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Introduction to Objective-Cand

iPhone Development

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Introduction

• Objective-C is implemented as set of extensions to the C language.

• It's designed to give C a full capability for object-oriented programming, and to do so in a simple and straightforward way.

• Its additions to C are few and are mostly based on Smalltalk, one of the first object-oriented programming languages.

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Why Objective C

• Objective-C incorporates C, you get all the benefits of C when working within Objective-C.

• You can choose when to do something in an object-oriented way (define a new class, for example) and when to stick to procedural programming techniques (define a struct and some functions instead of a class).

• Objective-C is a simple language. Its syntax is small, unambiguous, and easy to learn

• Objective-C is the most dynamic of the object-oriented languages based on C. Most decisions are made at run time.

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Object-Oriented Programming

• The insight of object-oriented programming is to combine state and behavior, data and operations on data, in a high-level unit, an object, and to give it language support.

• An object is a group of related functions and a data structure that serves those functions. The functions are known as the object's methods, and the fields of its data structure are its instance variables.

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The Objective-C Language

• The Objective-C language is fully compatible with ANSI standard C

• Objective-C can also be used as an extension to C++.

• Although C++ itself is a Object-Oriented Language, there are difference in the dynamic binding from Objective-C

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Objective-C Language (cont.)

• Objective-C source files have a “.m” extension

• “.h” file is the interface file

• For example:– main.m

– List.h (Interface of List class.)

– List.m (Implementation of List class.)

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ID

• “id” is a data type used by Objective-C to define a pointer of an object (a pointer to the object’s data)

• Any type of object, as long as it is an object, we can use the id data type.

• For example, we can define an object by:id anObject;

• nil is the reserved word for null object

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Dynamic Typing

• “id” data type has no information about the object

• Every object carries with it an isa instance variable that identifies the object's class, that is, what kind of object it is.

• Objects are thus dynamically typed at run time. Whenever it needs to, the run-time system can find the exact class that an object belongs to, just by asking the object

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Messages

• To get an object to do something, you send it a message telling it to apply a method. In Objective-C, message expressions are enclosed in square brackets

[receiver message]

• The receiver is an object. The message is simply the name of a method and any arguments that are passed to it

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Messages (cont.)

• For example, this message tells the myRect object to perform its display method, which causes the rectangle to display itself

[myRect display];

[myRect setOrigin:30.0 :50.0]; • The method setOrigin::, has two colons, one for

each of its arguments. The arguments are inserted after the colons, breaking the name apart

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Polymorphism

• Each object has define its own method but for different class, they can have the same method name which has totally different meaning

• The two different object can respond differently to the same message

• Together with dynamic binding, it permits you to write code that might apply to any number of different kinds of objects, without having to choose at the time you write the code what kinds of objects they might be

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Inheritance

• Root class is NSObject

• Inheritance is cumulative. A Square object has the methods and instance variables defined for Rectangle, Shape, Graphic, and NSObject, as well as those defined specifically for Square

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Inheritance (cont.)

• Instance Variables: The new object contains not only the instance variables that were defined for its class, but also the instance variables defined for its super class, all the way back to the root class

• Methods: An object has access not only to the methods that were defined for its class, but also to methods defined for its super class

• Method Overriding: Implement a new method with the same name as one defined in a class farther up the hierarchy. The new method overrides the original; instances of the new class will perform it rather than the original

Primitive data types from C

• int, short, long

• float,double

• char

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Operators same as C

• +

• -

• *

• /

• ++

• --

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Address and Pointers

• Same as C

• To get address of a variable i

&i

• Pointer

int *addressofi = &I;

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Conditionals and Loops

• Same as C/C++• if / else if/ else• for• while• break• ontinue• do-while

for(int i=0; i< 22; i++) {

NSLog(“Checking i =“%d”, i); if(i == 12)

{ break;}}

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for in loop

• Introduced in Objective-C 2.0 (“fast enumeration”)

for(Item_Type *item in Collection_of_Items) {

//do whatever with the item Nslog(@” Looking now at %@”, item);}

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Functions

• Same as C/C++• return_type functionName(type v1, type v2, ….) { //code of function

}

Examplevoid showMeInfo(int age){ printf(“You are %d years old”, age); //or use NSLog()}

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Global and static variables

• Same as C/C++• Global variables defined at top of file• For static variables use keyword static before it

static in CurrentYear = 2013;

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Automatic Reference Counting

• Objective C uses ‘AUTOMATIC reference counting' as memory management

• keeps an internal count of how many times an Object is 'needed'.

• System makes sure that objects that are needed are not deleted, and when an object is not needed it is deleted.

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Defining a Class

• In Objective-C, classes are defined in two parts: – An interface that declares the methods and

instance variables of the class and names its super class

– An implementation that actually defines the class (contains the code that implements its methods)

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The Interface

• The declaration of a class interface begins with the compiler directive @interface and ends with the directive @end

@interface ClassName : ItsSuperclass { instance variable declarations

} method declarations

@end

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Declaration

• Instance Variablesfloat width;float height; BOOL filled; NSColor *fillColor;

• Methods:• names of methods that can be used by class objects,

class methods, are preceded by a plus sign+ alloc

• methods that instances of a class can use, instance methods, are marked with a minus sign: - (void) display;

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Declaration (cont.)

• Importing the Interface: The interface is usually included with the #import directive

#import "Rectangle.h"

• To reflect the fact that a class definition builds on the definitions of inherited classes, an interface file begins by importing the interface for its super class

• Referring to Other Classes: If the interface mentions classes not in this hierarchy, it must declare them with the @class directive:

@class Rectangle, Circle;

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Creating class instances

ClassName *object = [[ClassName alloc] init];

ClassName *object = [[ClassName alloc] initWith* ];– NSString* myString = [[NSString alloc] init];– Nested method call. The first is the alloc method called on NSString itself.

This is a relatively low-level call which reserves memory and instantiates an object. The second is a call to init on the new object. The init implementation usually does basic setup, such as creating instance variables. The details of that are unknown to you as a client of the class. In some cases, you may use a different version of init which takes input:

ClassName *object = [ClassName method_to_create];– NSString* myString = [NSString string];– Some classes may define a special method that will in essence call alloc followed by

some kind of init

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Example: Hello World#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

// insert code here... NSLog(@"Hello, World!"); [pool drain]; return 0; }

NSLog() is equivalent to a C printf()Very similar to CC programs are valid in Objective-C

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Intro to HelloWorld

• Objective-C uses a string class similar to the std::string or Java string. It is called NSString.

• Constant NSStrings are preceded by @ for example: @”Hello World”

• You will notice that NSLog() also outputs time and date and various extra information.

• NSAutoreleasePool* pool =[[NSAutoreleasePool• alloc] init]; allocates a lump of memory• Memory must be freed with [pool drain];

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Some things to note

• No line break needed at end of NSLog statements.

• NSString constants use C-style variables.

• Test this program to see results.

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@interface

@interface ClassName : ParentClassName{declare member variable here;declare another member variable here;}declare method functions here;@end

• Equivalent to C class declaration• Goes in header (.h) file• Functions outside curly braces• Don’t forget the @end tag

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@implementation

#import <Foundation/Foundation.h>#include "ComputerScience.h“

@implementation ClassNamedefine method function here;define another method function here;define yet another method function here;@end

● Equivalent to C class function definitions● Goes in source (.m) file● Don't forget the @end tag

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Some things to note

• NSObject is the “root” object in Objective-C• No direct access to instance variables so we write

some get/set “instance methods”• Instance methods (affect internal state of class)

are preceded with a minus sign• “Class methods” (higher level functions) are

preceded with a plus sign e.g. create new class• Method return type in parentheses• Semicolon before list of parameters• Parameter type in parenthesis, followed by name

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Data Structures• Objective-C arrays are similar to C arrays, but

you can initialize whole array in a list.• Or just a few indices of the array. The rest are

set to 0.• Or mix and match; the example will create an

array of size [8].int values[3] = { 3, 4, 2 };char letters[3] = { 'a', 'c', 'x' };float grades[100] = {10.0,11.2,1.1};int array[] = {[3]=11,[2]=1,[7]=0};

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Multi-dimensional Arrays

• Can be initialized by using subset notation with braces. Note no comma after second subset.

• Subset braces are optional, the compiler will fill in blanks where it can as with 1D arrays.

int array[2][3] = {

{ 0, 3, 4},

{ 1, 1, 1}

};

int array[2][3] = {0, 3, 4, 1, 1, 1};

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Frameworks

• UIKit.framework for developing standard iOS GUIs (buttons etc.)

• UITextField (user-entered text that brings up the keyboard)

• UIFont• UIView• UITableView• UIImageView• UIImage• UIButton (a click-able button)• UILabel (a string of text on-screen)• UIWindow (main Window on iPhone)