Avoid copying NSMutableArray for reading with multithreaded writes

Question

I have a class that uses a mutable array that is modified once after a lot of reads (new items arrive).

The problem is that when times comes to mutate the array, rea

Answer 1

The simplest approach is to use @synchronized, like this:

-(void) accessTheArray {
    MyClass *obj;
    @synchronized(theArray) {
        obj = [theArray objectAtIndex:...];
    }
    [obj someMessage];
}

EDIT: If not using ARC, you might want to retain/autorelease the object, otherwise it might be removed from the array (and released) before someMessage is called (thanks for omz for this excellent comment).

Answer 2

in this case, you would consider using a read/write lock. Cocoa doesn't provide them, but pthread_rwlock_t is available in pthreads interfaces -- declared in pthread.h. note that this is going to be much more efficient (for your usage) than @synchronized, or even a plain lock.

Answer 3

Put all your array accesses onto a serial dispatch queue. That will prevent any two operations from occuring at the same time. See "Eliminating Lock-based Code" in the Concurrency Programming Guide.

If you can require iOS >= 4.3, you can use a concurrent custom queue and dispatch barriers for the mutation operations. This will allow the reads to happen simultaneously, but when a write is necessary they'll be held up until the write finishes. The block submitted as a barrier essentially executes serially on a concurrent queue -- it will not begin until all previous blocks have finished, nor will any subsequent blocks begin until the barrier block completes. (This is the GCD version of the read-write lock that Justin mentions.) I direct you to the inimitable Mike Ash for samples of this.