change UI at runtime from view controller - ios

Question

I have a view controller that loads a custom view(which in turn draws UI elements, then spawns a thread to do some things in the back ground.
If the "things that run in the background", encounter an error, my view controller catches it, and at this time I want to change UI elements, like bgcolor or add a new label.
But any changes I make are not showing up. This is what I'm trying:
[self performSelectorOnMainThread:#selector(onCompleteFail) withObject:nil waitUntilDone:YES];
- (void)onCompleteFail
{
NSLog(#"Error: Device Init Failed");
mLiveViewerView.backgroundColor= [UIColor whiteColor];
//self.view.backgroundColor = [UIColor whiteColor];
UILabel *tmpLabel = [[UILabel alloc] initWithFrame:CGRectMake(20, 20, 200, 30)];
tmpLabel.text = #"Failed to init";
[self.view addSubview:tmpLabel];
}

Answer 1

You need to make any UI-related calls on the main thread: UIKit is not thread-safe and you’ll see all kinds of weird behavior from acting as if it is. That could be as simple as switching something from
[self onCompleteFail];
to
[self performSelectorOnMainThread:#selector(onCompleteFail) withObject:nil waitUntilDone:NO];
…or if -onCompleteFail has to be called on the background thread for other reasons, you can wrap your UI calls in a dispatch to the main queue, like this:
- (void)onCompleteFail
{
NSLog(#"Error: Device Init Failed");
dispatch_async(dispatch_get_main_queue(), ^{
mLiveViewerView.backgroundColor= [UIColor whiteColor];
//self.view.backgroundColor = [UIColor whiteColor];
UILabel *tmpLabel = [[UILabel alloc] initWithFrame:CGRectMake(20, 20, 200, 30)];
tmpLabel.text = #"Failed to init";
[self.view addSubview:tmpLabel];
});
}