Locking an object from being accessed by multiple threads - Objective-C

Question

I have a question regarding thread safety in Objective-C. I\'ve read a couple of other answers, some of the Apple documentation, and still have some doubts regarding this, s

Answer 1

Subclass NSMutableArray to provide locking for the accessor (read and write) methods. Something like:

@interface MySafeMutableArray : NSMutableArray { NSRecursiveLock *lock; } @end

@implementation MySafeMutableArray

- (void)addObject:(id)obj {
  [self.lock lock];
  [super addObject: obj];
  [self.lock unlock];
}

// ...
@end

This approach encapsulates the locking as part of the array. Users don't need to change their calls (but may need to be aware that they could block/wait for access if the access is time critical). A significant advantage to this approach is that if you decide that you prefer not to use locks you can re-implement MySafeMutableArray to use dispatch queues - or whatever is best for your specific problem. For example, you could implement addObject as:

- (void)addObject:(id)obj {
    dispatch_sync (self.queue, ^{ [super addObject: obj] });
}

Note: if using locks, you'll surely need NSRecursiveLock, not NSLock, because you don't know of the Objective-C implementations of addObject, etc are themselves recursive.