Introduction to Objective - C

INTRODUCTION TO
OBJECTIVE-C
COMPILED BY: ASIM RAIS SIDDIQUI

Goals of the Lecture

 Present an introduction to Objective-C 2.0
 Coverage of the language will be INCOMPLETE
 We’ll see the basics… there is a lot more to learn
 There is a nice Objective-C tutorial located here:
 http://cocoadevcentral.com/d/learn_objectivec

History (I)
Brad Cox created Objective-C in the early 1980s
It was his attempt to add object-oriented programming
concepts to the C programming language
NeXT Computer licensed the language in 1988; it was
used to develop the NeXTSTEP operating system,
programming libraries and applications for NeXT
In 1993, NeXT worked with Sun to create OpenStep,
an open specification of NeXTSTEP on Sun hardware

History (II)
In 1997,Apple purchased NeXT and transformed
NeXTSTEP into MacOS X which was first released in the
summer of 2000
Objective-C has been one of the primary ways to
develop applications for MacOS for the past 11 years
In 2008, it became the primary way to develop
applications for iOS targeting (currently) the iPhone
and the iPad and (soon, I’m guessing) the AppleTV

Objective-C is“C plus Objects” (I)
Objective-C makes a small set of extensions to C which
turn it into an object-oriented language
It is used with two object-oriented frameworks
The Foundation framework contains classes for basic
concepts such as strings, arrays and other data
structures and provides classes to interact with the
underlying operating system
The AppKit contains classes for developing applications
and for creating windows, buttons and other widgets

Objective-C is“C plus Objects” (II)
Together, Foundation and AppKit are called Cocoa
On iOS,AppKit is replaced by UIKit
Foundation and UIKit are called Cocoa Touch
In this lecture, we focus on the Objective-C language,
we’ll see a few examples of the Foundation framework
we’ll see examples of UIKit in Lecture 13

C Skills? Highly relevant
Since Objective-C is“C plus objects” any skills you have in
the C language directly apply
statements, data types, structs, functions, etc.
What the OO additions do, is reduce your need on
structs, malloc, dealloc and the like
and enable all of the object-oriented concepts we’ve
been discussing
Objective-C and C code otherwise freely intermix

DevelopmentTools (I)
Apple’s XCode is used to develop in Objective-C
Behind the scenes, XCode makes use of either gcc or
Apple’s own LLVM to compile Objective-C programs
The latest version of Xcode, Xcode 4, has integrated
functionality that previously existed in a separate
application, known as Interface Builder
We’ll see examples of that integration next week

DevelopmentTools (II)
XCode is available on the Mac App Store
It is free for users of OS X Lion
Otherwise, I believe it costs $5 for previous versions of
OS X
Clicking Install in the App Store downloads a program
called“Install XCode”.You then run that program to get
XCode installed

HelloWorld
As is traditional, let’s look at our first objective-c program
via the traditional HelloWorld example
To create it, we launch XCode and create a New Project

Similar to what we saw
with Eclipse, XCode
creates a default project
for us;

There are folders for this
program’s source code (.m
and .h files), frameworks,
and products (the application itself)

Note: the Foundation
framework is front and
center and HelloWorld is
shown in red because it
hasn’t been created yet

The resulting project structure on disk does not map completely to what is shown in Xcode; The source file, man page, and pre-
compiled header file are all stored in a sub-directory of the main directory.

The project file HelloWorld.xcodeproj is stored in the main directory. It is the file that keeps track of all project settings and the

location of project files.

XCode project directories are a lot simpler now that files generated during a build are stored elsewhere.

Where is the actual application?
After you ran the application, HelloWorld switched from
being displayed in red to being displayed in black
You can right click on HelloWorld and select“Show in
Finder” to see where XCode placed the actual
executable
By default, XCode creates a directory for your project in
~/Library/Developer/XCode/DerivedData
For HelloWorld, XCode generated 20 directories
containing 31 files!

Objective-C programs start with a function called main, just like C programs; #import is
similar to C’s #include except it ensures that header files are only included once and only
once. Ignore the “NSAutoreleasePool” stuff for now
Thus our program calls a function, NSLog, and returns 0

The blue arrow indicates that a breakpoint has been set; gdb
will stop execution on line 7 the next time we run the program

Objective-C classes
Classes in Objective-C are defined in two files
A header file which defines the attributes and method
signatures of the class
An implementation file (.m) that provides the method
bodies

Header Files
The header file of an Objective-C class traditionally has
the following structure
<import statements>

@interface <classname> : <superclass name> {

<attribute definitions>

}

<method signature definitions>

@end

Header Files
With Objective-C 2.0, the structure has changed to the
following (the previous structure is still supported)
<import statements>

@interface <classname> : <superclass name>

<property definitions>

<method signature definitions>

@end

What’s the difference?
In Objective-C 2.0, the need for defining the attributes of a
class has been greatly reduced due to the addition of
properties
When you declare a property, you automatically get
an attribute (instance variable)
a getter method
and a setter method
synthesized (automatically added) for you

New Style
In this class, I’ll be using the new style promoted by
Objective-C 2.0
Occasionally we may run into code that uses the old
style, I’ll explain the old style when we encounter it

Objective-C additions to C (I)
Besides the very useful #import, the best way to spot an
addition to C by Objective-C is the presence of this symbol

Objective-C additions to C (II)
In header files, the two key additions from Objective-C are
@interface
and
@end
@interface is used to define a new objective-c class
As we saw, you provide the class name and its superclass;
Objective-C is a single inheritance language
@end does what it says, ending the @interface compiler directive

We’ve added one property: It’s called greetingText. Its type is
NSString* which means “pointer to an instance of NSString”

We’ve also added one method called greet. It takes no
parameters and its return type is “void”.

(By the way, NS stands for “NeXTSTEP”! NeXT lives on!)

Objective-C Properties (I)
An Objective-C property helps to define the public interface
of an Objective-C class
It defines an instance variable, a getter and a setter all
in one go
@property (nonatomic, copy) NSString* greetingText
“nonatomic” tells the runtime that this property will never be
accessed by more than one thread (use“atomic” otherwise)
“copy” is related to memory management and will be
discussed later

Objective-C Properties (III)
@property (nonatomic, copy) NSString* greetingText
If you have an instance of Greeter
Greeter* ken = [[Greeter alloc] init];
You can assign the property using dot notation
ken.greetingText = @“Say Hello, Ken”;
You can retrieve the property also using dot notation
NSString* whatsTheGreeting = ken.greetingText;

Objective-C Properties (IV)
Dot notation is simply“syntactic sugar” for calling the
automatically generated getter and setter methods
NSString* whatsTheGreeting = ken.greetingText;
is equivalent to
NSString* whatsTheGreeting = [ken greetingText];
The above is a call to a method that is defined as
- (NSString*) greetingText;

Objective-C Properties (V)
Dot notation is simply“syntactic sugar” for calling the
automatically generated getter and setter methods
ken.greetingText = @“Say Hello, Ken”;
is equivalent to
[ken setGreetingText:@”Say Hello, Ken”];
The above is a call to a method that is defined as
- (void) setGreetingText:(NSString*) newText;

Objective-C Methods (I)
It takes a while to get use to Object-C method signatures
- (void) setGreetingText: (NSString*) newText;

defines an instance method (-) called setGreetingText:
The colon signifies that the method has one parameter
and is PART OFTHE METHOD NAME
newText of type (NSString*)
The names setGreetingText: and setGreetingText refer to
TWO different methods; the former has one parameter

Let’s implement the method bodies
The implementation file of a class looks like this
<import statements>

<optional class extension>

@implementation <classname>

<method body definitions>

@end

Let’s ignore the “optional class extension” part
for now

But first, calling methods (I)
The method invocation syntax of Objective-C is
[object method:arg1 method:arg2 …];
Method calls are enclosed by square brackets
Inside the brackets, you list the object being called
Then the method with any arguments for the methods
parameters

But first, calling methods (II)
Here’s a call using Greeter’s setter method; @“Howdy!” is a shorthand
syntax for creating an NSString instance
[greeter setGreetingText: @“Howdy!”];

Here’s a call to the same method where we get the greeting from
some other Greeter object
[greeterOne setGreetingText:[greeterTwo greetingText]];

Above we nested one call inside another; now a call with multiple args
[rectangle setStrokeColor: [NSColor red] andFillColor: [NSColor green]];

Memory Management (I)
Memory management of Objective-C objects involves the
use of six methods
alloc, init, dealloc, retain, release, autorelease
Objects are created using alloc and init
We then keep track of who is using an object with retain
and release
We get rid of an object with dealloc (although, we never
call dealloc ourselves)

Memory Management (II)
When an object is created, its retain count is set to 1
It is assumed that the creator is referencing the object
that was just created
If another object wants to reference it, it calls retain to
increase the reference count by 1
When it is done, it calls release to decrease the
reference count by 1
If an object’s reference count goes to zero, the runtime
system automatically calls dealloc

Memory Management (III)
I won’t talk about autorelease today, we’ll see it in action soon
Objective-C 2.0 added a garbage collector to the language
When garbage collection is turned on, retain, release, and
autorelease become no-ops, doing nothing
However, the garbage collector is not available when
running on iOS, so the use of retain and release are still
with us
Apple recently released“automatic reference counting” which
may make all of this go away (including the garbage collector)

Some things not (yet) discussed
Objective-C has a few additions to C not yet discussed
The type id: id is defined as a pointer to an object
id iCanPointAtAString = @“Hello”;
Note: no need for an asterisk in this case
The keyword nil: nil is a pointer to no object
It is similar to Java’s null
The type BOOL: BOOL is a boolean type with valuesYES
and NO; used throughout the Cocoa frameworks

Wrapping Up (I)
Basic introduction to Objective-C
main methods
class and method definition and implementation
method calling syntax
creation of objects and memory management
More to come as we use this knowledge to explore the
iOS platform in future lectures

Primitive data types from C

 int, short, long

 float,double

 char

Classes from Apple

 NSString is a string of text that is immutable.
 NSMutableString is a string of text that is mutable.
 NSArray is an array of objects that is immutable.
 NSMutableArray is an array of objects that is mutable.
 NSNumber holds a numeric value.

Working with Objects & Messaging

 The majority of work in an
Objective-C application happens
as a result of messages being sent
back and forth across an
ecosystem of objects. Some of
these objects are instances of
classes provided by Cocoa or
Cocoa Touch, some are
instances of your own classes.

NSArray

Initialize an NSArray by including a comma-separated list of objects
between @[ and ]. The compiler converts that line of code into a call to
the +[NSArray arrayWithObjects:count:] class method

NSDictionary
Dictionaries are very
common in iOS application
development.

A dictionary is nothing more
than a collection of key-
value pairs. The keys are
represented as strings and
the values are objects.

The keys in a dictionary must
be unique. As with other
collections, dictionaries have
two variants, mutable and
non-mutable.

Let’s look at a few examples. The code below creates a mutable dictionary and
adds a series of objects, including a string, a number and an array (with various
embedded object types):

NSNumber

 You can’t put scalar types like ints or floats directly into Foundation
collections. You have to “box” them first by wrapping them in an
NSNumber object.

NSNumber versus NSInteger

 NSInteger is nothing more than a synonym for a long integer.
 NSNumber is an Objective-C class, a subclass of NSValue to be
specific. You can create an NSNumber object from a signed or
unsigned char, short int, int, long int, long long int, float, double or
BOOL.
 One of the primary distinctions is that you can use NSNumber in
collections, such as NSArray, where an object is required. For
example, if you need to add a float into an NSArray, you would first
need to create an NSNumber object from the float:

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

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

Introduction to Objective - C

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