UIActivityIndicatorView subview disappears before NSOperationQueue is finished - ios

Question

I'm trying to get a UIActivityIndicatorView to show on screen while my CSV import method is running, but I can't get it right. With the code below, the ActivityIndicator subview shows for a second or so then disappears, even if the import operation is still running. How can I make it stay on screen until the NSOperationQueue is finished? I'm using iOS 7.1 on my test device.
User taps 'Yes' on an UIAlertView to import the data:
-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
if(buttonIndex == 0){
//clicked Yes
[self loadingSpinner];
operationQueue = [NSOperationQueue new];
NSInvocationOperation *importOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(importCSVData:) object:self.importURL];
[operationQueue addOperation:importOperation];
}
else if(buttonIndex == 1){
//clicked No
}
}
Method to show a UIActivityIndicatorView on top of everything else on screen:
-(void)loadingSpinner{
self.overlayView = [[UIView alloc] init];
self.overlayView.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:0.5];
UIView *topView = [UIApplication sharedApplication].keyWindow.rootViewController.view;
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad) {
self.overlayView.frame = [UIScreen mainScreen].bounds;
}
else{
self.overlayView.frame = topView.frame;
}
[self.overlayView setUserInteractionEnabled:NO];
self.spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
self.spinner.center = self.overlayView.center;
[self.overlayView addSubview:self.spinner];
[self.spinner startAnimating];
[topView addSubview:self.overlayView];
}
At the end of the import operation to remove the activity indicator:
[self.overlayView removeFromSuperview];

Answer 1

Use addOperationWithBlock: method on NSOperationQueue with which you will have more control. You would edit your code like this to use block version,
NSOperationQueue *queue = [NSOperationQueue new];
[queue addOperationWithBlock:^{
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
[self loadingSpinner];
}];
[self importCSVData:self.importUrl];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
[self.overlay removeFromSuperView];
}];
}];

Answer 2

You can also use GCD to implement this task.
Create dispatch queue:
dispatch_queue_t queue = dispatch_queue_create("Other Q", NULL);
or you can use the main queue like:
dispatch_get_main_queue()
Finally replace your code with the following:
dispatch_async(queue, ^{
dispatch_sync(dispatch_get_main_queue(),^{
[self loadingSpinner];
});
[self importCSVData:self.importURL];
});
You might want to check for the title of you button clicked on your UIAlertView.
To do so change this:
if(buttonIndex == 1)
With this:
if([[alertView buttonTitleAtIndex:buttonIndex]isEqual:#"YES"])
Let me know if this helps.