Cocoa Development

... an introduction

by Lieven Dekeyser

Part 1

• What is Cocoa?
• Objective-C
• C
• Foundation Framework

What is Cocoa

• Collection of frameworks
  • Mac OS X: Foundation and AppKit
  • iPhone OS: Foundation and UIKit
• Frameworks enable tools
  • Interface builder (views)
  • Interface builder (controllers)
  • Xcode (code)
  • Xcode (data models)

Cocoa on Mac OS X

Cocoa on Mac OS X

Cocoa on iPhone OS

Objective-C

• Geschiedenis:
  • Begin jaren '80: Brad Cox at StepStone
  • 1988: gekozen door NeXTStep
  • 1992: deel van de GCC collectie
  • Heden: GNUStep, Mac OS X, iPhone OS, Cocotron, inspiratie voor Objective-J,...
• Net zoals C++ een objectgeoriënteerde uitbreiding op ANSI C, maar:
  • Syntax geïnspireerd op SmallTalk
  • Slechts 1 echte syntactische toevoeging aan C
  • Dynamic typing and linking

C: Hello World

#include <stdio.h>

int main(int argc, const char * argv[])
{
    printf("hello world");
    return 0;
}

C: Hello World, explained

• #include <stdio.h>

  Tells the preprocessor to replace this line with the the contents of the file stdio.h

• int main(int argc, const char * argv[])

  • int: return type
  • main: entry point of the application. Every C-application needs a main function
  • int argc: first argument of main, number of elements in argv
  • const char * argv[]: array of elements of type const char *

C: Types

• int: Native integer format of the architecture
• char: Character, usually 1 byte
• float, double: Single and double precision floating point numbers
• enum: Enumeration
• struct: "aggregates a fixed set of labelled objects, possibly of different types, into a single object"
• union: "a datastructure that holds a value that may have any of several representations or formats"

C: Pointers

• 
• Seriously, pointers aren't scary!

C: Pointers

• A pointer is just a variable that contains an address of a location in memory
• In other words, it points to that location
• Addresses are expressed as numbers. Size is architecture-dependent.
• Use the & operator to get the address of a variable
• Use the * operator to get the data a pointer points to (dereferencing)

C: Pointer gotchas

• Pointers allow you to manipulate any memory location in any way you'd want
• Dereferencing an invalid pointer can lead to crashes
• Stick to your own memory.
• Give pointers (but actually any variable) an initial value!
• Always reset pointers you don't need anymore to NULL

C: Strings

• There is no string type in C
• Just pointers to a location in memory which is interpreted as a 0-termintated block of characters.
• [address]->['a','b','c','\0'];

C: Memory allocation

• Variables are allocated on the stack.
  • easy
  • out of scope => deallocation
  • limited in size (architecture-dependent)
  • not flexible
• You can also allocate memory on the heap
  • very flexible
  • limited only by available (virtual) memory
  • manual (de)allocation (malloc/free)
  • requires discipline (who "owns" the memory?)

Objective-C

• C: basically a cross-architecture assembler with a nicer syntax
• You can write modern code in it though, see CoreFoundation
• Objective-C: let's get object-oriented!

Objective-C: Class interface

@interface Circle : NSObject
{
    float mX;
    float mY;
    float mRadius;
}

- (id)init;
- (id)initWithRadius:(float)inRadius x:(float)inX y:(float)inY;

- (float)surface;

@end

Objective-C: Class implementation

@implementation Circle

- (id)init
{
	return [self initWithRadius:0.0f x:0.0f y:0.0f];
}

- (id)initWithRadius:(float)inRadius x:(float)inX y:(float)inY
{
	self = [super init];
	if (self != nil)
	{
		mX = inX;
		mY = inY;
		mRadius = inRadius;
	}
	return self;
}

- (float)surface
{
	return M_PI * mRadius * mRadius;
}

@end

Objective-C: Instances & messages

#import "Circle.h"

int main(int inNumArgs, const char * inArgs[])
{
    Circle * circle = [[Circle alloc] initWithRadius:100.0f
        x:10.0f y:5.0f];
       
    float surface = [circle surface];
    
    NSLog(@"Circle surface = %.2f", surface);
    
    return 0;
}

Objective-C: Memory management

• All objects are allocated on the heap
• Allocation: [ClassName alloc], followed by an init message
• Deallocation: [someObject release]
• Actually, that's not really true: Retain Count
• After alloc, retain count is 1
• release => retainCount--
• When retain count == 0, object is deallocated
• retain => retainCount++

Objective-C: Autorelease Pools

• [someObject autorelease] same as release but...
• ... if retain count == 0, put in autorelease pool in stead of being deallocated immediately
• Deallocated if autorelease pool is cleared
• Event-driven apps: every event-cycle
• In practice: ideal for temporary objects
• similar to variables on stack

The End

Questions?