iOS关联对象

#import "AFImageDownloader.h"
@interface UIImageView (_AFNetworking)

@property (readwrite, nonatomic, strong, setter = af_setActiveImageDownloadReceipt:) AFImageDownloadReceipt *af_activeImageDownloadReceipt;

@end

@implementation UIImageView (_AFNetworking)
- (AFImageDownloadReceipt *)af_activeImageDownloadReceipt {

    return (AFImageDownloadReceipt *)objc_getAssociatedObject(self, @selector(af_activeImageDownloadReceipt));
}
- (void)af_setActiveImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt {

    objc_setAssociatedObject(self, @selector(af_activeImageDownloadReceipt), imageDownloadReceipt, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end