// 异步下载图片

        dispatch_queue_t queue =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

        dispatch_group_t group =dispatch_group_create();

        dispatch_group_async(group, queue, ^{

            NSData *data = http://www.mamicode.com/[NSDatadataWithContentsOfURL:[NSURLURLWithString:imageUrl]];

            UIImage *image = [UIImageimageWithData:data];

                //  保存图片到相册

                [PDPhotoGraphToolsaveImageToPhotos:image];

        });

@interface PDPhotoGraphTool: NSObject

+ (void)saveImageToPhotos:(UIImage *)savedImage;

+ (void)image: (UIImage *) image didFinishSavingWithError: (NSError *) errorcontextInfo: (void *) contextInfo;

@end

@implementationPDPhotoGraphTool

+(void)saveImageToPhotos:(UIImage *)savedImage

{

    UIImageWriteToSavedPhotosAlbum(savedImage,self,@selector(image:didFinishSavingWithError:contextInfo:),NULL);

}

+(void)image:(UIImage *)image didFinishSavingWithError:(NSError *)errorcontextInfo:(void *)contextInfo

{

    NSString *msg = nil ;

    if(error != NULL){

        msg = @"保存图片失败" ;

    }else{

        msg = @"保存图片成功" ;

    }

    UIAlertView *alert = [[UIAlertViewalloc] initWithTitle:@"保存图片结果提示"

                                                   message:msg

                                                  delegate:self

                                          cancelButtonTitle:@"确定"

                                          otherButtonTitles:nil];

    [alert show];

}

@end