Updating UI after completion of background thread

Question

I'm implementing a simple twitter client for the iPhone using a UITableView. I fetch the picture of each twitter user in my feed when their cell appears in tableView: cellForRowAtIndexPath:

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    UIImage *profileImage = [tweet.user getProfileImageDataInContext:self.fetchedResultsController.managedObjectContext];
    dispatch_async(dispatch_get_main_queue(), ^{
        cell.imageView.image = profileImage;
    });
});

Here is the code to fetch the image:

    if (!self.profileImage) 
{
    sleep(2);
    self.profileImage = [NSData dataWithContentsOfURL:[NSURL URLWithString:self.profileImageURL]];
    //// if we recently scheduled an autosave, cancel it
    [TwitterUser cancelPreviousPerformRequestsWithTarget:self selector:@selector(autosave:) object:context];
    // request a new autosave in a few tenths of a second
    [TwitterUser performSelector:@selector(autosave:) withObject:context afterDelay:0.2];
}
return [UIImage imageWithData:self.profileImage];

Here is the error I'm getting:

twitterClient[10743:15803] bool _WebTryThreadLock(bool), 0x59bac90: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...

I think it's also worth mentioning that this happens when I scroll through the tableview very quickly when it hasn't been populated yet.

I would like the main UI to update upon completion of the download. The actual twitter app for iPhone does this quite well.

Any suggestions?

Can you show us the call that crashes? Hopping onto the main thread (via the main queue if you're doing this with GCD) to push an image should be correct. — Tommy, Aug 10 '11 at 22:52
Thanks, Tommy. I edited the question with my actual calls and the crash. — toddbranch, Aug 11 '11 at 04:45

score 4 · Answer 1 · answered Aug 10 '11 at 22:52

4

What’s the crash? A pretty standard pattern for things like this is

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // do background stuff
    dispatch_async(dispatch_get_main_queue(), ^{
        // do main-queue stuff, like updating the UI
    });
});

answered Aug 10 '11 at 22:52

Noah Witherspoon

57,021
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Thanks, Noah. I edited the question above with my actual implementation and the crash. – toddbranch Aug 11 '11 at 04:45

score 1 · Accepted Answer · edited May 23 '17 at 12:11

1

You're using GCD fine, your crash is happening because you're calling dataWithContentsOfURL in a background thread, which is not thread safe.

See: Does -dataWithContentsOfURL: of NSData work in a background thread?

edited May 23 '17 at 12:11

Community

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1

answered Dec 14 '11 at 23:08

Sahil

1,268
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19

Updating UI after completion of background thread

2 Answers2