Objective-C slides

7/27/2019 Objective-C slides

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Vitor Carreira

ESTG/IPLJuly 24-28, 2012

Leiria, Portugal

Objective-CDay 1 - Session 1


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July 24-28, 2012 iOS Game Development - International Summer School, ESTG/IPL

Quick Introduction

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Requirements

Requirements for this session

Moderate knowledge of the C programminglanguage

Basic knowledge of Object OrientedProgramming concepts (instance vs class,encapsulation, inheritance and polymorphism)

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Why Objective-C?

Selected as the main language by NeXT(company founded by Steve Jobs) to develop

the NeXTStep OS In 1996, Apple acquires NeXT and starts

developing Mac OS X (2001) usingObjective-C

Objective-C is not a choice but rather alegacy :)

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Syntax

Objective-C 2.0 is a superset of the C99 standard. All features of the Clanguage are preserved

Case sensitive, strongly typed, every variable should be declared beforeused, separates function declaration (header file) from function

implementation (implementation file), vars declared inside a functionare local, pass by reference using pointers, etc

Same set of operators and control flow structures New features

Extensions to the C language are identified with the @ symbol

New file extensions: .m (implementation file), .h (header file) Var names: lowerCamelCase Function names: UpperCamelCase

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Data value types

Value types: char, int, float, double

Modifiers: unsigned, short, long By default, are integers are signed

Enumerations: keyword enum

Structures: keyword struct

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Pre-processor

All lines that start with # are executed by the pre-processor. Justto recap:

#define - defines a macro

#if, #else, #endif - conditional includes code or macros #import - includes the file only once (should be used instead

of #include)

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Hello World

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main.m


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Demo timeLets tr it on Xcode

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Classes

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Classes 1/2

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In Objective-C, a class has two parts:

@interface - part where the class interface isdeclared (public instance/class methods andpublic properties). The public interface must beplaced on an header file (.h)

@implementation - part where the methods areimplemented. The implementation must beplaced on a code file (.m)


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Classes 2/2

Some conventions: Class name: UpperCamelCase

Members and properties: lowerCamelCase Objective-C doesnt have namespaces. It is

recommended to use a prefix of 2 to 4 letters

to avoid collisions between class names Some prefixes are already reserved: NS, IB,

CG, etc

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Interface part 1/3

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Example (ComplexNumber.h)

Syntax


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Interface part 2/3

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Every class should be a direct or indirectdescendant ofNSObject

Instance variables (ivars) that are protected orpublic (not recommended) are declared between {} If no scope is given, ivars are @protected.

Available scopes: @public, @protected,

@private and @package

Syntax to declare an instance variable:Type instanceVarName;


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Interface part 3/3

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Method declaration:MethodType (ReturnType)namePart1: (Type)param1

namePart2: (Type)param2, ...;

Method type: Use the - sign for instance methods

Use the + sign for class methods


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Method overloading

Method overloading is not supported Methods with the same name but different

parameter types

How the name of the method is represented?

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// Name = modulus- (double)modulus;

// Name = debug- (void)debug;

// Name = setRadius:phase:- (void)setRadius:(double)aRadius phase:(double)aPhase;

// Name = setRadius:- (void)setRadius:(double)aRadius


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Syntax

Implementation part 1/2

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Example (ComplexNumber.m)

Implementation part 2/2

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Putting all together

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[[ComplexNumber alloc] init] Allocates e initializes a new instance of ComplexNumber An object is always represented by a pointer

main.m


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Getters and setters 1/3

By convention:

The getter method name is equal to the ivar.Theget prefix is not used

The setter has the following name:- (void)set:(IVarType)value

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Getters and setters 2/3

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Properties 1/4

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Objective-C 2.0 has introduced a new syntax to implementgetters and setters called dot syntax

Invoking getters and setters becomes familiar. Some examples:

int age = [person age];

int age = person.age; // getter dot syntax

[person setAge: 39];

person.age = 39;// setter dot syntax

[[person father] setAge: 68];

person.father.age = 68; // setter dot syntax


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Properties 2/4

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Declaring a property: On the interface part use the @propertydirective

@property (optional attributes) PropertyType propertyName


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Properties 3/4

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Synthesizing a property: On the implementation part use the @synthesize directive

@synthesize propertyName[=optional_ivarname];

By default, @synthesize follows the same conventions forgetters and setters and injects the following pair ofmethods:-(void)setIVarName:(Type)value;

-(Type)varIName;


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Properties 4/4

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Easier to type. Easierto remember


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Inheritance 1/2

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Objective-C only supports single inheritance There is no such thing as a protected method

To redefine a parent method you just have toprovide the new implementation (keeping themethod signature)

The keyword selfis a reference for the currentinstance

The keyword super refers to the parent of thecurrent instance


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Inheritance 2/2

In Objective-C method invocation follows the dynamicbinding mechanism. One a message is sent to a receptor:

The runtime checks if the receptor implements the methodinvoked. If the method is founded it is executed.

If the method is not founded, the message is sent to thereceptors parent

The receptor hierarchy is traversed until one of thefollowing conditions is verified:

The method is founded and invoked The method is not found when reaching the hierarchy top and an

exception is thrown

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Some LanguageConsiderations

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Exceptions 1/3

Exception handling is similar to C# or Java

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@try {[f noSuchMethod];

}@catch (NSException *exception) {

NSLog(@Caught %@%@, [exception name], [exception reason]);}@finally {

NSLog(@Always called);

}


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Exceptions 2/3

You can have multiple @catch blocks to catchdifferent types of exceptions

Use @throw to throw an exception

If inside a @catch block the exception instance can beomitted@throw; // Re throws the exception caught

NSException is the base class for all exceptions The runtime doesnt impose this kind of check

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Exceptions 3/3

In languages like C# or Java, exceptions is fairly commonplaceand used intensively to signal errors that affect the executionflow

Exceptions are resource-intensive in Objective-C. You shouldnot use exceptions for general flow-control, or simply tosignify errors. Instead you should use the return value of amethod or function to indicate that an error has occurred,and provide information about the problem in an error object

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Veryimportant


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BOOL vs bool

Objective-C 1.0 didnt have a boolean data type So the type BOOL was introduced to the language with

the macros YES (true) and NO (false)

Objective-C 2.0 is a superset of C99 so it also has theboolean type bool that is almost never used due thelegacy code

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BOOL isLessonComplete = NO;bool newButSeldomUsed = true;


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Static vs dynamic typing

Static typing - the data type is alwaysspecified

ComplexNumber *aNumber;

Dynamic typing - the data type is notspecified. So the variable can represent any

object (equivalent to the void * in C)id anyObject;

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Nil instance

The keyword nil represents a null instance. It can be used: To assign an null object to a variable

aComplexNumber = nil;

As part of a conditionif (aComplexNumber == nil) or if (!aComplexNumber)

As a method argument:[slider setTarget: nil];

As a receptor (message is ignored and returns 0)aComplexNumber = nil;

value = [aComplexNumber modulus];

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Selectors 1/2

SEL is a data type called selector thatrepresents a method (similar to a functionpointer)

A selector is declared using the methods fullname. The following method- (void)setRadius:(double)r phase:(double)p

Could be represented with the followingselector:SEL setAll = @selector(setRadius:phase:);

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Selectors 2/2

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The NSObject class provides several methodsthat work with selectors (e.g.respondsToSelector, performSelector, etc). For

example:

id receiver = self.target;

SEL sel = self.action;

if ([receiver respondsToSelector:sel]) {

[receiver performSelector:sel withObject:self];

}


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Allocation andInitialization

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There is no such thing asconstructors


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Allocation and

initialization 1/5

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Two steps are required to instantiate an class:

1. Allocate memory for the instance.NSObject provides twomethods for that: alloc e allocWithZone

When memory is allocated all ivars are set to zero (objects are set to nil)

2. Initialize the instance with the desired values. The class isresponsible to provide proper initializers to initialize the instance.

By convention (this is very important) all initializers methods muststart with init

All classes that have instance variables should provide at least oneinitializer

AClass *anObject = [[AClass alloc] init];


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Allocation and

initialization 2/5

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An initializer must initialize all ivars of thereceptor and returns its own instance

Exceptionally a different instance or even nil canbe returned by an initializer. So the valuereturned by the initializer should always be kept

By convention, the initializer should be called onlyonce (very important for memory management)

-(id)init {...}


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Allocation and

initialization 2/5

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AClass *anObject = [AClass alloc];[anObject init];[anObject someMethod];

AClass *anObject = [AClass alloc];anObject = [anObject init];[anObject someMethod];

Incorrect Correct

AClass *anObject = [[AClass alloc] init];[anObject someMethod];

Typical code


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Allocation and

initialization 4/5

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Rules to implement an initializer By convention, the method should start with the name init The method must return id

If the class has more than one initializer it is required to state whichone is the designated initializer. This is the initializer that all the otherinitializers should invoke

The designated initializer is the only one to invoke the parent initializer You must assign to self the result of calling the designated or parent

initializer. You should check if the result is nil (exceptionally an initializercan return nil or an different instance of the one currently allocated)

You should not use getters and setters to initialize ivars The initializer must always return self (or nil if an error occurs)


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Allocation and

initialization 5/5

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Example


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Combining allocation

and initialization Some classes combine allocation and

initialization in a single step

This methods are called convenienceconstructors and usually start with the nameof the class

Examples from NSString:

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+ (id)stringWithCString:(const char *)cString encoding:(NSStringEncoding)enc;+ (id)stringWithFormat:(NSString *)format, ...;


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Before moving to the next session.Any questions?

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