Multithreading problem(UI not updated)

Question

Hey, I have a problem in Multithreading. To describe this, I have two threads. On main thread I have some logic And on another thread, I have a logic and some logic to UI which will update UI. What I want to do is that I want to call some method after the UI on another thread is updated. I think I am supposed to use NSOperationQueue or something. or background queue. but I have read the concurrency programming and threading guide and I still have no idea how to do it.

So how to do it so that I can call a method after a thread is finished? thanks!

Answer 1

You should never touch the UI from any thread other than the main thread. Two easy ways of sending messsages between threads:

performSelector:

[someObject performSelectorOnMainThread:@selector(someSelector) withObject:nil waitUntilDone:NO];

Notifications:

NSNotification *notification = [NSNotification notificationWithName:@"myNotification" object:nil];
[[NSNotificationCenter defaultCenter] performSelectorOnMainThread:@selector(postNotification:) withObject:notification waitUntilDone:NO];

You can of course also use an NSOperationQueue and Key-Value-Observe the job's finished property, but unless you'd need it for other reasons it just adds unnecessary complexity.

Answer 2

Here's the nicest way, if you're using iOS 4.0:

dispatch_async(dispatch_get_main_queue(), ^{
    // update the UI here...
});