IOS - KVO

#import <Foundation/Foundation.h>

@interface MYClass : NSObject
@property(assign,nonatomic)int number;
@end


-----------------------

#import "ViewController.h"
#import "MYClass.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIButton *plusButton;
@property (weak, nonatomic) IBOutlet UILabel *showLabel;
@property(nonatomic,strong)MYClass *myClass;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
     _myClass = [[MYClass alloc] init];
    
    [_myClass addObserver:self forKeyPath:@"number" options:NSKeyValueObservingOptionNew context:nil];
    
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{
    if ([keyPath isEqualToString:@"number"]&&object == _myClass) {
        _showLabel.text = [NSString stringWithFormat:@"%@",[change valueForKey:@"new"]];
    }
}

- (IBAction)makeCountPlus:(id)sender {
    
    ++_myClass.number;
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
  
    [self removeObserver:self forKeyPath:@"number"];
}

@end