Property not a class member... why?

Question

I'm looking at this sample code... metronome.h:
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File: MetronomeView.h
Abstract: MetronomeView builds and displays the primary user interface of the
Metronome application.
Version: 1.6
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#import <UIKit/UIKit.h>
#import <QuartzCore/QuartzCore.h>
#import "MetronomeViewController.h"
#import "Constants.h"
@class SoundEffect, MetronomeViewController;
@interface MetronomeView : UIView {
IBOutlet MetronomeViewController *metronomeViewController;
UIImageView *metronomeArm;
UIImageView *metronomeWeight;
UILabel *tempoDisplay;
NSThread *soundPlayerThread;
SoundEffect *tickSound;
SoundEffect *tockSound;
CGFloat duration;
NSUInteger beatNumber;
BOOL armIsAnimating;
BOOL tempoChangeInProgress;
BOOL armIsAtRightExtreme;
CGPoint lastLocation;
// Property for beats per minute. Getting and setting this property alters the duration.
// The accessors for this property are implemented, rather than synthesized by the compiler.
@property NSUInteger bpm;
@property (nonatomic, retain) NSThread *soundPlayerThread;
@property (nonatomic, assign) MetronomeViewController *metronomeViewController;
// Invoked when the user changes the time signature
- (IBAction)updateAnimation:(id)sender;
@end
I'm looking at the property bpm. It isn't a class variable. I'm confused why this is... does it still work like the other class variables? I see that the the getter and setter methods are implemented rather than synthesized... But I still don't understand why it isn't a class member...
Thanks.

Answer 1

First, in Objective-C there is no "class member", only instance variable (and no class variable).
Second, @property allows you to declare accessor methods without writing those :
- (void)setName:(NSString *)aName;
- (NSString *)name;
That's just some line economizing.
On the implementation side. @synthesize allows you to implement methods without writing those :
- (void)setName:(NSString *)aName {
if(name != aName)
[name release];
name = [aName retain];
- (NSString *)name {
return [[name retain] autorelease];
Here it economize code again... Nothing more.
@synthesize creates a default implementation for a specific property. However if you want to make specific modifications in your getter/setter you're forced to implement the methods yourself. In this case the reason is that there's no bpm ivar to link the properties to.
But it could be another possibility, like, for example, when the user set the name, you want a nick-name ivar to be set as well. Here you need to use a specific method implementation and @synthesize can't help you.
However, that are just implementation things... It doesn't change the behavior of the properties... And you can even use the dot-syntax on non-property methods if you want, it's just need to be KVC-compliant.