在iOS中，将NSOperation与嵌套完成块一起使用会导致重复EXC_BAD_ACCESS

我正在尝试使用NSOperation和完成块获取远程Web图像。本质上，接收对象（视图控制器）将调用SGImageManager的fetchImageWithUrlString：completionBlock方法，该方法将依次设置一个具有自己的完成块的SGFetchImageOperation。最后，该操作在完成块内调用完成块。

该应用程序不会崩溃，但会在指示的行上反复中断，并且在检查器中，存在与operationImage和operationUrlString相关联的奇怪值。我不确定如何调试它。我唯一的理论是由于某种原因发生了循环调用。

//SGFetchImageOperation.h
typedef void(^SGFetchImageCompletionBlock)(UIImage *image, NSString *urlString);

@interface SGFetchImageOperation : NSOperation
@property (nonatomic, strong) NSString *urlString;
@property (copy) SGFetchImageCompletionBlock completionBlock;
@end


//SGFetchImageOperation.m
#import "SGFetchImageOperation.h"

@implementation SGFetchImageOperation

- (void)main {
    @autoreleasepool {
        if (self.isCancelled) {
            return;
        }

        UIImage *image = [self image];

        if (self.isCancelled) {
            return;
        }

        if(self.completionBlock  &&  self.urlString  && image) {
            dispatch_async(dispatch_get_main_queue(), ^{
                self.completionBlock(image, self.urlString);
            });
        }
    }
}

- (UIImage *)image{
    UIImage *image;
    if(self.urlString){
        NSURL *url = [NSURL URLWithString:self.urlString];
        NSError *error = nil;
        NSData *data = [NSData dataWithContentsOfURL:url options:NSDataReadingMappedAlways error:&error];
        if (data) {
            image = [UIImage imageWithData:data];
        } else {
            NSLog(@"Error downloading image. %@", error.localizedDescription);
        }
    }
    return image;
}

@end



//SGImageManager.h
#import "SGFetchImageOperation.h"

@interface SGImageManager : NSObject
- (void)fetchImageWithUrlString:(NSString *)urlString completionBlock:(SGFetchImageCompletionBlock)completionBlock;
@end


//SGImageManager.m
- (void)fetchImageWithUrlString:(NSString *)urlString completionBlock:(SGFetchImageCompletionBlock)completionBlock {
    SGFetchImageOperation *operation = [SGFetchImageOperation new];
    operation.urlString = urlString;

    //Keeps breaking on this line with "Thread x: EXC_BAD_ACCESS (code=2, address=0x1)", but doesn't seem to crash.
    operation.completionBlock = ^(UIImage *operationImage, NSString *operationUrlString){

        completionBlock(operationImage, operationUrlString);
    };
    [self.queue addOperation:operation];
}