can not receive notification of change, key value observing, KVO model-IOS

Question

Im registering an observer for on of my class named myClassA ( subclass of uiview ) from one of my class named UIScrollView(Sample)( a category for uiscroll view )

@implementation UIScrollView (myClass)
    // Register an observer       
    - (void)registerAsObserver {        
         [self addObserver:[self getMyClassA] forKeyPath:@"notification" options:NSKeyValueObservingOptionNew context:nil];
    }

In myClassA

@implementation myClassA

- (id)initWithFrame:(CGRect)frame {
    if(self = [super initWithFrame:frame]) {
        // Initialization goes here        
    }
    return self;
}

#pragma mark - Receive notification of change 
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
    CGPoint newPoint    =   [[change objectForKey:NSKeyValueChangeNewKey] CGPointValue];

    if ( [keyPath isEqualToString:@"notification"] ) {
        [self doSomething:newPoint];

    }

}

However, it seems to be that myClassA is not receiving any notification from UIScrollView(myClass). After google it, i found out that the reason is the keyword forKeyPath. If i change it to contentOffset, everything works like charm.

Did a quick research at here but still dont understand why.

Answer 1

See Apple's basic documentation on KVO:

With KVO, one object can observe any properties of another object

Well, UIScrollView has a contentOffset property. So it is key value coding compliant for contentOffset. So you can observe changes to that property, and that is what you are asking to do if you set the key path to contentOffset.

UIScrollView has no notification property, so its notification property can never change, so there is nothing to observe if you set the key path to notification.

Here's the explanation from my book:

http://www.apeth.com/iOSBook/ch13.html#_key_value_observing