Objective-C Classes

Objective-C (Obj-C) is C with Object Oriented Programming tacked on. The object syntax takes some time to get used to, as it is decidedly different from the normal C++/Java syntax.

There are two parts for an object in Obj-C, which are the interface and the implementation. The parts should be stored separately, the interface in a header file (*.h) and the implementation is a *.m file.

The interface stores the instance variables and the prototypes for the class and instance methods. The implementation defines those methods.

Sample code:

*** CounterController.h ***
#import < Cocoa/Cocoa.h >
@interface CounterController : NSObject {
IBOutlet id display_update;
int count;
}
- (IBAction)minus_button_pushed:(id)sender;
- (IBAction)p10_button_pushed:(id)sender;
- (IBAction)plus_button_pushed:(id)sender;
@end

*** CounterController.m ***
#import "CounterController.h"
@implementation CounterController
- (IBAction)minus_button_pushed:(id)sender {
count--;
[display_update setIntValue:count];
}
- (IBAction)p10_button_pushed:(id)sender {
count+=10;
[display_update setIntValue:count];
}
- (IBAction)plus_button_pushed:(id)sender {
count++;
[display_update setIntValue:count];
}
@end

Things of note:

• Obj-C uses import instead of include.
• The class CounterController is a subclass of NSObject.
• Inside the brackets are the instance variables display_update and count.
• The IBOutlet before display_update means the display_update is an output on the GUI display in the Interface Builder.
• id is a general type, meaning any object.
• (IBAction)minus_button_pushed:(id)sender translates to IBAction minus_button_pushed(id sender).
• The - before the methods classify them as instance methods, + for class methods.
• The brackets [] indicate messaging, [display_update setIntValue:count] translates to display_update.setIntValue(count).

http://www.gmonline.demon.co.uk/cscene/CS7/CS7-05.html
http://en.wikipedia.org/wiki/Objective-C
http://www.otierney.net/objective-c.html