Simple KVO example

KVO works with setter and getter and in incNumber you are directly accessing iVar so instead of that use self.number

- (IBAction)incNumber:(id)sender{    self.number++;    NSLog(@"%d", self.number);}

Rather than:

_number++;

Try:

[self willChangeValueForKey:@"number"];_number++;[self didChangeValueForKey:@"number"];

or ever better just:

self.number++

And let the system take care of the willChangeValueForKey: and didChangeValueForKey: methods.

@interface TraineeLocationCell : UIView@property (strong, nonatomic) NSString *traineeAddress;@end@implementation// in textview delgate method  i am setting traineeAddress string value- (void)textViewDidEndEditing:(UITextView *)textView{    if (textView.text.length >0)    [self setValue:textView.text forKey:@"traineeAddress"];}@end

and in other class where i am using this class

 TraineeLocationCell *locationView;//create object here [locationView addObserver:self forKeyPath:@"traineeAddress" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];//observing the key in this class

//this is the delegate method which take care of value is changed or not

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{   if([keyPath isEqualToString:@"traineeAddress"]){        if (!settingsValues.address)            settingsValues.address = [[NSMutableArray alloc]initWithObjects:[change valueForKey:@"new"], nil];        else            [settingsValues.address replaceObjectAtIndex:0 withObject:[change valueForKey:@"new"]];    }}