Question

I could not find any examples how to deal with the same (class) variable when operation queue is used. In C & threads its about mutexes. So, what happens when NSOperationQueue starts a thread for operation and class variable is modified? Is it thread safe? Thank you.

@interface MyTest {
    NSMutableArray *_array;
}
@end

-(id)init
{
    ...
    _array = [NSMutableArray new]; // class variable

        // queue time consuming loading
    NSOperationQueue *queue = [NSOperationQueue new];
    NSInvocationOperation *operation =
        [NSInvocationOperation initWithTarget:self
                                     selector:@selector(populate)
                                       object:nil];
    [queue addOperation:operation];

        // start continuous processing
    [NSTimer scheduledTimerWithTimeInterval:0.1
                                     target:self
                                   selector:@selector(processing)
                                   userInfo:nil
                                    repeats:YES];
    ...
}

-(void)populate
{
    while (...)
    {
        id element = ...; // time consuming

            // modify class variable "_array" from operation's thread (?)
        [_array addObject:element];

            // Ok, I can do instead of addObject
            // performSelectorOnMainThread:withObject:waitUntilDone:
            // but is it the only way? Is it needed?
    }
}

    // access and/or modify class variable "_array"
-(void)processing
{
    NSLog(@"array.count = %d", array.count);
    for (id i in _array)
    {
        [_array addObject:[NSNumber numberWithInt:rand() % 100]];
            // etc...
    }
}

Answer 1

No, this is not thread safe, if you start a thread that does some work on a class variable that can be modified by some other thread then its not thread safe, if processing is called from some thread while populate is running on another then you might get an exception when the foreach loop sees that the array has been modified, though you will get that exception anyway as you are modifying the array inside the foreach loop in your example (you shouldnt do that, and the program will throw an exception )… One way to get around this can be with a synchronized block on the array, it will ensure that the synchronized blocks wont be executed at the same time, the thread blocks until one synchronized block finishes, for example

    -(void)populate
    {


        while (...)
        {
            id element = ...; // time consuming

                // modify class variable "_array" from operation's thread (?)
      @synchronized(_array)
        {
            [_array addObject:element];

        }          // Ok, I can do instead of addObject
                // performSelectorOnMainThread:withObject:waitUntilDone:
                // but is it the only way? Is it needed?
        }

    }

        // access and/or modify class variable "_array"
    -(void)processing
    {


          @synchronized(_array)
         {
            NSLog(@"array.count = %d", array.count);
            for (id i in _array)
            {
                //you shouldnt modify the _array here you will get an exception
                    // etc...
            }
        }
    }