NSOperationQueue becomes suspended at random? - objective-c

Question

I have an app that uses NSOperationQueue intensively.
Sometimes I've noticed that some of the NSOperationQueues would "lock up" or enter a "isSuspended" state randomly even though my code never calls the setSuspended: method.
It's impossible to replicate and very hard to debug because whenever I hook the device up to Xcode to debug it, the app would reload and the bug would go away.
I added a lot of NSLogs at all possible points that might have a problem and just had to use the app for a few days until the bug reemerged.
Looking at the device system logs, I've discovered that [myOperationQueue operationCount] would increment but the opeations in queue will not execute.
I haven't tried manually setting "setSuspended:NO" yet but is that really necessary?
What could be causing this?
Here is a little bit of my code
The view controller that calls operations
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Custom initialization
self.operationQueue = [[[NSOperationQueue alloc] init] autorelease];
[self.operationQueue setMaxConcurrentOperationCount:2];
self.sendOperationQueue = [[[NSOperationQueue alloc] init] autorelease];
[self.sendOperationQueue setMaxConcurrentOperationCount:2];
self.receiveOperationQueue = [[[NSOperationQueue alloc] init] autorelease];
[self.receiveOperationQueue setMaxConcurrentOperationCount:1];
}
}
- (void)dealloc
{
[[NSNotificationCenter defaultCenter] removeObserver:self];
[operationQueue cancelAllOperations];
[sendOperationQueue cancelAllOperations];
[receiveOperationQueue cancelAllOperations];
[operationQueue release];
[sendOperationQueue release];
[receiveOperationQueue release];
}
- (IBAction)sendMessage
{
if(![chatInput.text isEqualToString:#""])
{
NSString *message = self.chatInput.text;
SendMessageOperation *sendMessageOperation = [[SendMessageOperation alloc] initWithMatchData:matchData andMessage:message resendWithKey:nil];
[self.sendOperationQueue addOperation:sendMessageOperation];
[sendMessageOperation release];
}
}
NSOperation subclass SendMessageOperation
- (id)initWithMatchData:(MatchData*)data andMessage:(NSString*)messageString resendWithKey:(NSString*)resendKey
{
self = [super init];
if(self != nil)
{
if(data == nil || messageString == nil)
{
[self release];
return nil;
}
appDelegate = (YongoPalAppDelegate *) [[UIApplication sharedApplication] delegate];
context = [[NSManagedObjectContext alloc] init];
[context setPersistentStoreCoordinator:[appDelegate persistentStoreCoordinator]];
[context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(mergeContextChanges:) name:NSManagedObjectContextDidSaveNotification object:context];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(mergeMainContextChanges:) name:NSManagedObjectContextDidSaveNotification object:appDelegate.managedObjectContext];
self.matchData = (MatchData*)[context objectWithID:[data objectID]];
matchNo = [[matchData valueForKey:#"matchNo"] intValue];
partnerNo = [[matchData valueForKey:#"partnerNo"] intValue];
self.message = messageString;
self.key = resendKey;
apiRequest = [[APIRequest alloc] init];
}
return self;
}
- (void)dealloc
{
[[NSNotificationCenter defaultCenter] removeObserver:self];
self.matchData = nil;
self.message = nil;
self.key = nil;
[context release];
[apiRequest release];
[super dealloc];
}
- (void)start
{
if([self isCancelled] == YES)
{
[self willChangeValueForKey:#"isFinished"];
finished = YES;
[self didChangeValueForKey:#"isFinished"];
return;
}
else
{
[self willChangeValueForKey:#"isExecuting"];
executing = YES;
[self main];
[self didChangeValueForKey:#"isExecuting"];
}
}
- (void)main
{
#try
{
NSAutoreleasePool *pool = [NSAutoreleasePool new];
bool taskIsFinished = NO;
while(taskIsFinished == NO && [self isCancelled] == NO)
{
NSDictionary *requestData = nil;
if(key == nil)
{
requestData = [self sendMessage];
}
else
{
requestData = [self resendMessage];
}
NSDictionary *apiResult = nil;
if(requestData != nil)
{
apiResult = [self sendMessageToServer:requestData];
}
if(apiResult != nil)
{
[[NSNotificationCenter defaultCenter] postNotificationName:#"shouldConfirmSentMessage" object:nil userInfo:apiResult];
}
taskIsFinished = YES;
}
[self willChangeValueForKey:#"isFinished"];
[self willChangeValueForKey:#"isExecuting"];
finished = YES;
executing = NO;
[self didChangeValueForKey:#"isFinished"];
[self didChangeValueForKey:#"isExecuting"];
[pool drain];
}
#catch (NSException *e)
{
NSLog(#"Exception %#", e);
}
}
- (BOOL)isConcurrent
{
return YES;
}
- (BOOL)isFinished
{
return finished;
}
- (BOOL)isExecuting
{
return executing;
}
- (void)mergeContextChanges:(NSNotification *)notification
{
[[NSNotificationCenter defaultCenter] postNotificationName:#"mergeChatDataChanges" object:nil userInfo:[notification userInfo]];
}
- (void)mergeMainContextChanges:(NSNotification *)notification
{
NSSet *updated = [[notification userInfo] objectForKey:NSUpdatedObjectsKey];
for(NSManagedObject *thing in updated)
{
[[context objectWithID:[thing objectID]] willAccessValueForKey:nil];
}
[context mergeChangesFromContextDidSaveNotification:notification];
}

Answer 1

You're using:
setMaxConcurrentOperationCount:X
My hunch is that X operations in the queue have not finished, therefore causing any subsequent operations added to the queue to not run until this is the case.

Answer 2

From the NSOperationQueue's isSuspended method documentation:
If you want to know when the queue’s suspended state changes, configure a KVO observer to observe the suspended key path of the operation queue.
Another idea that you can implement in parallel with the observer is to create a subclass of NSOperationQueue that redefines the setSuspended: method. In the redefined version you can set a breakpoint if you are running on the debugger, or print a stacktrace to the log if you are running w/o debugger.
Hope this helps.

Answer 3

take a look at ASIHTTPRequestConfig.h, it has some flags that you can turn on to see what is really happening behind the scenes.