1. Introduction to Objective-C and Xcode (Part 2) FA 175 Intro to Mobile App Development

2. Agenda Object-oriented programming OOP in Objective-C – Classes – Instances and objects – Properties and methods

3. Definition of a program, revisited Traditional definition of a program: sequence of instructions to be executed on a computer Under the object-oriented programming (OOP) paradigm: a program, when it executes, is a collection of interacting objects Programming in the OOP paradigm means specifying what data are in these objects and how these objects behave

4. So… what is an object? An object is a thing that has type, identity, state, and behavior – Type: it belongs to a class of similar things – Identity: it is an instance distinct from other objects – State: it has a set of properties that take on values – Behavior: it can act or carry out methods

5. Object examples Light Bulb -state? -behavior ? Bank Account -state? -behavior? Car -state? -behavior ?

6. Class: Light Bulb State – lit or not (on or off) Behavior – turn on – turn off – check whether lit

7. Class: Bank Account State – balance Behavior – deposit – withdraw – inquire balance

8. Class: Car State – distance travelled – gas left Behavior – drive – load gas – check gas level – check odometer

9. Objective-C: interface versus implementation.h file contains an interface declaring properties and methods of a class – the interface is the public façade of an object.m file contains an implementation of the class – code for the methods plus other “private” data – the implementation contains details encapsulated within the object, hidden from users To create files in Xcode while project is open, File->New->File, then choose Obective-C class

10. Interface for the Car class (Car.h) #import @interface Car : NSObject @property double gasLeft; @property int distanceTravelled; -(void) driveDistance:(int) dist; -(void) loadGas:(double) gas; @end

11. Implementation (Car.m) #import "Car.h" @implementation Car -(void) driveDistance:(int) dist { self.distanceTravelled = self.distanceTravelled + dist; double gasUsed = dist/8.5; self.gasLeft = self.gasLeft - gasUsed; } -(void) loadGas:(double) gas { self.gasLeft = self.gasLeft + gas; } @end

12. Using the Car class #import "Car.h"... NSLog(@"Test code for a car object"); Car *myCar = [Car alloc]; [myCar loadGas:50.0]; [myCar driveDistance:10]; NSLog(@"gas: %6.2f, distance: %d", myCar.gasLeft, myCar.distanceTravelled); [myCar driveDistance:94]; [myCar loadGas:10.0]; NSLog(@"gas: %6.2f, distance: %d", myCar.gasLeft, myCar.distanceTravelled);

13. Encapsulation and direct data update Oftentimes, it is appropriate for properties to be “readonly” and updated only through appropriate methods Example: – gasLeft property should be updated only as a result of driveDistance or loadGas – Try adding: myCar.gasLeft = 100.0; at the end of the code, and then print myCar.gasLeft

14. Solution: specify property attributes in.h and.m files In.h file, replace property declarations with: – @property(readonly) double gasLeft; @property(readonly) int distanceTravelled; In.m file, add the following after #import line: – @interface Car() @property(readwrite) double gasLeft; @property(readwrite) int distanceTravelled; @end Notice that direct update of properties are no longer allowed

15. Syntax Property declaration – @property(,…) ; Method declaration (no arguments) – -( ) ; Method declaration (one argument) – -( ) :( ) ;

16. Syntax Object creation/instantiation – = [ alloc]; Referring to a property of an object – From within the class: self. – For an object variable:. Invoking methods: – [ ]; – [ : ];

17. Naming conventions Variable names and method names – Camel case: begin with small letter, capitalize first letters of succeeding words – Examples: distanceTravelled, myCar, loadGas Class names – Capitalize first letters of all words within the name – Examples: BankAccount, Car, LightBulb

18. void The methods loadGas and driveDistance have the following signatures – -(void)driveDistance:(int) dist – -(void)loadGas:(double) gas Here, void means “no return value” Some methods return a value Example: add a method to the Car class with the following signature – -(double) distanceTravelledInMiles

19. Returning a value from a method In Car.h: – -(double) distanceTravelledInMiles; In Car.m – -(double) distanceTravelledInMiles { double miles = self.distanceTravelled*0.62; return miles; } In your test code, – NSLog(@”my car travelled %6.2 miles”, [myCar distanceTravelledInMiles]);

20. Initialization It is common to provide initialization code intended for a newly created object – Set initial values for properties By convention, these methods should begin with the word init Also by convention (and for reasons too technical to discuss at this point), use _ (e.g., _gasLeft) instead of self. (e.g. self.gasLeft), when referring to the properties

21. init method examples -(id) init { self = [super init]; _gasLeft = 0; _distanceTravelled = 0; return self; } -(id) initWithGas:(double) amt { self = [super init]; _gasLeft = amt; _distanceTravelled = 0; return self; }

22. Implementing initialization methods Make sure to place method declarations of init and initWithGas in Car.h Method implementations should be in Car.m In your test code, add the following: – Car *car2 = [[Car alloc] init]; Car *car3 = [[Car alloc] initWithGas:20.0]; [car3 driveDistance:15]; // some code to print gas levels of car2 & car3

23. Summary Introduction to Object-Oriented Programming Objective-C class creation in Xcode (Car class: Car.h, Car.m) OOP concepts tackled – classes, objects, properties, methods, initialization Naming conventions