Help with Key-Value-Observing

Question

I need a bit of help with KVO, I'm about half way there. What I'm trying to do is trigger a method when something in an Tree Controller changes.

So I 'm using this code to register as a KVO.

[theObject addObserver: self
            forKeyPath: @"myKeyPath"
               options: NSKeyValueObservingOptionNew
               context: NULL];

But how do i trigger a method when the Key Path I am observing changes?

One extra question, when I add my self as an Observer I want the Key Path to be a Property in my Core Data model, Have I done that correctly?

Answer 1

Override observeValueForKeyPath:ofObject:change:context: to dispatch the method you wish to call.

@interface Foo : NSObject {
    NSDictionary *dispatch;
    ...
}
@end
@implementation Foo
-(id)init {
    if (self = [super init]) {
        dispatch = [[NSDictionary dictionaryWithObjectsAndKeys:NSStringFromSelector(@selector(somethingHappenedTo:with:)),@"myKeyPath",...,nil] retain];
        ...
    }
}
...
- (void)observeValueForKeyPath:(NSString *)keyPath
            ofObject:(id)object
            change:(NSDictionary *)change
            context:(void *)context
{
    SEL msg = NSSelectorFromString([dispatch objectForKey:keyPath]);
    if (msg) {
        [self performSelector:msg withObject:object withObject:keyPath];
    }
    [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
}
...

See "Receiving Notification of a Change" for details.