Main Scene and Game State - Learn SpriteBuilder for iOS Game Development - page 240

Now add the musicVolume property setter and getter methods of Listing 7-12 below
the sharedGameState method.
Listing 7-12 . The musicVolume property setter and getter methods
static NSString* KeyForMusicVolume = @"musicVolume";
-(void) setMusicVolume:(CGFloat)volume
{
[[NSUserDefaults standardUserDefaults] setDouble:volume
forKey:KeyForMusicVolume];
}
-(CGFloat) musicVolume
{
NSNumber* number = [[NSUserDefaults
standardUserDefaults]
objectForKey:KeyForMusicVolume];
return (number ? number.doubleValue : 1.0);
}
The NSUserDefaults key is declared as a static NSString* variable so that you
don't have to write the string twice. If you ever made a typo writing the same thing twice
over, you'll understand the value of declaring frequently used strings as static vari-
ables.
Property setter and getter methods follow a consistent naming scheme. The getter has the
same name as the property; the setter prefixes the property name with set , and the prop-
erty's first letter is capitalized.
NSUserDefaults is the class that persists any integral data type and so-called property
list objects: NSData , NSString , NSNumber , NSDate , NSArray , or NSDiction-
ary . So it won't save your custom class just like that. But you can save individual proper-
ties of your classes, like the volumes here.
The standardUserDefaults is a singleton accessor just like sharedGameState .
The setDouble:forKey: method stores a value of type double for the given key,
which must be an NSString* object. The same key is then used in objectForKey:
to receive the NSNumber object associated with that key. NSUserDefaults will al-
ways return integral data types wrapped in NSNumber objects so that it can return nil to
signal that there's no entry for the given key, which will be the case when you first launch
the app.