Question

So I’m using performSelectorInBackground:@selector(loadStuff) to load things in the background. It takes some time.

The user may want to re-load the items, while the method above is running in the background. If I performSelectorInBackground:@selector(loadStuff) again while the other method is already running, I get all kinds of errors.

Is there an easy way to handle this situation?

I’d like to stop the already-running-in-the-background method, then start the new one. (Or if there is a better way of accomplishing the end goal, that would be fine as well).

Answer 1

If you want to start over, you can cancel the connection and create a new one. Since you’re going to run this method on a background thread you need to make sure that only one thread can access the related instance variables at a time:

- (void)loadStuff
{
    @synchronized(self) {
        if (currentConnection != nil)
            [currentConnection cancel];
            [currentConnection release];
        }
        currentConnection = [[NSURLConnection alloc] initWithRequest:request 
                                                            delegate:self 
                                                    startImmediately:YES];
    }
}

Another way to do it is to use a flag to indicate a busy state. For example, if you wanted to just return if the long-running method were already in process on another thread, you could do this:

- (void)loadStuff
{
    @synchronized(self) {
        if (loadingStuff == YES)
            return;
        }
        loadingStuff = YES;
    }

    NSURLRequest *request = ...
    NSURLResponse *response;
    NSError *error;
    [NSURLConnection sendSynchronousRequest:request returningReseponse:&response error:&error];

    @synchronized(self) {
        loadingStuff = NO;
    }
}