By default, NSObject provides automatic key-value observing. Under this default behaviour, you in fact do not actually need to call either ‘willChangeValueForKey:’ or ‘didChangeValueForKey:’ as these will be called for you if a call is made to a KVC accessor method (e.g. ‘setTheString:’).

But, you can also disable this automatic notification such that you do need to notify observers manually; this is extremely useful in cases where you need to change large numbers of values and want to control when notifications are sent out. It’s also useful in that you can change the behaviour to send notifications only if the new value is different from its current value:

- (void)setTheString:(NSString *)value
{
	if (  ![_theString isEqualToString:value] )
	{
		[self willChangeValueForKey:@"theString"];
        	_theString = value;
        	[self didChangeValueForKey:@"theString"];
	}
}

You can override the class method +automaticallyNotifiesObserversForKey: to specify that certain keys do not provide the automatic behaviour:

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)theKey
{
	BOOL automatic = NO;
 
	if ( [theKey isEqualToString:@"theString"] )
	{
        	automatic=NO;
    	}
	else
	{
		automatic=[super automaticallyNotifiesObserversForKey:theKey];
	}
	return automatic;
}

Both these methods and further explanation can be found at: Apple Developer Connection : KVO Programming Guide : Automatic Versus Manual Support.

is not needed in the setter for a value. It is automatically generated.