Status application: sometimes panels are missing, multithreading possible issue - objective-c

Question

I have a status application, it's long to post it all so I'll describe it and post only part of the code:
In the xib file there are two objects: AboutController and PreferencesController;
The app delegate is able to launch AboutController's and PreferencesController's panels;
The panels are in the xib file too;
The user by selecting a status menu item is able to launch these two panels;
There is a timer that constantly downloads a HTML page and reads it;
When the page is downloaded, the stringValue of a label is changed.But the stringValue may also be changed from the PreferencesController.The page is downloaded from a background thread, but it's changed through the main queue.
Now I've got some questions:
Do I have to invalidate the timer when the application starts sleeping (computer goes in standby),and create another one when it returns on?
The label is updated in the main queue, so I still have to protect the label access with a mutex?
Sometimes the panels are missing: at the start of the application I am able to launch panels by clicking a menu item, but sometimes they are not launching.I don't know how to reproduce this bug always, it just happens randomly when the application is active for 2/3 hours usually, I have to relaunch the application to go around this.
The code is too long, that's only a piece of code:
- (void) checkPosts: (id ) sender
{
NSOperationQueue* queue=[NSOperationQueue new];
queue.maxConcurrentOperationCount=1;
[queue addOperationWithBlock:^
{
NSNumber* newPosts= [self updatePosts];
NSNumber* posts= [controller posts];
if([posts integerValue]!=[newPosts integerValue])
{
NSOperationQueue* queue=[NSOperationQueue mainQueue];
posts= newPosts;
[queue addOperationWithBlock:^
{
// This is where I may have a race condition
item.attributedTitle=[[NSAttributedString alloc]initWithString: [formatter stringFromNumber: posts] attributes: #{NSForegroundColorAttributeName : [controller color], NSFontAttributeName : [NSFont userFontOfSize: 12.5]}];
}];
// That's not so relevant:
NSUserNotification* notification=[NSUserNotification new];
notification.title= [NSString stringWithFormat: #"posts Changed to %#",posts];
notification.deliveryDate=[NSDate date];
notification.soundName= NSUserNotificationDefaultSoundName;
NSUserNotificationCenter* center=[NSUserNotificationCenter defaultUserNotificationCenter];
[center deliverNotification: notification];
center.delegate= self;
[controller setPosts: posts];
}
}];
}
A little background information:
This method works in a background thread;
[self updatePosts] downloads the HTML page and returns the number of posts;
[controller posts] reads the previous number of posts using NSUserDefaults;
item is a status menu's menu item.
More Details
This is how I get the timer:
// In the applicationDidFinishLaunching method
timer=[NSTimer scheduledTimerWithTimeInterval: [interval integerValue ] target: self selector: #selector(checkReputation:) userInfo: nil repeats: YES];
timer is a property:
#property (nonatomic, strong) NSTimer* timer;
interval is a NSNumber, for sure it's integer value is greater or equal than 1.

Answer 1

It's not entirely clear what's happening here. You've provided plenty of information but not everything needed to give a definitive answer. I'll try to address your questions first:
Do I have to invalidate the timer when the application starts sleeping
(computer goes in standby),and create another one when it returns on?
Have to? No. Should for cleanliness and certainty of the state? Yes, probably. You should probably specify exactly how you set up the timer since you can run into problems with it interacting with the run loop ... but I don't think this is your problem.
The label is updated in the main queue, so I still have to protect the
label access with a mutex?
As long as you update the UI from the main thread/queue, you should be fine. This is a standard design approach with blocks.
Sometimes the panels are missing: at the start of the application I am
able to launch panels by clicking a menu item, but sometimes they are
not launching.I don't know how to reproduce this bug always, it just
happens randomly when the application is active for 2/3 hours usually,
I have to relaunch the application to go around this.
If you don't know how to reproduce it, I'm not sure we can help you beyond "places to look." The first thought I've got is that you may be recreating multiple copies of your primary controllers when the app becomes active (since you asked about this earlier, I assume you've tried doing something with it). Make sure the same controllers are being reused.
Now on to the code.
NSOperationQueue* queue=[NSOperationQueue new];
The queue variable is local to the scope of the method. I see no retain/release, so I assume you're using ARC. In that case, you're not retaining the new queue you're creating and its life span is not guaranteed to survive as long as you need it to once the method completes and you've left its scope. You should make queue an instance variable so it sticks around. This way the queue can be reused every time the method is fired and it'll stay around long enough for other queues/threads to use.
I think it's likely this is your biggest culprit. Adjust it and update your question to reflect how it affects the condition of your app.