loading images from a background thread using blocks

Question

I have the following method, which basically calls a request to load an array of NSData of images in the background thread:

[query findObjectsInBackgroundWithBlock:^(NSArray * objects, NSError * error){

}];

In this case objects is an array of the NSData. The issue is that if I have 100 images to load (100 elements in the array). This means that the user will have to wait for quite some time to see any image showing up in a UITableView. What I want to do is for them to see an image once it is available/loaded.. do I have to then change the code so that it does 100 background threads to load the image?

try to intervene the code and once that Background thread is showing that it has an image, Start Displaying it. Like i video Game loading content in an update Method. — Keeano, Sep 21 '11 at 15:10

score 48 · Accepted Answer · answered Sep 21 '11 at 15:16

48

you could implement something like this in your cellForRowAtIndexPath:

That way you load each image in the background and as soon as its loaded the corresponding cell is updated on the mainThread.

      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void) {
        NSData *data0 = [NSData dataWithContentsOfURL:someURL];
        UIImage *image = [UIImage imageWithData:data0];

        dispatch_sync(dispatch_get_main_queue(), ^(void) {
            UIImageView* imageView = (UIImageView*)[cell viewWithTag:100];
            imageView.image = image;
        });
    });

answered Sep 21 '11 at 15:16

HeikoG

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Strictly speaking, UIKit (which UIImage is part of) isn't background-thread-safe, but it seems like you're able to get away with it here because you're only using 'UIImage imageWithData'. Nice hack! – Chris Jun 17 '12 at 01:31
This is an old answer, but since there are recent comments referencing it, I would like to add this warning: this solution will likely not work correctly in conjunction with cell updates in a table view, and it uses `dataWithContentsOfURL:` for remote access which is not reliable and inappropriate for that purpose. – CouchDeveloper Jul 17 '13 at 21:44
Hey. Thanks for your comment. I understand that using this method inside GCD might not be pure elegance but since I never encountered any issues in practice I would be interested in what problems you are seeing in it ? – HeikoG Jul 17 '13 at 22:37
For downloading remote resources you are better of using `NSURLConnection`. The methods family "xxxWithContentOfURL:" is really meant to load from file URLs. Then, every serious application will require to cancel a running request. Either this is the user you want enable to _cancel_ a connection when she considers appropriate. And it is you since in your code you often HAVE to cancel a running request when the user switches away from a view and the result isn't used anyway anymore. The latter also prevents accidentally escalation of starting requests, which can hog the CPU and consume memory. – CouchDeveloper Jul 17 '13 at 22:53
1

Note: when you have to cancel a request, you will need a `NSURLConnection` instance to send it the `cancel` message. This also implies that you _have_ to use `NSURLConnection` in _asynchronous_ mode and implement the delegates. – CouchDeveloper Jul 17 '13 at 22:56
I understand your point. However I am using this approach for very small images that serve as thumbnails in a table view. The download process takes a fraction of a second. I think for this purpose it's ok to use this code – HeikoG Oct 04 '13 at 20:41
`AFNetworking` had issues with concurrent `-imageWithData:` calls, so it might not be totally safe to call concurrently. https://github.com/AFNetworking/AFNetworking/issues/2572 – Heath Borders May 06 '16 at 20:39

score 0 · Answer 2 · edited Sep 21 '11 at 15:39

0

You can create NSInvocationOperation and set it to NSOperationQueue

for example: Initializing NSOperationQueue:

NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];

After create NSInvocationOperation:

NSInvocationOperation* downloadOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(selectorToDownloadImage:) object:YouData];    
[operationQueue addOperation:downloadOperation];
[downloadOperation release];

edited Sep 21 '11 at 15:39

Abizern

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answered Sep 21 '11 at 15:14

Dmytriy

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why don't you edit that in your answer and make it look nice? – xonegirlz Sep 21 '11 at 15:26

score -1 · Answer 3 · answered Sep 21 '11 at 15:10

-1

No, you don't have to create that many background threads. Use NSOperationQueue.

answered Sep 21 '11 at 15:10

Jim

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loading images from a background thread using blocks

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