iOS中KVO

KVO(Key Value Observing，即键值监听)

KVO机制NSKeyValueObserving协议提供支持，NSObject遵守了该协议，因此NSObject的子类都可使用该协议中的方法

例子

假设我们要监听UitextFild中的text值的变化

#import "ViewController.h"


@interface ViewController ()
@property (weak, nonatomic) IBOutlet UITextField *textField;//Main.storyboard的rootVC创建的

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    
    [self.textField addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    self.textField.text = @"123";
    
    
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"%s",__func__);
    NSLog(@"被修改的keyPath为%@",keyPath);
    NSLog(@"被修改的对象为：%@",object);
    NSLog(@"被修改的属性值为%@",[change objectForKey:@"new"]);
    
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    self.textField.text = @"520250";
}

- (void)dealloc {
    [self.textField removeObserver:self forKeyPath:@"text"];
}

Screenshot 2018-07-27_15-11-43.png

当点击屏幕的时候打印输出

 被修改的属性值为520250

注意：只有当用代码设置UitextFild的text属性时才会被监听