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Re: Key-Value-Observing and Collections

Subject: Re: Key-Value-Observing and Collections
From: David Spooner <email@hidden>
Date: Thu, 25 Oct 2007 07:51:54 -0600
Delivered-to: email@hidden
Delivered-to: email@hidden

Philip,

You can expose 'grades' as a to-many property by adding the following methods to Pupil

- (unsigned int) countOfGrades - (id) objectInGradesAtIndex:(unsigned int)index - (void) insertObjectInGrades:(id)grade atIndex:(unsigned int)index - (void) removeObjectFromGradesAtIndex:(unsigned int)index - (void) replaceObjectInGradesAtIndex:(unsigned int)index withObject: (id)grade

Implement these to manipulate your instance variable (which you must rename to something other than grades). Now all methods of NSMutableArray are applicable to your grades property and will automatically generate the appropriate key/value observations. For example

  NSMutableArray *myGrades = [me mutableArrayValueForKey:@"grades"];
  [myGrades removeObject:badGrade];
  [myGrades addObject:goodGrade];

You might add the following method for aesthetics...

- (NSMutableArray *) mutableGrades
  { return [self mutableArrayValueForKey:@"grades"]; }

 dave

On 25-Oct-07, at 7:23 AM, Philip Mötteli wrote:

There is something, I just don't get with KVO. Lets say, I have an object
@interface Pupil
{
	NSMutableArray	grades;
}
- (void)setGrades:(NSMutableArray *)theGrades;
- (NSMutableArray *)grades;
- (void)addGrade:(Grade *)aGrade;
@end
Using KVO, I want to be notified not only, when the old grades collection object (NSMutableArray) is replaced, but also, when a Grade instance is added to the grades collection (something like "[grades addObject:aGrade]"). The first case is easy, because I know the attribute name (grades), which gives the key. But for the second, I don't know the attribute name, so I can't add an observer. Can anybody explain me how to do this?
Thanks
Phil


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