update ui elements from a background thread in ios

Thu, Apr 11, 2013

This is a weird one, yet not weird at all really. I was playing around with Grand Central Dispatch (GCD) in viewDidLoad:

dispatch_queue_t queue = dispatch_queue_create("com.codebrane.onlineTimeQueue", NULL);

dispatch_async(queue, ^{

  … some code to retrieve the time from an online service

  self.textField.text = responseAsText;

});

It worked fine when textField was a label but when I changed to use a textField to get the full response displayed, it produced this error:

 

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..

Silly me, I should have remembered you should update UI elements from the main thread:

dispatch_queue_t queue = dispatch_queue_create("com.codebrane.onlineTimeQueue", NULL);

dispatch_async(queue, ^{

  … some code to retrieve the time from an online service

  dispatch_async(dispatch_get_main_queue(), ^{

    self.textField.text = responseAsText;

  });

});