一,概述
KVO,即:Key-Value Observing,它提供一种机制,当指定的对象的属性被修改后,则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后,KVO就会自动通知相应的观察者了。
二,使用方法
系统框架已经支持KVO,所以程序员在使用的时候非常简单。
1. 注册,指定被观察者的属性,
2. 实现回调方法
3. 移除观察
三,实例1:
检测本类中的属性age:
代码如下:
- (void)viewDidLoad { [super viewDidLoad]; self.age = 20; //创建一个button,来改变监听属性age的值 UIButton * btn = [UIButton buttonWithType:UIButtonTypeRoundedRect]; btn.frame = CGRectMake(0, 300, 100, 30); [btn setTitle:@"点我age+1" forState:(UIControlStateNormal)]; [btn addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:btn]; //添加观察对象 [self addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL]; } -(void)buttonAction { ++ self.age; } //改变观察属性的值 -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { if([keyPath isEqualToString:@"age"]) { NSLog(@"监听成功"); } } //增加观察与取消观察是成对出现的,所以需要在最后的时候,移除观察者 - (void)dealloc { [self removeObserver:self forKeyPath:@"age"]; }
运行结果:
实例2:
检测另外一个类中的属性:
假设一个场景,股票的价格显示在当前屏幕上,当股票价格更改的时候,实时显示更新其价格。
1.定义DataModel,
- @interface StockData : NSObject {
- NSString * stockName;
- float price;
- }
- @end
- @implementation StockData
- @end
2.定义此model为Controller的属性,实例化它,监听它的属性,并显示在当前的View里边
- - (void)viewDidLoad
- {
- [super viewDidLoad];
- stockForKVO = [[StockData alloc] init];
- [stockForKVO setValue:@"searph" forKey:@"stockName"];
- [stockForKVO setValue:@"10.0" forKey:@"price"];
- [stockForKVO addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];
- myLabel = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 30 )];
- myLabel.textColor = [UIColor redColor];
- myLabel.text = [stockForKVO valueForKey:@"price"];
- [self.view addSubview:myLabel];
- UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect];
- b.frame = CGRectMake(0, 0, 100, 30);
- [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
- [self.view addSubview:b];
- }
3.当点击button的时候,调用buttonAction方法,修改对象的属性
- -(void) buttonAction
- {
- [stockForKVO setValue:@"20.0" forKey:@"price"];
- }
4. 实现回调方法
- -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
- {
- if([keyPath isEqualToString:@"price"])
- {
- myLabel.text = [stockForKVO valueForKey:@"price"];
- }
- }
5.增加观察与取消观察是成对出现的,所以需要在最后的时候,移除观察者
- - (void)dealloc
- {
- [super dealloc];
- [stockForKVO removeObserver:self forKeyPath:@"price"];
- }