Objective-c – Avoiding “NSArray was mutated while being enumerated”

cocoacocoa-touchcollectionsnsmutablearrayobjective c

I have an NSMutableArray that stores mousejoints for a Box2d physics simulation. When using more than one finger to play I'll get exceptions stating

NSArray was mutated while being enumerated

I know this is because I'm deleting objects from the array while also enumerating through it, invalidating the enum.

What I want to know is what is the best strategy to solve this going forward? I've seen a few solutions online: @synchronized, copying the array before enumerating or putting the touch joint into a garbage array for later deletion (which I'm not sure would work, because I need to remove the mousejoint from the array straight after removing it from the world).

Best Answer

You can always iterate without an enumerator. Which means a regular for loop, and when you remove an object:- decrement the index variable and continue;. if you are caching the array's count before entering the for-loop, then make sure you decrement that too when removing an object.

Anyway, I do not see why an array with objects for later removal would be a problem. I do not know the exact situation which you are having and the technologies involved, but theoretically there should not be a problem. Because in most cases when using this method you can just do nothing in the first enumeration, and do the real work when enumerating the removal array. And if you have a situation where in the first enumeration you are checking something again against the same array and you need to know that the objects are not there anymore, you can just add a check to see if they are in the removal array.

Anyway, hope I helped. Good luck!

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OK, I think I've solved my problem and I must thank this blog post from Fred McCann's :

http://www.duckrowing.com/2010/03/11/using-core-data-on-multiple-threads/

The problem seems to come from the fact that I instantiate my background moc on the main thread instead of the background thread. When Apple tells that each thread needs to have its own moc, you have to take it seriously : each moc must be instantiated in the thread that will be using it !

Moving the following lines...

// We instantiate the background moc

self.backgroundMOC = [[[NSManagedObjectContext alloc] init] autorelease];

[self.backgroundMOC setPersistentStoreCoordinator:[self.managedObjectContext persistentStoreCoordinator]];

...in the _importData method (just before to register the controller as observer for the notification) solves the problem.

Thanks for your help, Peter. And thanks to Fred McCann's for its valuable blog post !

Php take users input write to file, and echo out the file for user to view

You want to actually generate a new HTML file for each user that logs in? That kind of defies the point of server-side scripting.

In fact, it's a bad idea security-wise to let your scripts have write access to your web server root.

What you'd want to do is link to "sometextfile.php?sessid=" or use a cookie containing their session ID, then generate that page dynamically.

If you just want to write a text file to disk somewhere, you can create a string then use file_put_contents (http://php.net/file_put_contents). This could be put in the web root, but I'm guessing that's not actually what you're trying to do.

Best Answer

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Cocoa – “Collection was mutated while being enumerated” on executeFetchRequest

Php take users input write to file, and echo out the file for user to view

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