- (instancetype)init
{
    self = [super init];
    if (self) {
        [self.layer addObserver:self
                           forKeyPath:@"borderWidth"
                              options:NSKeyValueObservingOptionNew
                              context:NULL];
    }
    return self;
}


// 监听 borderWidth 属性的变化
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary<NSKeyValueChangeKey,id> *)change
                       context:(void *)context {
    if ([keyPath isEqualToString:@"layer.borderWidth"] && [object isKindOfClass:[XXXXButton class]]) {
        // borderWidth 属性被修改了
        NSLog(@"XXXXButton's borderWidth was changed");
    }
}