Introduction to Objective - C

Introduc)on to Objec)ve-‐C Jussi Pohjolainen Tampere University of Applied Sciences

QUICKSTART

Background •  Objec)ve-‐C is layered on top of the C – language –  Based on SmallTalk-‐80 –  Designed in early 1980s •  NeXT SoLware licensed Objec)ve-‐C in 1988 •  Apple Computer acquired NeXT in 1996 •  Today: na)ve language for developing Mac OS X -‐ and iPhone -‐ apps

Class Declara)on: MyPoint.h

Class Implementa)on: MyPoint.m

Test the Class: main.m

Simple makeﬁle MyPoint : MyPoint.m main.m clang -fno-objc-arc -framework foundation MyPoint.m main.m -o MyPointrun : ./MyPointclean : rm MyPoint

Compiling and Running TB308POHJUS-L-2:point pohjus$ ls -altotal 32drwxr-xr-x 6 pohjus staff 204 18 Tou 15:32 .drwxr-xr-x 3 pohjus staff 102 18 Tou 14:52 ..-rw-r--r--@ 1 pohjus staff 196 18 Tou 15:25 MyPoint.h-rw-r--r--@ 1 pohjus staff 268 18 Tou 15:23 MyPoint.m-rw-r--r--@ 1 pohjus staff 339 18 Tou 15:25 main.m-rw-r--r--@ 1 pohjus staff 120 18 Tou 15:14 makefileTB308POHJUS-L-2:point pohjus$ makeclang -fno-objc-arc -framework foundation MyPoint.m main.m -o MyPointTB308POHJUS-L-2:point pohjus$ make run./MyPoint2009-05-18 15:32:46.339 MyPoint[8725:807] X = 0 and Y = 02009-05-18 15:32:46.341 MyPoint[8725:807] X = 8 and Y = 7TB308POHJUS-L-2:point pohjus$

Instan)a)ng an Object // Declare a pointer to the objectMyPoint* point;// Allocate memory for the objectpoint = [MyPoint alloc];// Initialize the objectpoint = [point init];

Instan)a)ng an Object: One Liner // Allocate and initialize the objectMyPoint* point1 = [[MyPoint alloc] init];

Messages (Methods) - (void) setX: (int) n;! method type: return type selector name argument type argument + = class method name -‐ = object method

Messages, with Two Arguments Declara=on - (void) setXAndY: (int) x: (int) yUsage [object setXAndY: 5: 6];Declara=on, be@er way - (void) setX: (int) x andY: (int) yUsage [object setX: 5 andY: 6];

EXERCISES

MEMORY MANAGEMENT AND OBJECT LIFECYCLE

Memory Handling in General •  When allocate something it must be released –  Memory consump)on •  In Java, garbage collector takes care of the release. Separate thread looking for objects that can be released •  In Obj-‐C and C/C++, programmer is responsible about the release.

About Pointers •  int a = 5; –  Holds one integer value –  The integer is stored in some memory address •  Where? –  You can get the memory address by using & in front of the variable: &a

Example of Memory Address int main(int argc, char *argv[]){ int a = 5; // prints 5 NSLog(@"%i", a); // prints something like 0x7fff5fbff9cc NSLog(@"%p", &a); return 0;}

Pointers •  Pointer is a variable that stores memory address •  int a; –  holds integer variable •  int* b; –  holds memory address that points to integer value •  int* b = &a; –  Now b has the memory address of a

Example about Pointers #import "MyPoint.h"int main(int argc, char *argv[]){ int a = 5; // Store as memory address to variable b int *b = &a; // prints 5 NSLog(@"%i", a); // prints something like 0x7fff5fbff9cc NSLog(@"%p", &a); // prints something like 0x7fff5fbff9cc NSLog(@"%p", b); // prints 5 NSLog(@"%i", *b); return 0;}

What is the result now? int main(int argc, char *argv[]){ int a = 5; // Store as memory address to variable b int *b = &a; if(b == &a) { NSLog(@"Do we go here?"); } if(*b == a) { NSLog(@"What about here?"); } return 0;}

What is the result now? int main(int argc, char *argv[]){ int a = 5; int *b = &a; *b = 10; NSLog(@"%i", a); return 0;}

Memory Areas •  Memory can be divided into three categories 1.  Global or sta)c 2.  Stack 3.  Heap

Sta)c •  When something is in sta)c memory, its there all the )me when app is running •  So when star)ng the app, the memory is allocated an when the app closes, the memory is deallocated •  The variable is stored in the same memory address all the )me.

Example of Sta)c Memory int thisIsStoredInStaticMemory = 5;int main(int argc, char *argv[]){ static int thisIsAlsoStoredInStaticMemory = 10; NSLog(@"My number = %i", thisIsStoredInStaticMemory); NSLog(@"My number = %i", thisIsAlsoStoredInStaticMemory); return 0;}

Stack -‐ Memory •  Stack – memoryarea is usually small •  If you put too much "stuﬀ" into stack, you might get stack overﬂow •  Local variables are stored in stack! •  Variables are released when out of scope

Example of Stack Memory int main(int argc, char *argv[]){ int stack1 = 3; NSLog(@"My number = %i", stack1); if(YES) { int stack2 = 4; NSLog(@"My number = %i", stack2); // stack2 is released from memory now. } // stack1 is released from memory now. return 0;}

Example of Stack Memory int main(int argc, char *argv[]){ if(YES) { int stack2 = 4; } // Does this work? NSLog(@"My number = %i", stack2); return 0;}

Heap -‐ Memory •  Heap – memory is the large memory area where almost all of the objects go. •  Programmer is responsible for releasing the objects!

Example of Heap -‐ memory #import "MyPoint.h"int main(int argc, char *argv[]){ MyPoint* point = [MyPoint alloc]; //... [point release]; return 0;}

Crea)ng a Object •  The crea)on of an object is done in two parts •  1) Alloca)ng memory –  MyPoint* point = [MyPoint alloc];•  2) Ini)alize object state –  point = [point init];•  Combined –  MyPoint* point = [[MyPoint alloc] init];•  ó –  MyPoint* point = [MyPoint new];

What happens in Memory? •  What happens in here? –  MyPoint* p = [MyPoint alloc];•  Two things! –  MyPoint* p; –  p = [MyPoint alloc];•  p is in stack – memory! •  MyPoint object is in heap memory!

Problem? #import "MyPoint.h"int main(int argc, char *argv[]){ if(YES) { MyPoint* point = [MyPoint alloc]; } [point release]; return 0;}

Problem? #import "MyPoint.h"int main(int argc, char *argv[]){ MyPoint* point = [MyPoint alloc]; point = [MyPoint alloc]; [point release]; return 0;}

init-‐method? •  init-‐method is implemented in NSObject•  You can however implement your own init-‐ method - (id) initWithName: (NSString*) aName{ if(self = [super init]) { name = aName; } return self;}

Other init-‐methods •  Like in Java and C++, one can have mul)ple constructors •  In Obj-‐C, one can have mul)ple init-‐methods -  (id) init-  (id) initWithX: (int) aX;-  (id) initWithX: (int) aX andY: (int) aY

Deallocing Object and Reference Count •  Rules about reference coun=ng –  When object is created its reference count is set to 1 –  Incremen)ng the reference count: •  [point retain]; –  Decreasing reference count •  [point release]; –  When reference count reaches to 0, dealloca=ng occurs •  ARC will save you from this!

Reference Count Example #import <Foundation/Foundation.h>@interface Cat : NSObject{}- (void) printMyRetainCount;@end@implementation Cat- (void) printMyRetainCount{ NSLog(@"Retain count = %i", [self retainCount]);}@end

Reference Count Example int main(int argc, char *argv[]){ Cat* myCat = [[Cat alloc] init]; [myCat printMyRetainCount]; // Retain count = 1 Cat* reference = myCat; [reference retain]; [myCat printMyRetainCount]; // Retain count = 2 [myCat release]; [myCat printMyRetainCount]; // Retain count = 1 [myCat release]; // Deallocation return 0;}

dealloc•  You can implement a dealloc method (void) dealloc{ // Some code [super dealloc];}

Reference Count Example #import <Foundation/Foundation.h> int main(int argc, char *argv[]) {@interface Cat : NSObject ....{ Cat* mirri = [[Cat alloc] init]; NSString* name; [mirri setName: someName];} // What happens now? [someName release];- (void) setName: (NSString *) theName }@end Since reference count is 0, dealloca)on occurs. @implementation Cat This means, that the Cat does not have a name - (void) setName: (NSString *) theName anymore. { name = theName;}@end

Reference Count Example #import <Foundation/Foundation.h> int main(int argc, char *argv[]) {@interface Cat : NSObject ....{ Cat* mirri = [[Cat alloc] init]; NSString* name; [mirri setName: someName];} // What happens now? [someName release];- (void) setName: (NSString *) theName }@end Since reference count is 1, dealloca)on does not @implementation Cat occur and the Cat s)ll has its name. - (void) setName: (NSString *) theName{ [name release]; name = theName; [name retain];}@end

Copying a Object - (void)setName:(NSString *)theName} [name release]; name = [theName copy];}

Autorelease Pool •  Every Founda)on program must set up autorelease pool for the Founda)on objects •  Pool keeps track of your objects for later release –  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; –  ... –  [pool drain];

Method Names •  If method name includes alloc or copy, it returns a object that must be released –  // Must be released –  NSObject* object = [[NSObject alloc] init]; –  // Must be released –  NSObject* copy = [object copy]; –  // Do not have to release –  NSMutableString* string = [NSMutableString string];•  The above string is released in Autorelease Pool!

Problem // Programmer A code[[someObject giveCat] eat];// Programmer B code- (Cat*) giveCat{ // Must be released! Cat* myCat = [[Cat alloc] init]; // But where? Should the programmer who calls this method be // responsible for deallocation of the Cat? How does the programmer // know this? return myCat;}..

Solu)on // Programmer A code[[someObject giveCat] eat];// Programmer B code- (Cat*) giveCat{ // Must be released! Cat* myCat = [[Cat alloc] init]; // But where? When autopool is drained! [myCat autorelease]; return myCat;}

Delayed Release •  Autorelease means "send release message later". •  Release message is sent when Autorelease Pool is released •  Autorelease Pool is created and released in UIKit programs automa=cally! –  Pool is created at the beginning of an event cycle –  Pool is released at the end of an event cycle

Autorelease Pools Event loop Pool released Pool created App Loaded Wait for Event Handle event Exit app

Autorelease Pool Usage // Method- (Cat*) giveCat{ // Must be released! Cat* myCat = [[Cat alloc] init]; // But where? When autopool is drained! [myCat autorelease]; return myCat;}// UsageCat* someCat = [object giveCat];// someCat will be released in some time, so if you want to hold it, use// retain[someCat retain];

Rules •  If method name contains "alloc", "new" or "copy", you must remember to use release or autorelease –  Example: alloc, newObject, mutableCopy•  If you retain something, you must use release or autorelease•  Instance Variables: retain or copy •  autorelease means "send release later"

Cat.h #import <Foundation/Foundation.h>@interface Cat : NSObject{ @private NSString* name;}- (id) initWithName: (NSString*) aName;- (void) setName: (NSString*) aName;- (NSString*) getName;- (void) dealloc;@end

Cat.m #import "Cat.h" - (void) setName: (NSString*) aName {@implementation Cat if(aName != name) {- (id) initWithName: (NSString*) aName [name release];{ name = aName; if(self = [super init]) [name retain]; { } [self setName: aName]; } } return self;} - (void) dealloc {- (NSString*) getName [name release];{ [super dealloc]; return name; }} @end

main.m #import "Cat.h" // Get the name NSString* name = [cat getName];int main(int argc, char *argv[]){ // Print the name NSAutoreleasePool *pool = NSLog(name); [[NSAutoreleasePool alloc] init]; // Release name and cat // Create the string [cat release]; NSString* catName = [[NSString alloc] initWithString: @"Jack"]; [pool drain]; // Create cat with the string return 0; Cat* cat = [[Cat alloc] } initWithName: catName]; // Just testing. This does not deallocate catName! [catName release];

ARC to the rescue! MANAGING MEMORY WITH ARC

ARC? •  ARC (Automa=c Reference Coun=ng) –  Compiler does automa/c reference coun/ng by examining the source code and then add the retain and release messages to objects •  Not garbage collec)on, no background process of dealloca)on of objects! •  Inserts retain and release statements based on some ﬁxed rules •  OS X 10.7 and iOS 5 for all features

Object lose owners // Scenario 1Person* jack = [[Person alloc] init];jack = [[Person alloc] init];// Scenario 2Person* tina = [[Person alloc] init];tina = nil;// Scenario 3if(yes) { Person* dave = [[Person alloc] init];}

Some Fixed rules •  If object is allocated and local to method, release statement is added near the end of that method •  If allocated object is class a@ribute, release is added to dealloc •  If the object is return value, it gets an autorelease statement

Guidelines •  Don’t call! –  retain, release, retainCount, autorelease or dealloc•  You must use @autoreleasepool syntax •  You must enable ARC –  clang -fobjc-arc -framework foundation Car.m Motor.m main.m -o App

makeﬁle MyPoint : Car.m Motor.m main.m clang -fobjc-arc -framework foundation Car.m Motor.m main.m-o Apprun : ./Appclean : rm App

main.m

motor.h

motor.m

PROPERTIES

Objec)ve-‐C 2.0: @property

Objec)ve-‐C 2.0: @synthesize

Objec)ve-‐C 2.0: Dot Syntax

Autosynthesize

Property Declara)on Aoributes: Writability •  You can decorate a property with aoributes, example: –  @property (readonly) int x;•  readwrite –  Indicates that the property is read/write. Default •  readonly –  Only read –  Generates only geoer method

Seoer Seman)cs •  assign –  Simple seoer. Default. •  weak –  Non-‐owning rela)onship with an object –  If object is deallocated, the property is set to nil •  strong –  Owning rela)onship with an object •  copy –  Speciﬁes that a copy of the object should be used for assignment

Seoer Seman)cs Examples // assignproperty = newValue;// copyif (property != newValue){ [property release]; property = [newValue copy];}

Atomicity •  nonatomic –  Speciﬁes that accessor are non-‐atomic. •  Proper)es are atomic by default: –  [_internal lock]; –  id result = [[value retain] autorelease]; –  [_internal unlock]; –  return id;

Motor.h

Main.m

Change! Car.h

STRINGS

About Strings •  C String –  char * // Array of characters•  NSString –  Object, that holds array of Unicode characters –  Is immutable, contents cannot be changed aLerwards! •  NSMutableString –  String that can be modiﬁed aLerwards

Crea)ng Strings // Simple wayNSString *temp1 = @"Hello World!";// Appending, notice that this produces new stringNSString *beginning = @"beginning";NSString *alphaAndOmega = [beginning stringByAppendingString:@" and end"];

Formapng •  Formapng –  NSString *string1 = [NSString stringWithFormat:@"A string: %@, a float: %1.2f", @"string", 31415.9265]; –  // string1 is "A string: string, a float: 31415.93"•  Format Speciﬁers? –  http://developer.apple.com/iphone/ library/documentation/Cocoa/Conceptual/ Strings/Articles/formatSpecifiers.html#// apple_ref/doc/uid/TP40004265-SW1

NSString methods •  See: –  http://developer.apple.com/documentation/ Cocoa/Reference/Foundation/Classes/ NSString_Class/Reference/NSString.html

NSMutableString methods •  NSMutableString inherites NSString•  With NSMutableString you can modify the string with these methods – appendFormat: – appendString: – deleteCharactersInRange: – insertString:atIndex: – replaceCharactersInRange:withString: – replaceOccurrencesOfString:withString:options:range: – setString:

PROTOCOLS

Protocols? •  Compared to Java, protocols are interfaces •  You deﬁne methods that some object must implement

Using Protocols // MyProtocolName.h// Notice that the protocol inherites NSObject// protocol!@protocol MyProtocolName <NSObject> //Method declarations go here@end// MyObject@interface Class: NSObject <MyProtocolName>

Protocol as Variable •  In Java –  MyInterface object = new MyObject();•  In Obj-‐C –  id<MyProtocolName> object = [[MyObject alloc] init];•  As a method argument –  (void) doSomethingWithThisObject: (id<MyProtocolName>) aObject•  ID is a predeﬁned pointer type for an arbitrary object

FOUNDATION CLASSES

NSObject •  NSObject is the root class of Most Obj-‐C classes •  Crea)ng, copying, dealloca)ng objects

Collec)ons •  Array: Ordered Collec/ons •  Dic)onary: Collec/ons of Keys and Values •  Set: Unordered Collec/ons of Objects •  Counted Sets: Unordered Collec/on of Indis/nct Objects •  Enumera)on: Traversing a Collec/ons Elements •  Mutable and immutable versions!

Other Classes •  NSNumber, wrapper for standard number types •  NSDate, NSCalendarDate

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Introduction to Objective - C

by Jussi Pohjolainen on Feb 24, 2010

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