dispatch_async and frame rate - ios

Question

I'm trying to learn GCD, so I don't have a full grasp on how it works yet. For some reason, I experience a permanent drop in frame rate after I call the following method. If I don't use the dispatch functions and simply write the data on the main loop, the frame rate stays at 60. I don't know why.
-(void)saveDataFile {
_hud = [MBProgressHUD showHUDAddedTo:self.parentView animated:YES];
_hud.labelText = NSLocalizedString(#"Saving data...", nil);
dispatch_queue_t myQueue = dispatch_queue_create("myQueueName", NULL);
dispatch_async(myQueue, ^(void) {
#autoreleasepool {
id data = [self.model getData];
if (data != nil) {
NSString *filePath = #"myPath";
[data writeToFile:filePath atomically:YES];
}
}
dispatch_async(dispatch_get_main_queue(), ^(void) {
[_hud hide:YES];
});
});
}

Answer 1

Solved. I followed the implementation of HUD from this question: MBProgressHUD not showing
Basically, I need to remove the HUD rather than simply hide it. Otherwise the HUD animation continued, invisible to me, hence causing the drop in frame rate.
-(void)saveDataFile {
// create HUD and add to view
MBProgressHUD *hud = [[MBProgressHUD alloc]initWithView:self.parentView];
hud.labelText = NSLocalizedString(#"Saving data...", nil);
hud.delegate = self;
[self.parentView addSubview:hud];
// define block for saving data
void (^saveData)() = ^() {
#autoreleasepool {
id data = [self.model getData];
if (data != nil) {
NSString *filePath = #"myPath";
[data writeToFile:filePath atomically:YES];
}
}
}
// use HUD convenience method to run block on a background thread
[hud showAnimated:YES whileExecutingBlock:saveData];
}
// remove hud when done!
//
- (void)hudWasHidden:(MBProgressHUD *)hud {
[hud removeFromSuperview];
}