Objective-C A Beginner's Dive

transcript

Objective-CA Beginner’s Dive

Saturday, March 9, 13

I Assume

• A Background in Java or C++

• An understanding of OOP

• A basic understanding of C

What is Objective-C?

• Superset of C

• Smalltalk style Object-Oriented

• The Official Language for iOS and OS X

Cool!But before we dive in...

Hello World!//// main.m

#include <stdio.h>#include <Foundation/Foundation.h>

int main(int argc, const char **argv){ NSString *message = @"Hello World!"; printf("%s\n", [message cString]); return 0;}

Hello World!• Objective-C String Literal

• @”Hello World!”

• C Function

• printf()

• main()

• Objective-C message

• [message cString]

//// main.m

#include <stdio.h>#include <Foundation/Foundation.h>

int main(int argc, const char **argv){ NSString *message = @"Hello World!"; printf("%s\n", [message cString]); return 0;}

Ok.Now, on to the teaching!

Objective-C Features

• C Functions

• C Structs

• C Unions

• C Pointers

• C Arrays

• C Everything Else

Objective-C Features

• Objects

• Methods

• Inheritance

• Properties

• Protocols

• Categories

• Data and Collections

• Object Literals

• Object Subscripts

• Forin loops

• Blocks and Closures

• ARC

You Will Learn About

• Making and Using Objects

• Using Foundation Framework Data Types

• Automatic Reference Counting (ARC)

Writing ObjectsA Quick Run-Through

Objects

• Interface

• like Java’s Interface, but every object has one

• placed in the header file

• Implementation

• must fill minimum requirements of Interface

• placed in the implementation file

Example Object//// MyObject.h// Interface

#import <Foundation/Foundation.h>

@interface MyObject : NSObject { int _myInt;}

// method declarations go here

//// MyObject.m// Implementation

#import “MyObject.h”

@implementation MyObject

// method implementations go here

Objects (More Info)

• Objects have Instance Variables (ivars)

• No Class variables, use C static globals

• No enforced “public” and “private”

• Object Instances can be of type:

•Class *

Object Methods

• Class Method or Instance Method

• Variables are “named” in the method’s signature (fancy word for method name)

• Default return and variable type is id

Format of a Method

+/- (return type)methodName;

+/- (return type)methodWithVar:(var type)var;

+/- (return type)methodWithVar1:(var type)var1 ! ! ! ! ! ! ! ! var2:(var type)var2;

Method Examples//// MyObject.m// Implementation

// setter- (void)setMyInt:(int)myInt { _myInt = myInt;}

// getter- (int)myInt { return _myInt;}

//// MyObject.h// Interface

// setter- (void)setMyInt:(int)myInt;

// getter- (int)myInt;

Object Method Calling

• Not like C, C++, or Java

• Based on Smalltalk message passing

• The Square Brackets [] are your friend!

[object method];[object methodWithVar:value];[object methodWithVar1:val1 ! ! ! ! ! ! ! ! var2:val2];

Testing Responsiveness to a Selector

• The name of a method is its SelectorSEL mySelector = selector(myMethodWithParameter:)

• Every object inherits respondsToSelector:

• Takes the selector to test for

• Returns YES when the object can respond to that method

• Returns NO when the object cannot respond to that method

Object Constructor

• Not a special method (unlike Java)

• Just an instance method to set up the Object’s Instance Variables

• Generally named init

Generic Constructor

- (id)init { if (self = [super init]) { // initialize variables } return self;}

Constructor Example

// default constructor- (id)init { // calls my custom constructor return [self initWithInt:0];}

// custom constructor- (id)initWithInt:(int)myInt { if (self = [super init]) { _myInt = myInt; } return self;}

Inheritance

• Single Inheritance from Objects

• Method Overloading Supported

• Superclass defined in the Interface

• Super class referenced with super

Inheritance Example//// MyObject.m// Implementation

- (id)init { if (self = [super init]) { _myInt = 5; } return self;}

- (id)init;

Properties

• Syntactic sugar for variable, accessor, and mutator declarations all-in-one

• Properties are declared in the Interface and synthesized in the Implementation

• Let you use “dot syntax” with getters and setters! myObjectInstance.myInt = 5;! int x = myObjectInstance.myInt;

Property Example//// NSString+Reverse.m// Category

#import "NSString.h"

// creates variable _myInt// creates getter myInt// creates setter setMyInt:@synthesize myInt = _myInt;

//// NSString+Reverse.h// Category

@interface MyObject : NSObject

@property (assign, nonatomic) int myInt;

Categories

• The ability to add new methods to an Object

• Changes apply to all instances of the object

• Overwrites any methods that already exist in the class

• Cannot add Properties and ivars

• Can be dangerous

Example Category//// NSString+Reverse.m// Category

#import "NSString+Reverse.h"#import <stdlib.h>

@implementation NSString (Reverse)

- (NSString *)reverse { int length = [self length]; char *newString = ! ! calloc(length+1, sizeof(char)); int current = 0; const char *cstr = [self cString]; for (int i=length-1; i >= 0; i--) { newString[current] = cstr[i]; current++; } NSString *new = ! ! [NSString stringWithCString:newString]; free(newString); return new;}

//// NSString+Reverse.h// Category

@interface NSString (Reverse)

- (NSString *)reverse;

Class Extension

• Like Categories, but with the ability to add ivars and Properties

• Implementations of methods required in the main Implementation block

• Can be used to declare “private” methods

Class Extension Example

//// MyObject.m

// Class Extension@interface MyObject ()

@property (assign) int myPrivateInt;

// Implementation@implementation MyObject

@synthesize myPrivateInt = _myPrivateInt;

// more implementation here

Ok.Now, lets take a look at MyObject

MyObject//// MyObject.m

#import "MyObject.h"

@interface MyObject ()@property (assign, nonatomic) int myPrivateInt;@end

@synthesize myInt = _myInt;@synthesize myPrivateInt = _myPrivateInt;

- (id)init { return [self initWithInt:0];}

- (id)initWithInt:(int)myInt { if (self = [super init]) { _myInt = myInt;! ! _myPrivateInt = 5; } return self;}

- (NSString *)stringMyInt { return [NSString stringWithFormat:@"%d", _myInt];}

//// MyObject.h

#import <Foundation/Foundation.h>#import "MyProtocol.h"

@interface MyObject : NSObject <MyProtocol>

// default constructor- (id)init;

// custom constructor- (id)initWithInt:(int)myInt;

GreatBut how do I use it?

Using MyObject

MyObject *obj = [[MyObject alloc] init];obj.myInt = 5;NSLog(@"%i\n", obj.myInt);

MyObject *other = [MyObject alloc];other = [other initWithInt:5];[other setMyInt:10];NSLog(@"%i\n", [obj myInt]);

Using MyObject• alloc class method

• use of init and initWithInt: methods

• splitting up the alloc and init method calls

• use of dot syntax and generated methods

• NSLog() for printing messages to the console

MyObject *obj = [[MyObject alloc] init];obj.myInt = 5;NSLog(@"%i\n", obj.myInt);

MyObject *other = [MyObject alloc];other = [other initWithInt:5];[other setMyInt:10];NSLog(@"%i\n", [obj myInt]);

One More ThingImports and Forward Declarations

Imports

• Objective-C introduces the #import Preprocessor Directive#import "MyHeaderFile.h"

#import <SystemHeader.h>

• Guarantees the file is only included once

• Better Practice than C’s #include

Forward Declarations

• A Promise to the Compiler that a Class or Protocol will exist at Compile Time

@class PromisedObject;

@protocol PromisedProtocol;

• Minimizes the amount of #includes

Objective-C DataWhat you get with the Foundation Framework

• Represents the absence of an object

• Messages passed to nil return nil

• nil is for objects, NULL is for C pointers

YES and NO

• Boolean type BOOL for “true” and “false”

• Use BOOL with YES and NO

• Dont use _Bool or bool with true and false from stdbool.h

NSNumber

• Object Representation for Integers, Booleans, Floats, Doubles, Characters, etc.

• Object Literal Syntax@1 @1.0 @YES @NO @(1+2) @'a'

NSString

• Immutable string

• NSMutableString for mutable strings

• Object Literal Syntax@"Hello World!" @("Hello World!")

NSArray

• Immutable object array

• NSMutableArray for mutable arrays

• Object Literal Syntax@[object1, object2, ..., objectN];

• Object Subscripting Syntaxarray[0] = object;id object = array[0];

NSDictionary

• Immutable object dictionary

• NSMutableDictionary for mutable

• Object Literal Syntax@{key1:value1, key2:value2, ...};

• Object Subscripting Syntaxdictionary[key] = object;id object = dictionary[key];

Implementing Array Subscripting

• Accessing ObjectsobjectAtIndexedSubscript:

• Setting ObjectssetObject:atIndexedSubscript:

• The index subscript must be an integral

Implementing Dictionary Subscripting

• Accessing Object values with key ObjectsobjectForKeyedSubscript:

• Setting Object values with key ObjectssetObject:forKeyedSubscript:

Forin Loops

• Loop over the contents of a collection

• Foundation Framework collectionsNSArray NSDictionary

• Any implementation of the NSFastEnumeration Protocol

• Any Subclass of NSEnumerator

Forin Loops Example

NSArray *array = @[@1, @2, @3, @4, @5];

for (NSNumber *num in array) { NSLog(@"%@", num);}

for (id num in [array reverseObjectEnumerator]) { NSLog(@"%@", num);}

Objective-C Blocks

• Functions you can declare within functions

• Can close-over variables for later use

• Can pass block functions around like data

Block Syntax

• Similar to C Function Pointer Syntax

return_type (^name)(parameter types) =! ^(parameter list) {! ! // do stuff! };

return_type (^name)() = ^{! ! // do stuff! ! // takes no parameters! };

Block Exampleint multiplier = 5;int (^mult)(int) = ^(int num){ return num * multiplier;};

int num = mult(5); // num = 25

__block int number = 0;void (^increment)() = ^{ number++;};

increment(); // number = 1increment(); // number = 2

The block itself has read access to variables defined in the lexical scope at the creation of the block.

The block can close over outside variables and modify their values by declaring the variable with the __block modifier.

Type-Safe Enums• Enums that you can declare the type they enumerate over

• int, char, unsigned char, NSUInteger, ...

• Syntax similar to Object Subclassingtypedef enum MyEnum : NSUInteger { A, B, C} MyEnum;

enum MyUnsignedCharEnum : unsigned char { FIRST, SECOND, THIRD};

typedef enum MyUnsignedCharEnum MyUnsignedCharEnum;

typedef enum : NSUInteger { ONE, TWO, THREE} AnotherEnum;

Automatic Reference Counting

What it is, and what it means to you

Referencing Counting

• Nearly-Manual Memory Management

• Objects have a counter showing how many references are using them

• Retain Objects when you receive them[object retain];

• Release Objects when you’re done using them[object release];

• Objects deallocate themselves when their retain count reaches 0

Automatic Reference Counting (ARC)

• Almost Compile Time Garbage Collection

• Retain and Release messages are added at compile time by the compiler

• ARC manages when to do this for you

• strong variables are Retained when assigned

• weak variables are not Retained on assignment and are zeroed out when deallocated

Retain Cycle

• Two Objects Reference Each Other

• Their retain counts can never reach 0

• Set one of the references to weak to prevent one of the objects from retaining the other and causing a cycle

So there you have itObjective-C thrown at you

Useful References• Programming with Objective-C

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html

http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/ProgrammingWithObjectiveC.pdf

• Objective-C Cheat Sheet and Quick Referencehttp://cdn5.raywenderlich.com/downloads/RW-Objective-C-Cheatsheet-v1_2.pdf

• Coding Guidelines for Cocoahttp://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.html

http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.pdf

• Ry’s Objective-C Tutorialhttp://rypress.com/tutorials/objective-c/index.html

Objective-C A Beginner's Dive

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