KVO- key listener

Monitor key is that you can monitor changes in the value of a property of the object;

First of all, in the project, create a new class of Person

@interface Person : NSObject

@property (nonatomic, copy) NSString *name;

@end

ViewController introduced in the header file "Person", began testing the use of KVO

- (void)viewDidLoad {
    [super viewDidLoad];
   
    Person *person = [[Person alloc] init];
    person.name = @"珠珠";
    [person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:@"testKVO"];
    
    person.name = @"小雅";
    [person removeObserver:self forKeyPath:@"name"];
    
}

- (void) the observeValueForKeyPath: (NSString *) keyPath ofObject: (ID) Object Change: (NSDictionary <NSKeyValueChangeKey, ID> *) Change context :( void * ) context { 
    NSLog ( @ " keypaht% = @, @% = Object, Change = % @, context =% @ " , keyPath, Object , Change, context);
     // NSKeyValueChangeOldKey can use the" old "instead, but get it, if the system modifies old as the" Old ", then it would not obtain, so suggested NSKeyValueChangeOldKey 
    (NSLog @ " Get value before being modified =% @ " , Change [NSKeyValueChangeOldKey]);
     // NSKeyValueChangeNewKey may be "new" instead, but acquired, and if the system modified new is "new", then will get less, it is recommended to use NSKeyValueChangeNewKey 
    NSLog ( @ "= Modified value after acquisition% @", change[NSKeyValueChangeNewKey]);
}

Run the program to give a value out of the following: