Running method on background thread making animations stop after a while - ios

Question

I have two methods on my app delegate to start and stop an activity indicator. I need to call them on a background thread so that it is always visible. Like this:
[self.delegate performSelectorInBackground:#selector(acionarActivityIndicator) withObject:nil];
or
[NSThread detachNewThreadSelector:#selector(acionarActivityIndicator) toTarget:self.delegate withObject:nil];
Both work the same.
Suppose this is VCA button click, then, on VCB after loading everything, I call another method in my app delegate to stop animating. I perform other actions in the background on both views to get my data.
The activity indicator works fine. The problem is that after a while, all animations in my app stop working. I feel like it is related to this, but I'm not sure.
If I try to make screen transitions faster, I get this bug earlier. Does that make sense?
App Delegate methods:
- (void)acionarActivityIndicator {
// Show the activity indicator
self.view = [[UIView alloc] init];
self.activityIndicator = [[UIActivityIndicatorView alloc] init];
self.view.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:0.5];
self.view.frame = self.window.bounds;
self.activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
CGRect frame = self.view.frame;
self.activityIndicator.center = CGPointMake(frame.size.width/2, frame.size.height/2);
[self.view addSubview:self.activityIndicator];
[self.activityIndicator startAnimating];
[self.window addSubview:self.view];
[self.window makeKeyAndVisible];
}
- (void)pararActivityIndicator
{
// metodo parar activity indicator
[self.view removeFromSuperview];
[self.activityIndicator stopAnimating];
}

Answer 1

I would say that your problem is that you are manipulating your UI on a background thread. The documentation clearly says that you shouldn't do so:
Note: For the most part, UIKit classes should be used only from an application’s main thread. This is particularly true for classes derived from UIResponder or that involve manipulating your application’s user interface in any way.
UIActivityIndicatorView, which you are using, inherits from UIResponder so what you are doing falls under the "particularly true" case.
You are probably doing long synchronous work on the main thread which is what split the work into multiple threads in the beginning. For this I have 2 things to say:
1. You divided the work in the wrong way
It sound like you kept the work / data processing / networking / other long running synchronous task on the main thread and chose to dispatch UI updates to another thread. As the documentation (quoted above) says, this is the wrong order. You should do any long running synchronous task in the background and call back to the main thread when the UI needs to update.
2. There are technologies that are much easier to use than NSThread
You chose to use NSThread or other thread related technologies to divide your work. While this is the case on other platforms. iOS prefers to use queues for concurrency.
At a lower level you have grand central dispatch (GCD for short) that allows you to queue up a block on the main queue or on a background queue. You also have NSOperations to divide the work into operations that can have dependencies in between each other.
For what you are doing, I would try something along these lines:
- (IBAction)doHeavyWork:(id)sender
{
/* start the activity indicator (you are now on the main queue) */
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
/* Do some heavy work (you are now on a background queue) */
dispatch_sync(dispatch_get_main_queue(), ^{
/* stop the activity indicator (you are now on the main queue again) */
});
});
}
And also, take another look at the UIKit documentation and perhaps the iOS App Programming Guide.