Is loading UI in background thread okay? - ios

Question

My app is presently made of a TableViewController, and a ViewController. When a cell in the TableView is selected, the ViewController is pushed, which is the main app. This View Controller previously loads all its UIViews in the main thread, which caused the screen to freeze as the code ran, often leading users to believe it has crashed. To prevent this issue, and improve the user experience, I have changed my code to the following overall format:
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view.
[self initialiseApp];
}
- (void) initialiseApp {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Initialising views
imageView = [[UIImageView alloc] initWithImage:[self getImageFromUrl:currentWallpaperURL]];
[imageView setFrame:CGRectMake(0.0, 100, screenWidth, (screenWidth/7)*4)];
[imageView setContentMode:UIViewContentModeScaleAspectFit];
// etc etc for other views
dispatch_async(dispatch_get_main_queue(), ^{
//Add subviews to UI
[[self view] addSubview:imageView];
});
});
}
When app running in the simulator, this causes the ViewController to load as a blank screen, and later load the UI after some time. During loading, I would have some form of spinner, or text on the screen. I would therefore like clarification on this topic:
Is loading an app's UI when the ViewController is opened (or alternatively, when the app is launched) conventional? And if not, what is a better alternative to prevent having the app freeze for 10 seconds on launch?
Thanks.

Answer 1

I tend to experience issues when creating UI elements on background threads, so I would avoid this if possible (Apple says the same). In your situation, rather than loading the UI elements in the background, load the image in the background and then create the UI elements when the image has been loaded. As an example,
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
UIImage *image = [self getImageFromUrl:currentWallpaperURL];
dispatch_async(dispatch_get_main_queue(), ^{
//Add subviews to UI
imageView = [[UIImageView alloc] initWithImage:image];
[imageView setFrame:CGRectMake(0.0, 100, screenWidth, (screenWidth/7)*4)];
[imageView setContentMode:UIViewContentModeScaleAspectFit];
[[self view] addSubview:imageView];
});
});