NSOperation and NSOperationQueue does not work

Question

I have the following code and it does not work. Is there something working behind it.

[operationQueue addOperationWithBlock:^{
        imageData =  [NSData dataWithContentsOfURL:imageURL];
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            UIImage *image = nil;

            if(imageData){
                UIImage *image = [UIImage imageWithData:imageData];
                cell.imageView.image = image;
            }
        }];
    }];

Even I create a subclass of NSOperation and then alloc init it, it does not work the way I think it to. I always have to invoke start to the NSOperation subclass to run but I suppose sending start message to NSOperation runs it in the main thread rather than running in the background thread.

What is the output? IF you want help, tell us what error was triggered. We are not mind-readers. — Steven, Oct 30 '12 at 15:24
Have you tried putting breakpoints at the lines `imageData = [NSData dataWithContentsOfURL:imageURL];` and `UIImage *image = nil;`? Knowing this will allow us to know whether your program is ever executing the blocks which you've added to the two different `NSOperationQueue`s. If you do break at `UIImage *image = nil;`, please print `[imageData length]`. — Nate Chandler, Oct 30 '12 at 15:26
This does not work with me, I could not load image this way. I have empty response and it seems that the block for downloading the image is not being called at all. — Sandeep, Oct 30 '12 at 15:28
@koyla If `dataWithContentsOfURL:` returns `nil`, then there is something amiss with your URL. If, on the other hand, after setting your breakpoint, you never break at the line `imageData = [NSData dataWithContentsOfURL:imageURL];`, then there is something wrong with your `operationQueue`. How is it initialized? — Nate Chandler, Oct 30 '12 at 15:33
I am not really sure the whole context, but it seem you are trying to do a `cell.imageView.image` inside a queue (which runs on another thread). Only the main thread is allowed to make changes to the UI. You may want to call a `@selector` and perform that on the main thread and then set the image — Steven, Oct 30 '12 at 15:33
FYI, `NSOperationQueue` is not thread safe. Look at this post for explanation on thread safe: http://stackoverflow.com/questions/8597832/thread-safety-nsoperationqueue-array-addobject — Steven, Oct 30 '12 at 15:36
@Steven he is running that UI operation on [NSOperationQueue mainQueue], so that should be OK. — Graham Perks, Oct 30 '12 at 19:54
What is 'operationQueue'. Has it been init'd properly - it's not null, right? — Graham Perks, Oct 30 '12 at 19:55
operationQueue is simple a property which is initialized inside viewDidLoad method as; self.operationQueue = [[NSOperationQueue alloc] init]; And then I keep on adding the operation to this instance, is there something wrong with this approach ? — Sandeep, Oct 30 '12 at 20:57
operationQueue is simple a property which is initialized inside viewDidLoad method as; self.operationQueue = [[NSOperationQueue alloc] init]; — Sandeep, Oct 30 '12 at 20:58

subhash kumar singh · Accepted Answer · 2012-10-30T19:05:01.860

I want to add an alternative solution using GCD :

backgroundQueue = dispatch_queue_create("com.razeware.imagegrabber.bgqueue", NULL);
dispatch_async(backgroundQueue, ^{
                          /* put the codes which makes UI unresponsive like reading from network*/ 
                         imageData =  [NSData dataWithContentsOfURL:imageURL];
                         .....   ;
                         dispatch_async(dispatch_get_main_queue(),^{
                                         /* do the UI related work on main thread */
                                         UIImage *image = [UIImage imageWithData:imageData];
                                         cell.imageView.image = image;
                                         ......;   });
                                  });
dispatch_release(backgroundQueue);

Let me know whether this one helped you ;)

Reference

Hard to decide which implementation looks cleaner. I like both. The downside with this one is the need to remember dispatch_release(). — Graham Perks, Oct 30 '12 at 19:57

NSOperation and NSOperationQueue does not work

1 Answers1