Key-value observing (KVO) is a programming feature in Cocoa and Cocoa Touch frameworks that allows one object (the observer) to observe changes to specific properties in other objects (the observed). The observer receives notifications when the observed property changes, enabling the observer to respond.

KVO is mainly useful for implementing reactive programming patterns, in which updates in one part of an application can trigger updates in another, which ultimately helps in maintaining synchronization between objects.

Which is widely used in macOS and iOS development to facilitate communication between different components of an application.

Key-Value Observing (KVO) in a Cocoa Application

Implementing Key-Value Observing (KVO) in a Cocoa application involves several key steps. In this guide, we will specifically use Objective-C.

Steps to Implement KVO

Step 1 − Define the Observable Property

In the model class first define the property which you want to observe. Use @property to declare it.

@interface MyModel : NSObject

@property (nonatomic, strong) NSString *myString;

@end

@implementation MyModel

@end

Step 2 − Register for Observing

In the class in which you want to observe the property (e.g., a view controller), register for KVO in an appropriate method like viewDidLoad.

@implementation MyObserver {
   MyModel *_model;
}

- (instancetype)initWithModel:(MyModel *)model {
   self = [super init];
   if (self) {
      _model = model;
      [_model addObserver:self 
         forKeyPath:@"myString" 
            options:NSKeyObservingOptionNew | NSKeyObservingOptionOld 
            context:nil];
   }
   return self;
}

Step 3 − Implement the Observer Method

Now, here you need to implement the observeValueForKeyPath:ofObject:change:context: method to handle or respond to changes to the observed property.

- (void)observeValueForKeyPath:(NSString *)keyPath 
   ofObject:(id)object 
   change:(NSDictionary<NSKeyValueChangeKey,id> *)change 
   context:(void *)context {
      if ([keyPath isEqualToString:@"myString"]) {
         NSLog(@"myString changed from %@ to %@", change[NSKeyValueChangeOldKey], change[NSKeyValueChangeNewKey]);
      } else {
         [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
      }
}

Step 4 − Remove the Observer

Its important to remove the observer when you no longer need it or before the observing object is deallocated to prevent crashes.

- (void)dealloc {
   [_model removeObserver:self forKeyPath:@"myString"];
}

Step 5 − Trigger Changes

You can now change the property in your model class, and the observer will be notified of the change to see the observer respond.

_model.myString = @"New Value"; // This will trigger the observer

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