UIAlertView with a UIActivityIndicator shows up too late/Threading issue - iphone

Question

In my app I have a couple places where I'm calling a web service and retrieving JSON, which is then parsed into arrays of objects. The time-consuming part is definitely the web service call. The behavior I desire is as follows:
User selects row of interest
On current view, AlertView with ActivtiyIndicator subview added
Data loads in new VC
New view appears
AV indicator gone
My VC's are equipped with doLoadData methods that basically load an array of data that is displayed by each VC. For example:
AnimalViewController has a button "View Dogs" that when pressed does:
DogViewController *vc = [[DogViewController alloc] init];[self.navigationController pushViewController:vc animated:YES];
[vc release];
DogViewController has property NSArray *dogs and method doLoadDogs, which takes a while to happen.
Currently I have in the "View Dogs" method:
Create an AlertView with an ActivityIndicator
Show it
Create the VC
Push it
Hide the AlertView
Release and nil it
This seems like it would not be difficult but I've yet to find a simple implementation of this with threading.

Answer 1

If your network call/array creation takes a long time and is on the main thread, then all of this will essentially happen at the same time. When you do things like show an alert view, push a navigation controller, etc., what you're really doing is scheduling these things to happen at the next turn of the run loop. Since your "View Dogs" method essentially blocks until everything is done, the run loop isn't executed again until after everything is needed.
The best solution is to factor out your network loading code and not block your main thread/main event loop. As a quick fix, you could factor out the pieces of your current "View Dogs" method using GCD. As an example (obviously this would need to be updated to your actual requirements/code):
- (void)viewDogs:(id)sender
{
dispatch_queue_t mainQueue = dispatch_get_main_queue();
// create and schedule the alert view
UIAlertView* alert = [[UIAlertView alloc] initWithTitle...]; // your real alert view
dispatch_async(mainQueue, ^{ [alert show]; });
dispatch_async(mainQueue, ^{
DogViewController* vc = [[DogViewController alloc] init];
[[self navigationController] pushViewController:vc animated:YES];
[vc release];
dispatch_async(mainQueue, ^{ [alert dismissWith...]; [alert release]; });
});
}