How do you safely lock a variable using GCD? - cocoa

Question

I have an NSMutableArray I need to add objects to from multiple blocks I have dispatched. Is this an acceptable way to make sure that the array is safely being changed? These are already being dispatched from inside and NSOperation and running in the background. I was loading the data from within that thread serially but it was getting very slow to load a list of locations at once.
NSMutableArray *weatherObjects = [[NSMutableArray alloc] init];
ForecastDownloader *forecastDownloader = [[ForecastDownloader alloc] init];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t serialQueue;
serialQueue = dispatch_queue_create("us.mattshepherd.ForecasterSerialQueue", NULL);
for (NSDictionary *theLocation in self.weatherLocations) {
// Add a task to the group
dispatch_group_async(group, queue, ^{
NSLog(#"dispatching...");
int i = 0;
WeatherObject *weatherObject = [forecastDownloader getForecast:[theLocation objectForKey:#"lat"] lng:[theLocation objectForKey:#"lng"] weatherID:[[theLocation objectForKey:#"id"] intValue]];
}
if(!weatherObject){
//need to implement delegate method to show problem updating weather
NSLog(#"problem updating weather data");
}else{
NSLog(#"got weather for location...");
dispatch_sync(serialQueue, ^{
[weatherObjects addObject:weatherObject];
});
}
});
}
// wait on the group to block the current thread.
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
NSLog(#"finished getting weather for all locations...");
//we will now do something with the weatherObjects

Answer 1

That's not going to work because you're making a new lock each time, rather than using a single lock for the variable (analogy: imagine a locked door to a room. If everyone gets their own door with a lock, it hardly matters that they lock it, since everyone else will come in their own door).
You can either use a single NSLock for all iterations, or (basically equivalently) a single serial dispatch queue.