iOS

KVO（键值监听）全称 Key Value Observing。使用KVO可以实现视图组件和数据模型的分离，视图作为监听器，当模型的属性值发生变化后，监听器可以做相应的处理。KVO的方法由NSKeyValueObserving协议提供，同样NSObject已经实现了该协议，因此几乎所有的对象都可以使用KVO。

使用KVO操作常用的方法如下：

注册制定路径的监听器： addObserver:  forKeyPath: option: context:

删除制定路径的监听器：removeObserver: forKeyPath:

触发监听时的方法：observeValueForKeyPath: ofObject: change: context: 

 

引入的三方头文件：

#import "ReactiveObjC.h"
#import "MJExtension.h"

声明的属性：

@interface ViewController ()

@property(nonatomic, strong) UITextField *textField;
@property(nonatomic, strong) UILabel     *lableRetValue;

@end

 实现方法：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.view.backgroundColor = [UIColor whiteColor];
    
    self.person = [[Person alloc] init];
    [self.person addObserver:self forKeyPath:@"str" options:NSKeyValueObservingOptionNew context:nil];
    
    [self.textField.rac_textSignal subscribeNext:^(NSString * _Nullable x) {
        [self.person setValue:x forKey:@"str"];
    }];
}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    if ([keyPath isEqualToString:@"str"]) {
        self.lableRetValue.text = [change valueForKey:@"new"];
    }
}

-(UILabel *)lableRetValue{
    if (!_lableRetValue) {
        _lableRetValue = [UILabel new];
        _lableRetValue.layer.borderColor = [UIColor redColor].CGColor;
        _lableRetValue.layer.borderWidth = 1.f;
        _lableRetValue.textAlignment = NSTextAlignmentCenter;
        [self.view addSubview:_lableRetValue];
        
        [_lableRetValue mas_makeConstraints:^(MASConstraintMaker *make) {
            make.top.equalTo(self.view).offset(100);
            make.centerX.equalTo(self.view);
            make.width.equalTo(self.view).multipliedBy(0.6);
            make.height.offset(50);
        }];
    }
    return _lableRetValue;
}

-(UITextField *)textField{
    if (!_textField) {
        _textField = [UITextField new];
        _textField.placeholder = @"请输入内容";
        _textField.keyboardType = UIKeyboardTypeNumberPad;
        _textField.textAlignment = NSTextAlignmentCenter;
        _textField.backgroundColor = [UIColor grayColor];
        [self.view addSubview:_textField];
        
        [_textField mas_makeConstraints:^(MASConstraintMaker *make) {
            make.top.equalTo(self.lableRetValue.mas_bottom).offset(40);
            make.centerX.equalTo(self.view);
            make.width.equalTo(self.view).multipliedBy(0.7);
            make.height.offset(50);
        }];
    }
    return _textField;
}