Error when using NSMutableSet

Question

I get the error

* Terminating app due to uncaught exception 'NSGenericException', reason: '* Collection <__NSCFSet: 0x6b66390> was mutated while being enumerated.'

when adding an new delegate to my class. Or at least, that's where I think the problem is.

This is my code: MyAppAPI.m

[...]
static NSMutableSet *_delegates = nil;

@implementation MyAppAPI

+ (void)initialize
{
    if (self == [MyAppAPI class]) {
        _delegates = [[NSMutableSet alloc] init];
    }
}

+ (void)addDelegate:(id)delegate
{
    [_delegates addObject:delegate];
}

+ (void)removeDelegate:(id)delegate
{
    [_delegates removeObject:delegate];
}
[...]

@end

MyAppAPI is a singleton which I can use throughout my application. Wherever I can (or should be able to) do: [MyAppAPI addDelegate:self] .
This works great, but only in the first view. This view has a UIScrollView with PageViewController which loads new views within itself. These new views register to MyAppAPI to listen to messages until they are unloaded (which in that case they do a removeDelegate ). However, it seems to me that it dies directly after I did a addDelegate on the second view in the UIScrollView.

How could I improve the code so that this doesn't happen?

Update
I'd like to clarify me a bit further. What happens is that view controller "StartPage" has an UIScrollView with a page controller. It loads several other views (1 ahead of the current visible screen). Each view is an instans PageViewController, which registers itself using the addDelegate function shown above to the global singleton called MyAppAPI. However, as I understand this viewcontroller 1 is still reading from the delegate when viewcontroller 2 registers itself, hence the error shows above.

I hope I made the scenario clear. I have tried a few things but nothing helps. I need to register to the delegate using addDelegate even while reading from the delegates. How do I do that?

Update 2 This is one of the reponder methods:

+ (void)didRecieveFeaturedItems:(NSArray*)items
{   
    for (id delegate in _delegates)
    {
        if ([delegate respondsToSelector:@selector(didRecieveFeaturedItems:)])
            [delegate didRecieveFeaturedItems:items];
    }
}

Answer 1

Scott Hunter is right. This error is thrown when you try to edit a list while iterating.

So here is an example of what you may be doing.

+ (void)iteratingToRemove:(NSArray*)items {   
    for (id delegate in _delegates) {
        if(delegate.removeMePlease) {
          [MyAppAPI removeDelegate:delegate];  //error you are editing an NSSet while enumerating
        }
    }
}

And here is how you should handle this correctly:

+ (void)iteratingToRemove:(NSArray*)items
{   
    NSMutableArray *delegatesToRemove = [[NSMutableArray alloc] init];
    for (id delegate in _delegates) {
        if(delegate.removeMePlease) {
          [delegatesToRemove addObject:delegate];
        }
    }

    for(id delegate in delegatesToRemove) {
         [MyAppAPI removeDelegate:delegate];  //This works better
    }

    [delegatesToRemove release];
}

Answer 2

The error suggests that, while some code somewhere is in the middle of going through your list, you are modifying the list (which explains the crash after addDelegate is called). If the code doing the enumerating is the one modifying the list, then you just have to put off the modifications until the enumeration is done (say, by collecting them up in a different list). Without knowing anything about the code doing the enumerating, can't say much more than that.

Answer 3

A simple solution, don't use a mutable set. They are dangerous for a variety of reasons, including this one.

You can use -copy and -mutableCopy to convert between mutable and non-mutable versions of NSSet (and many other classes). Beware all copy methods return a new object with a retain count of 1 (just like alloc), so you need to release them.

Aside from having less potential for bugs, non-mutable objects are faster to work with and use less memory.

[...]
static NSSet *_delegates = nil;

@implementation MyAppAPI

+ (void)initialize
{
    if (self == [MyAppAPI class]) {
        _delegates = [[NSSet alloc] init];
    }
}

+ (void)addDelegate:(id)delegate
{
    NSMutableSet *delegatesMutable = [_delegates mutableCopy];
    [delegatesMutable addObject:delegate];

    [_delegates autorelease];
    _delegates = [delegatesMutable copy];

    [delegatesMutable release];
}

+ (void)removeDelegate:(id)delegate
{
    NSMutableSet *delegatesMutable = [_delegates mutableCopy];
    [delegatesMutable removeObject:delegate];

    [_delegates autorelease];
    _delegates = [delegatesMutable copy];

    [delegatesMutable release];
}
[...]

@end

Answer 4

Scott Hunter is right - it's a problem with modifying the NSSet while you're enumerating over the set's items. You should have a stack trace from where the application crashes. It probably has a line where you're adding to/remove from the _delegates set. This is where you need to make the modification. It's easy to do. Instead of adding to/deleting from the set, do the following:

NSMutableSet *tempSet = [_delegates copy];
for (id delegate in _delegates)
{
    //add or remove from tempSet instead
}
[_delegates release], _delegates = tempSet;

Additionally, NSMutableSet is not thread safe , so you should call your methods always from the main thread. If you haven't explicitly added any extra threads, you have nothing to worry about.

Answer 5

A thing to always remember about the Objective-C "fast enumeration".
There is 2 big difference between "fast enumeration" and a for loop.

"fast enumeration" is quicker than a for loop.
BUT
You can't modify the collection your enumerating over.

You can ask your NSSet for - (NSArray *)allObjects and enumerate over that array while modifying your NSSet.

Answer 6

You get this error when a thread tries to modify (add,delete) the array while other thread is iterating over it.

One way to solve this using NSLock or synchronizing the methods. That ways add, remove and iterate methods cannot be called in parallel. But this will have effect on performance and/or responsiveness because any add/delete will have to wait for the thread that was iterating over the array.

A better solution inspired from Java's CopyOnWriteArrayList would be to create a copy of the array and iterate over the copy. So the only change in your code will be:-

//better solution
+ (void)didRecieveFeaturedItems:(NSArray*)items
{   
    NSArray *copyOfDelegates = [_delegates copy]
    for (id delegate in copyOfDelegates)
    {
        if ([delegate respondsToSelector:@selector(didRecieveFeaturedItems:)])
            [delegate didRecieveFeaturedItems:items];
    }
}

Solution using locks with performance impact

//not a good solution

+ (void)addDelegate:(id)delegate
{
    @synchronized(self){
        [_delegates addObject:delegate];
    }
}

+ (void)removeDelegate:(id)delegate
{
    @synchronized(self){
        [_delegates removeObject:delegate];
   }
}

+ (void)didRecieveFeaturedItems:(NSArray*)items
{   
    @synchronized(self){
        for (id delegate in _delegates)
        {
            if ([delegate respondsToSelector:@selector(didRecieveFeaturedItems:)])
                [delegate didRecieveFeaturedItems:items];
        }
    }
}