Replacing performSelectorInBackground with GCD

asynchronous, grand-central-dispatch, objective-c

Solution

You use dispatch_async like suggested.

For example:

    // Create a Grand Central Dispatch (GCD) queue to process data in a background thread.
dispatch_queue_t myprocess_queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

// Start the thread 

dispatch_async(myprocess_queue, ^{
    // place your calculation code here that you want run in the background thread.

    // all the UI work is done in the main thread, so if you need to update the UI, you will need to create another dispatch_async, this time on the main queue.
    dispatch_async(dispatch_get_main_queue(), ^{

    // Any UI update code goes here like progress bars

    });  // end of main queue code block
}); // end of your big process.
// finally close the dispatch queue
dispatch_release(myprocess_queue);

That's the general gist of it, hope that helps.

Problem

In my iOS app I'm running a computationally intensive task on a background thread like this: ``` // f is called on the main thread - (void) f { [self performSelectorInBackground:@selector(doCalcs) withObject:nil]; } - (void) doCalcs { int r = expensiveFunction(); [self performSelectorOnMainThread:@selector(displayResults:) withObject:@(r) waitUntilDone:NO]; } ``` How can I use GCD to run an expensive calculation such that it doesn't block the main thread? I've looked at `dispatch_async` and some options for the GCD queue choice but I'm too new to GCD to feel like I understand it sufficiently.

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