KVO

static NSString *kNoteObservationContext = @"NoteObservationContext";
static NSString *kNoteCompletedKey       = @"completed";


- (void)setTest:(Test *)test
{
    if (m_test == test)
    {
        return;
        // Nothing to do for the same object
    }

    [m_test removeObserver:self forKeyPath:kNoteCompletedKey context:&kNoteObservationContext];

    m_test = test;

    [test addObserver:self forKeyPath:kNoteCompletedKey options:NSKeyValueObservingOptionNew context:&kNoteObservationContext];
}


#pragma mark - KVO

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    // observing changes to the model object so that background image can be changed.

    if (!(context == &kNoteObservationContext)) {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }

    if (self.test.completedStatus)
    {
        self.cellBackgroundImage.image = [self completedBackgroundImage];
    }
    else
    {
        self.cellBackgroundImage.image = [self defaultBackgroundImage];
    }
}