IOS. OC.03 KVO-key-value observer, ios. oc.03kvo-

IOS. OC.03 KVO-key-value observer, ios. oc.03kvo-

KVO Key, Value, Observing, that is, Key-Value observer. It provides such a mechanism that when the attributes of a specified object change, KVO automatically notifies the corresponding observer.

Unlike NSNotification, the key-value observation does not have any so-called central object to provide change notifications for all observers. Instead, when a change occurs, the notification is sent directly to the object in the observed state.

Three steps: register the observer, receive the change notification, and remove the observer. Example:

Create a project and two classes, one Baby and one Mother. Baby has a property hungryNum, called hunger value. When the hunger value changes, mother receives the hunger value change.

Baby. h

# Import <Foundation/Foundation. h>

@ Interface Baby: NSObject

@ Property (nonatomic, assign) NSInteger hungryNum;

@ End

 

Mother. m

# Import "Mother. h"

@ Implementation Mother

-(Void) observeValueForKeyPath :( NSString *) keyPath ofObject :( id) object change :( NSDictionary *) change context :( void *) context {

If ([keyPath isw.tostring: @ "hungryNum"]) {

// Print the current value and the previous value of hungryNum whenever the monitored property hungryNum changes.

NSLog (@ "% @", [change objectForKey: @ "new"], [change objectForKey: @ "old"]);

}

}

@ End

 

ViewController. m

# Import "ViewController. h"

# Import "Baby. h" // introduce the header file

# Import "Mother. h"

@ Interface ViewController ()

@ End

@ Implementation ViewController

-(Void) viewDidLoad {

[Super viewDidLoad];

Baby * xiaoxiao = [[Baby alloc] init]; // create an object

Mother * xiaoMama = [[Mother alloc] init];

// Register the observer. xiaoxiao adds the observer xiaoxiaoMama to observe the attributes of hungryNum.

[Xiaoxiao addObserver: xiaoMama forKeyPath: @ "hungryNum" options: NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context: nil];

Xiaoxiao. hungryNum = 10; // attach an initial value to the hungryNum attribute of xiaoxiao. Use a for loop to change the value of this attribute. The Listener is notified every time the value is changed.

For (xiaoxiao. hungryNum; xiaoxiao. hungryNum> 0; xiaoxiao. hungryNum --){

}

// Remove the observer. Note: if you do not remove the observer, the program will crash.

[Xiaoxiao removeObserver: xiaoMama forKeyPath: @ "hungryNum"];

}

 

Print result:

17:11:29. 735 OMG [2366: 148640] 10 0

17:11:29. 736 OMG [2366: 148640] 9 10

17:11:29. 736 OMG [2366: 148640] 8 9

17:11:29. 736 OMG [2366: 148640] 7 8

17:11:29. 736 OMG [2366: 148640] 6 7

17:11:29. 736 OMG [2366: 148640] 5 6

17:11:29. 736 OMG [2366: 148640] 4 5

17:11:29. 737 OMG [2366: 148640] 3 4

17:11:29. 737 OMG [2366: 148640] 2 3

17:11:29. 737 OMG [2366: 148640] 1 2

17:11:29. 737 OMG [2366: 148640] 0 1

 

In case of any errors, please do not give me any further information.