = [super init] ) {name = [string copy];}return self;}

- init{return [self initWithName:"default"];}

Using Super

- negotiate{...return [super negotiate];}overrides existing method to modify or add and still incorporates the original method.

Using Super

Initialization with init- (id)init{ if (self = [super init]) {...}}

sends an init message to super to have the classes it inherits from initialize their instance variables.

Redefining self in Class MethodsClass methods are often concerned not with the class object, but with instances of the class.+ (Rectangle*)rectangleOfColor:(NSColor *) color

{self = [[Rectangle alloc] init]; // BAD[self setColor:color];return [self autorelease]; }self here refers to class object.

Redefining self in Class Methods

To avoid confusion, use a variable other than self to refer to an instance inside a class method:

+ (id)rectangleOfColor:(NSColor *)color{id newInstance = [[Rectangle alloc] init]; // GOOD[newInstance setColor:color];return [newInstance autorelease];}

Redefining self in Class Methods

Rather than sending the alloc message to the class in a class method, it’s often better to send alloc to self. This way, if the class is subclassed, and the rectangleOfColor: message is received by a subclass,the instance returned will be the same type as the subclass+ (id)rectangleOfColor:(NSColor *)color

{id newInstance = [[self alloc] init]; // EXCELLENT[newInstance setColor:color];return [newInstance autorelease];}

Super vs. Self

super - is simply a flag to the compiler telling it where to begin searching for the method to perform; it’s used only as the receiver of a message.self - is a local variable within method and it can be used in any number of ways, even assigned a new value

Declared Propertiesshorthand for declaring accessor methodsprovides a clear, explicit specification of how the accessor methods behavecompiler can synthesize accessor methods for youProperties are represented syntactically as identifiers and are scopedyou have less code to write and maintain

NSTextField *textField;@property (retain) IBOutlet NSTextField *textField;@synthesize textField;

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Property Declaration

@property(attributes) type name;

is equivalent to:- (float)value;- (void)setValue:(float)newValue;

Property Declaration

Attributes- readwrite- readonly- assign- retain- copy- nonatomic

Using Properties

Property Re-declarationRe-declare a property in a subclass, but you must repeat its attributes in whole in the subclassesIf you declare a property in one class as readonly, you can redeclare it as readwrite in a class extension

Re-declare readonly to readwrite// public header file@interface MyObject : NSObject {NSString *language;}@property (readonly, copy) NSString *language;@end// private implementation file@interface MyObject ()@property (readwrite, copy) NSString *language;@end@implementation MyObject@synthesize language;@end

using a class extension to provide a property that is declared as read-only in the public header but is redeclared privately as read/write

Copy Attribute

value is copied during assignment@property (nonatomic, copy) NSString *string;string = [newString copy];

synthesized method uses the copy methodCopy returns immutable version of collection.Provide your own implementation for mutable arrays or collections

Copy For mutable Array@interface MyClass : NSObject {NSMutableArray *myArray;}@property (nonatomic, copy) NSMutableArray *myArray;@end@implementation MyClass@synthesize myArray;- (void)setMyArray:(NSMutableArray *)newArray {

if (myArray != newArray) {[myArray release];myArray = [newArray mutableCopy];

}}@end

Objective C for iPhone Development

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