IOS: KVO overview and usage

I. Overview

KVO, that is, Key-Value Observing, provides a mechanism. When the attribute of a specified object is modified, the object will receive a notification. In short, KVO automatically notifies the corresponding observer after the attribute of the specified object to be observed is modified.

Ii. Usage

The system framework already supports KVO, so it is very simple for programmers to use.

1. Registration, specifying the properties of the observer,

2. Implement callback Methods

3. Remove observation

Iii. Example:

In this scenario, the stock price is displayed on the current screen. When the stock price changes, the stock price is updated in real time.

1. Define DataModel,

 

[Cpp]View plaincopy

1. @ Interface StockData: NSObject {
2. NSString * stockName;
3. Float price;
4. }
5. @ End
6. @ Implementation StockData
7. @ End

 

2. Define this model as the Controller attribute, instantiate it, listen to its attributes, and display it in the current View.

 

[Cpp]View plaincopy

1. -(Void) viewDidLoad
2. {
3. [Super viewDidLoad];
5. StockForKVO = [[StockData alloc] init];
6. [StockForKVO setValue: @ "searph" forKey: @ "stockName"];
7. [StockForKVO setValue: @ "10.0" forKey: @ "price"];
8. [StockForKVO addObserver: self forKeyPath: @ "price" options: NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context: NULL];
10. MyLabel = [[UILabel alloc] initWithFrame: CGRectMake (100,100,100, 30)];
11. MyLabel. textColor = [UIColor redColor];
12. MyLabel. text = [stockForKVO valueForKey: @ "price"];
13. [Self. view addSubview: myLabel];
15. UIButton * B = [UIButton buttonWithType: UIButtonTypeRoundedRect];
16. B. frame = CGRectMake (0, 0,100, 30 );
17. [B addTarget: self action: @ selector (buttonAction) forControlEvents: UIControlEventTouchUpInside];
18. [Self. view addSubview: B];
20. }

3. When you click a button, call the buttonAction method to modify the attributes of the object.

 

[Cpp]View plaincopy

1. -(Void) buttonAction
2. {
3. [StockForKVO setValue: @ "20.0" forKey: @ "price"];
4. }

 

 

4. Implement callback Methods

 

[Cpp]View plaincopy

1. -(Void) observeValueForKeyPath :( NSString *) keyPath ofObject :( id) object change :( NSDictionary *) change context :( void *) context
2. {
3. If ([keyPath isw.tostring: @ "price"])
4. {
5. MyLabel. text = [stockForKVO valueForKey: @ "price"];
6. }
7. }

 

 

5. Adding observation and canceling observation appear in pairs, so you need to remove the Observer at the end.

 

[Cpp]View plaincopy

1. -(Void) dealloc
2. {
3. [Super dealloc];
4. [StockForKVO removeObserver: self forKeyPath: @ "price"];
5. [StockForKVO release];
6. }

 

 

Iv. SummaryKVO is a simple encoding method. It is applicable to the change of the UIVIew caused by the modification of datamodel. As in the preceding example, after changing the attribute value, the listener will be notified immediately.

Source Network Resources: http://blog.csdn.net/yuquan0821/article/details/6646400