How to target the parent object when using performSelectorOnMainThread? - objective-c

Question

I have a custom NSOperation object that is instantiated from a UITableViewController subclass (MusicTableVC). The NSOperation object is supposed to populate an NSarray from a URL in the background so the UI doesn't freeze up, but then I need to send that array back to main thread so the MusicTableVC instance can do stuff with it.
I know I need to use performSelectorOnMainThread: to send the array back to the MusicTableVC but to do that I need a pointer to the instance of MusicTableVC.
I was thinking about creating an init method in the NSOperation e.g. initWithParent to pass on a pointer to self and use that but maybe I'm missing something?
#synthesize parent;
- (id)initWithParent:(MusicTableVC*) musicTableViewController
{
if(self = [super init])
{
self.parent = musicTableViewController;
}
return self;
}
-(void) main
}
[parent performSelectorOnMainThread:#selector(arrayFinishedLoading:)
withObject:playlist
waitUntilDone:YES];
}

Answer 1

I think you would do better with blocks and Grand Central Dispatch:
dispatch_async(dispatch_get_queue(DISPATCH_QUEUE_PRIORITY_NORMAL, 0), ^{
// This is called in background, not blocking the UI
[self populateArrayFromURL];
dispatch_async(dispatch_get_main_queue(), ^{
// This is called on the main thread
[self reportStuffDone];
});
});

Answer 2

that is one way of doing it, but a more common pattern is to have the parent of the NSOperation observe its state, and do something with the results when it is complete. so when you create your operation in the view controller, do something like this:
NSOperation *myOp = [[NSOperation alloc] init];
[myOp addObserver:self forKeyPath:#"isFinished" options:NSKeyValueObservingOptionsNew context:NULL];
then add the KVO callback method:
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
NSOperation *myOp = (NSOperation *)object;
[myOp removeObserver:self forKeyPath:keyPath];
dispatch_async(dispatch_get_main_queue(), ^{
// reload the table with the results here
});
}

Answer 3

Your way is fine, I would call it initWithDelegate and define a protocol thou. Just pass the delegate and then the operation is finished just ket it know if it succeded or not.
Recently I've been switching from useless delegates to GCD, so instead I would make something like initWithSuccessBlock and then dispatch it to the main queue. Attention that, if you decide to use this you would have to make sure the block was copied.