StackOverflow - https://stackoverflow.com/questions/55321440/do-i-have-to-removeobserver-in-kvo-manually-now

ViewController.m

//
//  ViewController.m
//  KVOC
//
//  Created by kingcos on 2019/3/24.
//  Copyright © 2019 kingcos. All rights reserved.
//  This is the demo code for my question:
//  https://stackoverflow.com/questions/55321440/do-i-have-to-removeobserver-in-kvo-manually-now
//

#import "ViewController.h"

@interface Computer : NSObject
@property (nonatomic, assign) double memoryPercentage;
@end

@implementation Computer
- (instancetype)init
{
    self = [super init];
    if (self) {
        [self addObserver:self forKeyPath:NSStringFromSelector(@selector(memoryPercentage)) options:NSKeyValueObservingOptionNew context:nil];
    }
    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"change - %@", change[NSKeyValueChangeNewKey]);
}

@end

@interface ViewController ()
@property (nonatomic, strong) Computer *cpt;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    
    _cpt = [[Computer alloc] init];
}

- (IBAction)updated:(id)sender {
    self.cpt.memoryPercentage += 0.1;
}

- (IBAction)removed:(id)sender {
    [self.cpt removeObserver:self.cpt forKeyPath:@"memoryPercentage"];
}

- (IBAction)setToNil:(id)sender {
    self.cpt = nil;
}

- (IBAction)initAgain:(id)sender {
    self.cpt = [[Computer alloc] init];
}

@end