Async UIView content not showing up when coming back to main thread - ios

Question

When loading my UIViewController, I basically put a spinner in the middle of the page until the content loads, then come back on the main thread to add the subviews :
- (void)viewDidLoad {
[super viewDidLoad];
UIActivityIndicatorView *aiv_loading = ... // etc
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Load content
NSString *s_checkout = [[BRNetwork sharedNetwork] getCheckoutInstructionsForLocation:self.locBooking.location];
UIView *v_invoice_content = [[BRNetwork sharedNetwork] invoiceViewForBooking:locBooking.objectId];
// Display the content
dispatch_sync(dispatch_get_main_queue(), ^{
if (s_checkout && v_invoice_content) {
[aiv_loading removeFromSuperview];
[self showContentWithText:s_checkout AndInvoice:v_invoice_content];
} else {
NSLog(#"No data received!"); // is thankfully not called
}
});
});
}
- (void) showContentWithText:(NSString *)s_checkout AndInvoice:(UIView *)v_invoice {
[self.view addSubview:[self checkoutTextWithText:s_checkout]]; // Most of the time displayed text
[self.view addSubview:[self completeCheckout]]; // always Displayed UIButton
[self.view addSubview:[self divider]]; // always displayed UIImageView
// Summary title
UILabel *l_summary = [[UILabel alloc] initWithFrame:CGRectMake(0, [self divider].frame.origin.y + 6 + 10, self.view.bounds.size.width, 20)];
l_summary.text = NSLocalizedString(#"Summary", nil);
[self.view addSubview:l_summary];
CGRect totalRect = CGRectMake([self divider].frame.origin.x, [self divider].frame.origin.y + 6 + 30, self.view.bounds.size.width - [self divider].frame.origin.x, 90);
_v_invoice = v_invoice;
_v_invoice.frame = totalRect;
[self.view addSubview:[self v_invoiceWithData:v_invoice]]; // THIS Pretty much never displayed
UITextView *l_invoice = [[UITextView alloc] initWithFrame:CGRectMake(0, _v_invoice.frame.origin.y + _v_invoice.frame.size.height + offset, 320.0, 50)];
l_invoice.text = NSLocalizedString(#"summary_emailed", nil);
[self.view addSubview:l_invoice]; // Always displayed
}
However, not all the content is displayed. The invoice is never there at first, but gets displayed after a couple of minutes. The other async-created string, s_content is sometimes not displayed.
This seems to be random with the content creation. The end result is pretty neat, but not reliable for a production version.
I used the undocumented [self.view recursiveDescription] to check if everything was there, and even if I don't see it, they are all there with what seems to be correct frames.
Any pointers?
- layoutSubviews did not help!
- putting a background color to the invoice view is showing the background color

Answer 1

I suspect this line is your problem:
UIView *v_invoice_content = [[BRNetwork sharedNetwork] invoiceViewForBooking:locBooking.objectId];
As you are calling this in a background dispatch queue. Any work involving UIKit should be done on the main queue/thread. Either move that into the main thread block, or if building the view is dependent on data from a network call, change your invoiceViewForBooking method to return the data first, and build your view in the main thread with that data.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Load content
NSString *s_checkout = [[BRNetwork sharedNetwork] getCheckoutInstructionsForLocation:self.locBooking.location];
id someData = [[BRNetwork sharedNetwork] invoiceDataForBooking:locBooking.objectId];
// Display the content
dispatch_async(dispatch_get_main_queue(), ^{
UIView *v_invoice_content = [invoiceViewWithData:someData];
});
});
I'd also suggest using dispatch_async instead of dispatch_sync on the main queue.

Answer 2

Solved! I ended up removing the dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
To only leave the async block on the main queue:
// Display the content from a new async block on the main queue.
dispatch_async(dispatch_get_main_queue(), ^{
[aiv_loading removeFromSuperview];
NSString *s_checkout = [[BRNetwork sharedNetwork] getCheckoutInstructionsForLocation:self.locBooking.location];
[self showContentWithText:s_checkout AndInvoice:[[BRNetwork sharedNetwork] invoiceViewForBooking:locBooking.objectId]];
});
which did the trick, viewcontroller's view can appear without waiting for the view to load the remote data!

Answer 3

You should put your code in viewDidAppear instead of viewDidLoad. Whatever is related to display (even launching async blocks) should always be in viewWillAppear or viewDidAppear.
Also, I recommend you to use dispatch_async(dispatch_get_main_queue(), ^{}) instead of your dispatch_sync since you still want your block to be performed async (but on main thread).
Dont forget to call super methods in your viewDidAppear.