Introduction

Objective-C is a strict superset of C, which means that it’s possible to seamlessly combine both languages in the same source file.

Basic Syntax

Comments

          // This is an inline comment

          /* This is a block comment.
          It can span multiple lines. */
        

Variables

  // main.m
  #import <Foundation/Foundation.h>

  int main(int argc, const char * argv[]) {
    @autoreleasepool {
      double odometer = 9200.8;
      int odometerAsInteger = (int)odometer;

      NSLog(@"You've driven %.1f miles", odometer);        // 9200.8
      NSLog(@"You've driven %d miles", odometerAsInteger); // 9200
    }
    return 0;
  }
        

Constants

          double const pi = 3.14159;
          pi = 42001.0;               // Compiler error
        

Operators

• Arithmetic Operators
• Relational Operators
• Logical Operators
• Bitwise Operators
• Assignment Operators
• Misc Operators

Data Types

C Primitives

• void
• Integer Types
• Floating-Point Types

id

The id type is the generic type for all Objective-C objects. Like a void pointer, it can store a reference to any type of object.

          id mysteryObject = @"An NSString object";
          NSLog(@"%@", [mysteryObject description]);
          mysteryObject = @{@"model": @"Ford", @"year": @1967};
          NSLog(@"%@", [mysteryObject description]);
        

Class

Objective-C classes are represented as objects themselves, using a special data type called Class. This lets you, for example, dynamically check an object’s type at runtime.

          Class targetClass = [NSString class];
          id mysteryObject = @"An NSString object";
          if ([mysteryObject isKindOfClass:targetClass]) {
            NSLog(@"Yup! That's an instance of the target class");
          }
        

SEL

The SEL data type is used to store selectors, which are Objective-C’s internal representation of a method name.

          SEL someMethod = @selector(sayHello);
        

Defined in Foundation Framework

• NSSet
• NSNumber
• NSArray
• NSDecimalNumber
• NSDictionary
• NSString
• Dates

Creating Objects

standard alloc/init pattern

factory methods provided by Foundation Framework

Comparing Objects

• Pointer comparison uses the == operator to see if two pointers refer to the same memory address (i.e., the same object).
• Value comparison uses methods like isEqualToNumber: to see if two objects represent the same value. It is possible for different objects to compare equal with this kind of comparison

Flow Control & Loop

Flow Control

          int modelYear = 1990;
          if (modelYear < 1967) {
            NSLog(@"That car is an antique!!!");
          } else if (modelYear <= 1991) {
            NSLog(@"That car is a classic!");
          } else if (modelYear == 2013) {
            NSLog(@"That's a brand new car!");
          } else {
            NSLog(@"There's nothing special about that car.");
          }
        

        // Switch statements (only work with integral types)
        switch (modelYear) {
          case 1987:
            NSLog(@"Your car is from 1987.");
            break;
          case 1988:
            NSLog(@"Your car is from 1988.");
            break;
          case 1989:
          case 1990:
            NSLog(@"Your car is from 1989 or 1990.");
            break;
          default:
            NSLog(@"I have no idea when your car was made.");
            break;
        }
        

Loop

  // For-in loops ("Fast-enumeration," specific to Objective-C)
  NSArray *models = @[@"Ford", @"Honda", @"Nissan", @"Porsche"];
  for (id model in models) {
    NSLog(@"%@", model);
  }
        

Class

Objective-C is similar to C++ in that it abstracts a class’s interface from its implementation. An interface declares the public properties and methods of a class, and the corresponding implementation defines the code that actually makes these properties and methods work.

Interfaces

          // Car.h
          #import <Foundation/Foundation.h>

          @interface Car : NSObject {
            // Protected instance variables
          }

          @property (copy) NSString *model;

          - (void)drive;

          @end
        

Implementations

          // Car.m
          #import "Car.h"

          @implementation Car {
            // Private instance variables
            double _odometer;
          }

          @synthesize model = _model;    // Optional for Xcode 4.4+

          - (void)drive {
            NSLog(@"Driving a %@. Vrooooom!", self.model);
          }

          @end
        

Instantiation and Usage

          // main.m
          #import <Foundation/Foundation.h>
          #import "Car.h"

          int main(int argc, const char * argv[]) {
          @autoreleasepool {
            Car *toyota = [[Car alloc] init];

            [toyota setModel:@"Toyota Corolla"];
            NSLog(@"Created a %@", [toyota model]);

            toyota.model = @"Toyota Camry";
            NSLog(@"Changed the car to a %@", toyota.model);

            [toyota drive];
          }
          return 0;
          }
        

Class Methods and Variables

          // Car.h
          + (void)setDefaultModel:(NSString *)aModel;
        

          // Car.m
          #import "Car.h"

          static NSString *_defaultModel;

          @implementation Car {
          ...

          + (void)setDefaultModel:(NSString *)aModel {
            _defaultModel = [aModel copy];
          }

          @end
        

          // main.m
          [Car setDefaultModel:@"Nissan Versa"];
        

Custom Initialization Methods

          // Car.h
          - (id)initWithModel:(NSString *)aModel;
        

          // Car.m
          - (id)initWithModel:(NSString *)aModel {
            self = [super init];
              if (self) {
              // Any custom setup work goes here
                _model = [aModel copy];
                _odometer = 0;
            }
            return self;
          }

          - (id)init {
            // Forward to the "designated" initialization method
            return [self initWithModel:_defaultModel];
          }
        

Class-Level Initialization

  // Car.m
  + (void)initialize {
    if (self == [Car class]) {
      // Makes sure this isn't executed more than once
      _defaultModel = @"Nissan Versa";
    }
  }
        

  // main.m
  #import <Foundation/Foundation.h>
  #import "Car.h"

  int main(int argc, const char * argv[]) {
    @autoreleasepool {
      // Instantiating objects
      Car *nissan = [[Car alloc] init];
      NSLog(@"Created a %@", [nissan model]);

      Car *chevy = [[Car alloc] initWithModel:@"Chevy Corvette"];
      NSLog(@"Created a %@, too.", chevy.model);
    }
    return 0;
  }
        

Property

@property

      // Car.h
      #import <Foundation/Foundation.h>
      @interface Car : NSObject
      @property BOOL running;
      @end
        

      // Car.m
      #import "Car.h"
      @implementation Car
      @synthesize running = _running;    // Optional for Xcode 4.4+
      @end
        

      - (BOOL)running {
        return _running;
      }
      - (void)setRunning:(BOOL)newValue {
        _running = newValue;
      }
        

  // main.m
  #import <Foundation/Foundation.h>
  #import "Car.h"

  int main(int argc, const char * argv[]) {
    @autoreleasepool {
      Car *honda = [[Car alloc] init];
      honda.running = YES;                // [honda setRunning:YES]
      NSLog(@"%d", honda.running);        // [honda running]
    }
    return 0;
  }
        

honda.running code actually calls setRunning: when you assign a value to it and the running method when you read a value from it.

getter= and setter= Attributes

If you don’t like @property’s default naming conventions, you can change the getter/setter method names with the getter= and setter= attributes.

  @property (getter=isRunning) BOOL running;
        

  Car *honda = [[Car alloc] init];
  honda.running = YES;            // [honda setRunning:YES]
  NSLog(@"%d", honda.running);    // [honda isRunning]
  NSLog(@"%d", [honda running]);  // Error: method no longer exists
        

readonly Attribute

The readonly attribute is an easy way to make a property read-only. It omits the setter method and prevents assignment via dot-notation, but the getter is unaffected.

        #import <Foundation/Foundation.h>

        @interface Car : NSObject

        @property (getter=isRunning, readonly) BOOL running;

        - (void)startEngine;
        - (void)stopEngine;

        @end
        

          // Car.m
          #import "Car.h"

          @implementation Car

          - (void)startEngine {
            _running = YES;
          }
          - (void)stopEngine {
            _running = NO;
          }

          @end
        

          Car *honda = [[Car alloc] init];
          [honda startEngine];
          NSLog(@"Running: %d", honda.running);
          honda.running = NO;   // Error: read-only property
        

nonatomic Attribute

          @property (nonatomic) NSString *model;
        

Method

Naming Conventions

Examples

// Accessors
- (BOOL)isRunning;
- (void)setRunning:(BOOL)running;

// Action methods
- (void)driveForDistance:(double)theDistance;
- (void)driveFromOrigin:(id)theOrigin toDestination:(id)theDestination;

// Constructor methods
- (id)initWithModel:(NSString *)aModel;
- (id)initWithModel:(NSString *)aModel mileage:(double)theMileage;

// Comparison methods
- (BOOL)isEqualToCar:(Car *)anotherCar;
        

Calling Methods

          [porsche initWithModel:@"Porsche"];
          [porsche initWithModel:@"Porsche" mileage:42000.0];
        

Nested Method Calls

          [[Car alloc] init];
        

Protected and Private Methods?

Selectors

Selectors are Objective-C’s internal representation of a method name. They let you treat a method as an independent entity, enabling you to separate an action from the object that needs to perform it.

  // main.m
  #import <Foundation/Foundation.h>
  #import "Car.h"

  int main(int argc, const char * argv[]) {
    @autoreleasepool {
      Car *porsche = [[Car alloc] init];
      porsche.model = @"Porsche 911 Carrera";

      SEL stepOne = NSSelectorFromString(@"startEngine");
      SEL stepTwo = @selector(driveForDistance:);
      SEL stepThree = @selector(turnByAngle:quickly:);

      // This is the same as:
      // [porsche startEngine];
      [porsche performSelector:stepOne];

      // This is the same as:
      // [porsche driveForDistance:[NSNumber numberWithDouble:5.7]];
      [porsche performSelector:stepTwo
      withObject:[NSNumber numberWithDouble:5.7]];

      if ([porsche respondsToSelector:stepThree]) {
        // This is the same as:
        // [porsche turnByAngle:[NSNumber numberWithDouble:90.0]
        //              quickly:[NSNumber numberWithBool:YES]];
        [porsche performSelector:stepThree
        withObject:[NSNumber numberWithDouble:90.0]
        withObject:[NSNumber numberWithBool:YES]];
      }
      NSLog(@"Step one: %@", NSStringFromSelector(stepOne));
    }
    return 0;
  }