#synchronized not working on NSMutableArray - objective-c

Question

I am trying to remove and object from an mutable array - an array which is iterated through every frame (see tick: method).
I am getting
* Collection <__NSArrayM: 0xaa99cb0> was mutated while being enumerated.
exceptions.
So I added #synchronized() to lock it from being touched by other threads, but its still failing.
- (void)addEventSubscriber:(id <EventSubscriber>)eventSubscriber
{
[_eventSubscribers addObject:eventSubscriber];
}
- (void)removeEventSubscriber:(id <EventSubscriber>)eventSubscriber
{
#synchronized(_eventSubscribers) // Not working.
{
[_eventSubscribers removeObject:eventSubscriber];
}
}
- (void)tick:(ccTime)dt
{
for (id <EventSubscriber> subscriber in _eventSubscribers)
{
if ([subscriber respondsToSelector:#selector(tick:)])
{
[subscriber tick:dt];
}
}
}

Answer 1

You need to lock updates to the array completely while iterating. Adding synchronized blocks to both methods addEventSubscriber: and removeEventSubscriber: will not work because the array can change while being iterated over because the iteration is not synchronized. Simply put, only one of those three methods can run at a time.
You can use #synchronized, or an NSLock to manually lock array updates while it is being iterated over.
Alternatively, you could use GCD with a serial dispatch queue to ensure that only one method is executing at a time. Here's how that would work:
You could also store the queue as a property of the class object in which you're doing this processing.
// Create the queue
dispatch_queue_t myQueue = dispatch_queue_create("myQueue", NULL);
- (void)addEventSubscriber:(id <EventSubscriber>)eventSubscriber
{
dispatch_sync(myQueue, ^{
[_eventSubscribers addObject:eventSubscriber];
});
}
- (void)removeEventSubscriber:(id <EventSubscriber>)eventSubscriber
{
dispatch_sync(myQueue, ^{
[_eventSubscribers removeObject:eventSubscriber];
});
}
- (void)tick:(ccTime)dt
{
dispatch_sync(myQueue, ^{
for (id <EventSubscriber> subscriber in _eventSubscribers)
{
if ([subscriber respondsToSelector:#selector(tick:)])
{
[subscriber tick:dt];
}
}
});
}

Answer 2

You are only obtaining a lock while removing items from your array, not while enumerating items. The error suggests that within an enumeration you're trying to remove an item, which is allowed by your locking but not enumeration.
Simply locking the array before enumerating may not work either. The same thread can lock an object recursively, but if your enumeration and remove are on different threads then trying to remove within an enumeration would cause deadlock. If you are in this situation you'll need to rethink your model.

Answer 3

I run into this problem a lot. I have no experience with thread handling / synchronization beyond an undergraduate OS course, so this is what I came up with.
Every time you iterate over the list of objects and want to remove something - instead add that object to a global "objectsToRemove" array. In your update method, remove everything from the objectsToRemove, then clean up the array to avoid over-removing an object on the next update.
Cocos2D has a CCArray which is essentially an NSMutableArray with some added functionality– like being able to remove an item while iterating. I haven't read through the code myself, so I'm not sure how it is implemented and therefore I don't use it.

Answer 4

you need add synchronized in this functin too.
- (void)tick:(ccTime)dt
{
#synchronized(_eventSubscribers){
for (id <EventSubscriber> subscriber in _eventSubscribers)
{
if ([subscriber respondsToSelector:#selector(tick:)])
{
[subscriber tick:dt];
}
}
}
}