objc Runtime 关联对象为什么不走 set 方法？

代码如下：

@interface UIView (Rotate )

@property (nonatomic, strong ) NSString * animating;

- (void ) startSpin;
- (void ) stopSpin;
@end
//--------------------------------------------

#import "UIView+Rotate.h"
#import <objc/runtime.h>

#define ANIMATING_TAG @"1"
#define ANIMATING_STOP_TAG @"0"

@implementation UIView (Rotate )

static void * mAnimating = &mAnimating;


-(NSString *)animating{
    return objc_getAssociatedObject (self,mAnimating );
}

-(void )setAnimating:(NSString *)aAnimating {
    objc_setAssociatedObject (self, mAnimating, aAnimating, OBJC_ASSOCIATION_RETAIN_NONATOMIC );
}



- (void ) spinWithOptions: (UIViewAnimationOptions ) options{
        [UIView animateWithDuration: 0.5f
                        delay: 0.0f
                        options: options
                        animations: ^{
                        self.transform = CGAffineTransformRotate (self.transform, M_PI / 2 );
                 }
                    completion: ^(BOOL finished ) {
                        if (finished ) {
                            if ([self checkIsAnimating]) {
                                // if flag still set, keep spinning with constant speed
                                [self spinWithOptions: UIViewAnimationOptionCurveLinear];
                            } else if (options != UIViewAnimationOptionCurveEaseOut ) {
                                // one last spin, with deceleration
                               [self spinWithOptions: UIViewAnimationOptionCurveEaseOut];
                           }
                       }
                 }];
    }

        - (void ) startSpin{
        if (![self checkIsAnimating]) {
            [self setAnimating:ANIMATING_TAG];
            [self spinWithOptions: UIViewAnimationOptionCurveEaseIn];
            }
    }

    - (void ) stopSpin {
        [self setAnimating:ANIMATING_STOP_TAG];
    }

    - (BOOL )checkIsAnimating{
        if (!self.animating ) {
            return NO;
            }

            if ([self.animating isEqualToString:ANIMATING_TAG]) {
            return YES;
             }else{
            return NO;
        }
     }

         @end

想给 UIView 增加一个 animating 的 String ，在增加的方法里面调用。但是不管用 self.animating 或者 [self setAnimating:ANIMATING_STOP_TAG];

animating 的 get 方法没事，但是 set 方法一直没有走.导致 animating 都一直为 nil 。