Question

Im executing a thread that keeps looking for updates from a web site. It should be possible to set the refresh rate in some view.

The update thread keeps checking for a updated interval. But I would like to avoid race conditions. (Do I even have to worry for this with GCD?)

//This variable is used to avoid race conditions, refreshRate is a instance variable
int threadRefreshRate = refreshRate;
BOOL autoRefresh = YES;


dispatch_async(autoUpdateQueue, ^ { 
    while(YES){
        NSLog(@"Runs autoupdate thread");
        dispatch_async(dispatch_get_main_queue(), ^{

            if(autoRefresh){
                [self checkForUpdate];
                //Trying to set thread variable to avoid race condition
                threadRefreshRate = refreshRate;
            }
            else
                NSLog(@"Should not auto refresh");
        });
        sleep(threadRefreshRate);
    }

});

I tried to implement this code. However it doesn’t work to assing the ‘thread’-variable within a block.

Answer 1

For the kind of code you have given, i would use the timer events raised on the queue instead doing explicit sleep in the code. This way you dont have to worry about the race conditions etc..

queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
if (!timer) return;
dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), refreshRate * NSEC_PER_SEC, 5 * NSEC_PER_SEC);
dispatch_source_t timer = self.timer;
//initialize self to blockSelf with __block
self.timerAction = ^{
[blockSelf checkForUpdate];
};

dispatch_source_set_event_handler(timer, timerAction);
dispatch_resume(timer);

When the autoRefresh is set to NO, you can cancel it by

dispatch_source_cancel(timer);