NSOperationQueue问题

Question

我有一个代码在ios 6中用作图像下载器，但是当我在使用ios 5的不同应用程序中使用相同的代码时，它给我错误NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance听到的是我正在使用的代码

[self.imageDownloadingQueue addOperationWithBlock:^{
            NSURL *imageUrl   = [NSURL URLWithString:ImageURL];
            NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];
            UIImage *images    = nil;
            if (imageData)
                images = [UIImage imageWithData:imageData];
            if (images){
                [self.imageCache setObject:images forKey:ImageURL];
                [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                    // make sure the cell is still visible
                    imageV.image = images;
                }];
            }
        }];

谁能告诉我我在做什么错，因为此代码在单独下载图像时对我有很大帮助。 谢谢

PS我正在使用的整个功能

-(void) setScrollView:(UIScrollView *)scrollview :(NSArray *)contentArray{

    int x = 0;

    int i = 0;

    NSString *ImageURL=@"http://www.online-image-editor.com/styles/2013/images/example_image.png";



    for(NSDictionary *str in contentArray){

        UIImageView *imageV = [[UIImageView alloc ] initWithFrame:CGRectMake(x, 0, 200,125)];
        @try {
            ImageURL = [str objectForKey:@"image"];
       }
        @catch (NSException * e) {
            NSLog(@"Exception: %@", [e description]);
        }


        imageV.userInteractionEnabled = YES;

        ImageTapGesture *singleTap = [[ImageTapGesture alloc] initWithTarget:self action:@selector(tapDetectedMain:)];
        singleTap.newItem = str;
        singleTap.currentItem = i ;
        singleTap.numberOfTapsRequired = 1;

        [imageV addGestureRecognizer:singleTap];

       UILabel *title = [[UILabel alloc] initWithFrame:CGRectMake(x,125,200,20)];

        [title setFont:[UIFont systemFontOfSize:11]];
        title.numberOfLines = 2;
       title.autoresizingMask = UIViewAutoresizingFlexibleHeight;
       [title setBackgroundColor:[UIColor blackColor]];
       [title setTextColor:[UIColor whiteColor]];
       title.text = [NSString stringWithFormat:@" %@",[str objectForKey:@"title"]];


       x += title.frame.size.width+2;

        UIImage *cachedImage = [self.imageCache objectForKey:ImageURL];

       if (cachedImage){
           imageV.image = cachedImage;
        }
        else{
            imageV.image = [UIImage imageNamed:@"vdo.png"];

           [self.imageDownloadingQueue addOperationWithBlock:^{
               NSURL *imageUrl   = [NSURL URLWithString:ImageURL];
                NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];
                UIImage *images    = nil;
                if (imageData)
                    images = [UIImage imageWithData:imageData];
                  if (images){
                      [self.imageCache setObject:images forKey:ImageURL];
                      [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                        // make sure the cell is still visible
                        imageV.image = images;
                   }];
                }
            }];

        }

        [scrollview addSubview:imageV];
        [scrollview addSubview:title];
    }

    scrollview.contentSize = CGSizeMake(x, scrollview.frame.size.height);
    i++;

}

Answer 1

您绝对应该检查要发送给操作的ImageURL 。 看来，这个ImageURL是[NSNull null] 。 如果它是局部变量，是否声明为__block NSString *ImageUrl ？

Answer 2

请尝试以下代码：

[self.imageDownloadingQueue addOperationWithBlock:^{
    NSURL *imageUrl ;
    if((ImageURL!=nil)&&(![ImageURL isKindOfClass:[NSNull class]]))
    {
        imageUrl= [NSURL URLWithString:ImageURL];
    }
    NSData *imageData;
    if((imageUrl!=nil)&&(![imageUrl isKindOfClass:[NSNull class]]))
    {
        imageData = [NSData dataWithContentsOfURL:imageUrl];
    }

    UIImage *images    = nil;
    if((imageData!=nil)&&(![imageData isKindOfClass:[NSNull class]]))
        images = [UIImage imageWithData:imageData];
    if((images!=nil)&&(![images isKindOfClass:[NSNull class]]))
    {
        [self.imageCache setObject:images forKey:ImageURL];
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            // make sure the cell is still visible
            imageV.image = images;
        }];
    }
}];

希望这会帮助你。