用KVO监听控件属性的改变（observeValueForKeyPath）

创建一个测试的UIButton

#import "ViewController.h"

@interface ViewController ()

@property(nonatomic, strong)UIButton *button;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.button = [[UIButton alloc] initWithFrame:CGRectMake(30, 50, 50, 30)];
    [self.button setTitle:@"测试" forState:UIControlStateNormal];
    [self.button setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    self.button.layer.borderWidth = 1.0f;
    [self.view addSubview:self.button];

//注册监听button的enabled状态
    [self.button addObserver:self forKeyPath:@"enabled" options:NSKeyValueObservingOptionNew context:@"test_button"];

    //  3秒钟后改变当前button的enabled状态
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0f * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        self.button.enabled = YES;
    });
}

添加监听观察者

/**
 *  监听按钮状态改变的方法
 *
 *  @param keyPath 按钮改变的属性
 *  @param object  按钮
 *  @param change  改变后的数据
 *  @param context 注册监听时context传递过来的值
 */

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    UIButton *button = (UIButton *)object;
    if (self.button == button && [@"enabled" isEqualToString:keyPath]) {
        NSLog(@"self.button的enabled属性改变了%@",[change objectForKey:@"new"]);
    }
}