Introduction to Objective-C Programming (Level: Beginner) Lecture 1

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Introduction toObjective-C Programming

(Level: Beginner)

Lecture 1

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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) (sort of like * in C++)

• 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]; C++ equiv: myRect.display();

[myRect setOrigin:30.0 :50.0]; C++ equiv: 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

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Class Objects

• Compiler creates one class object to contain the information for the name of class and super class

• To start an object in a class:id myRectx; myRect = [[Rectangle alloc] init];

• The alloc method returns a new instance and that instance performs an init method to set its initial state.

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Defining a Class

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

and instance variables of the class and names its super class

– An implementation (.m) 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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DeclarationInstance Variables: float 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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GCC and Objective-C• Objective-C is layered on top of the C language• iPhone & iPad “native” applications are written in

Objective C• The Apple dev kit for Objective-C is called

“Cocoa”• Can be written on any computer that has GCC

plus “GNUstep” plug-in• Apple computers have all of this pre-installed,

and also have an iPhone simulator in the XCode IDE

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Tools• Apple: pre-installed with the Cocoa frameworks

– XCode or GCC in terminal window• Ubuntu: GnuStep is a free clone of Cocoa

– sudo aptget– install buildessentials– gobjc gnustep– gnustepmake– gnustepcommon– libgnustepbasedev

• Windows: http://www.gnustep.org/

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File Overview

• Source code files have a .m extension• Header files have a .h extension• You can use gcc to compile in the same

way as C code• But, you will need to add the Cocoa or

GNUStep frameworks to the build.

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Uses ARC – Compile with Xcode#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]){ @autoreleasepool { // insert code here... NSLog(@"Hello, World!"); } return 0;}

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

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No ARC – Compile with GCC

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Compiling Hello World in Xcode(Mac)

• File->New Project– Mac OS X category – Select Application– Command Line Tool– Type - “Foundation”

• Name project helloworld and choose folder• Save.• Note that Foundation.framework has been included

for us already• “Build and run”.• The debugger console should show “Hello World”

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Compiling Hello World on Terminal(Mac)

• Write hello.m using a plain text editor. Compile:gcc –framework Foundation hello.m –o helloor use: “clang”• Foundation brings in the Foundation framework

(Cocoa) which bundles together a set of dependencies (header files and libraries). You will need this every time we compile Objective C code.

• Type: ./hello will run the program in the terminal• Note: this approach is fine for early examples

but the iPhone applications will be much easier to build in XCode.

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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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Example 2#import <Foundation/Foundation.h>

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

NSLog (@”Hello World”);

int undergrads = 120; int postgrads = 50; int students = undergrads + postgrads;

NSLog (@”Now featuring...\n %i iOS students”, students);

[pool drain];

return 0;}

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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 “Example.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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Example 3: Student.h

@interface Student : NSObject{ int mUndergrads; int mPostgrads;}-(void) print;-(void) setUndergrads: (int) undergrads;-(void) setPostgrads: (int) postgrads;@end

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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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Example 3: Student.m#include “Student.h”

@implementation Example-(void) print {

int totalStudents = mUndergrads + mPostgrads;

NSLog (@”Total students in CompSci = %i”, totalStudents);}

-(void) setUndergrads: (int) undergrads { mUndergrads = undergrads;}-(void) setPostgrads: (int) postgrads { mPostgrads = postgrads;}@end

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Some things to note

• Note prefix minus sign• Very similar to C• Same format as with the interface

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Example 3: main.m#import <Foundation/Foundation.h>#include “Student.h”

int main(int argc, char* argv[]) { NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init]; id myStudent; myStudent = [[Student alloc] init]; // allocate

and initialize all in one! Student* iOSstudent; // pointer

iOSstudent = [Student alloc]; // allocate memory iOSstudent = [iOSstudent init]; // initialize [iOSstudent setUndergrads: 120]; // apply method to instance [iOSstudent setPostgrads: 2000]; [iOSstudent print]; [myStudent print]; [iOSstudent release]; [myStudent release]; [pool drain];return 0;}

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Some things to note

• Applying methods to instance format: [receiver message];

• Semi-colon for variables or values passed to methods

• [pool drain] etc. are also applying methods.• Built-in messages; alloc, init, release.

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Some other things to notealloc is equivalent to C++ new but it also zeros

all variables

init should be applied to an instance before use

These are often combined in shorthand: Student* iOSstudent = [[Student alloc] init];

There is no garbage collection on iOS, so we should release all of our instance memory.

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To compile example 3

• Create a project in Xcode (don’t use “Auto Reference Counting” if

you cut-n-paste from this lecture)• Use ARC with the example code provided

separately.• Note: You will probably use “ARC” for all

of your other projects• Compile and run!

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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:

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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Arrays and Functions• Arrays can be passed as arguments to functions.• This function will print every integer in an array but it needs

to also be told how long the array is (arrayLength).• Statements with hard-coded indices such as array[4] are

potentially dangerous.

void function(int array[], int arrayLength) { int fourthValue = array[4]; for (int i = 0; i < arrayLength; i++) { NSLog(@”%i”, array[i]; }}

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Structs

• Similarly, the Objective C “struct” can be initialized in one go.

• Use the '.' member notation or just use values in the correct order or use some values. 'unknown.'

• In the following example we explicitly define the last member but the others are undefined.

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Structsstruct Student { int BSYears; int PhDYears; int WellSpentYears;};

Student myCareer = { .BSYears = 3, .PhDYears = 3, .WellSpentYears = 6};

Student poorLifeDecisions = { 3, 8, 3 };Student unknown = { 3 };Student andFurtherMore = { .WellSpentYears = 0 };

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Fitting into limited memory: Unions• Union data structures allow ambiguity.• Useful for allowing one storage area to hold different

variable types.• Can only hold one of the variables at a time.• Must be careful to ensure retrieval type matches last type

stored.• Might come in handy for storing lists of “data” that might be

of different types.• The next example gives ones potentially practical use;• A series of recordings store ‘Data.’• The struct has a char to indicate the type of data recorded

in Data.

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Memory ManagementMemory management in Objective-C is semi-automatic:The programmer must allocate memory for objects either

a) explicitly (alloc) or b) indirectly using a constructor

No need to deallocate

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Allocation• Allocation happens through the class method alloc.• The message ‘alloc’ is sent to the class of the

requested object. Alloc is inherited from NSObject. Every alloc creates a new instance (=object) [HelloWorld alloc];

• The class creates the object with all zeros in it and returns a pointer to it.

HelloWorld *p = [HelloWorld alloc];

• The pointer p now points to the new instance. Now we send messages to the instance through p.

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The reference counter• Every instance has a so called reference

counter. It counts how many references are retaining the object. The counter is 1 after allocation. It does not count how many references exist to the object

• Sending the retain message to the object increases the reference counter by 1.

• Sending the release message decreases the reference counter by 1.

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reference counter = retain counter• When the reference counter reaches zero,

the object is automatically de-allocated. The programmer does not deallocate.

• The programmer only does:

allocretainrelease

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Rules for memory management• The method that does an alloc or a retain

must also do a release, it must maintain the balance between:

(alloc or retain) and (release)• If a method does alloc and returns a

pointer to the created object then the method must do an autorelease instead of release. The calling code can do (retain – release) but this is not a must.

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Autorelease pool – if no ARC

• For autorelease to work the programmer must create an autorelease pool, using:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

• When Cocoa is used then the autorelease pool is created automatically, the programmer does not need to do it.

• To release the pool and all objects in it, do: [pool release];

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Constructors

• This is a class method that allocates and initializes an object. The programmer is neither doing alloc nor init.

• Example:

+(id)studentWithName :(NSString*)name AndGpa:(float)gpa{ id newInst = [[self alloc]initStudent:name :gpa]; return [newInst autorelease];}

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Constructors

• Essential: the method sends alloc to ‘self’ which is the Student class object

• Essential: the method autoreleases the instance, because it returns a pointer to the created instance

• Not essential: This example uses an existing initializer, it could use something else or initialize the Student data directly

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Constructors

Calling the constructor:

id stud = [Student studentWithName:@"Johnnie" AndGpa: 3.8];

The message is sent to the Student class object and returns a pointer to it, the pointer is assigned to stud

The calling code does neither alloc nor initAn autorelease pool must be in place

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Something to note!

• You can program for either iOS or OSX or both in the latest XCode.

• If you are going to do the examples for both simultaneously be aware that there are two sets of .h and .m files (one for each).

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FrameworksUIKit framework for developing standard iOS GUIs (buttons

etc.)UITextField (user-entered text that brings up the keyboard)UIFontUIViewUITableViewUIImageViewUIImageUIButton (a click-able button)UILabel (a string of text on-screen)UIWindow (main Window on iPhone)

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iOS programming

• Event driven framework• Interface Designer has some extra macros for

the code that act like hooks to variables;– IBAction - trigger events like mouse clicks– IBOutlet - captures outputs from events

• These tags are not compiled (don't affect the code) but sit as an extra before variables that the Interface Designer can see.

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End of Lecture 1

• Next, we will be doing more Object-C programming and building more applications.

• Make sure to practice writing Object-C source code so you can create highly functioning apps.