How to execute a method on another thread using Grand Central Dispatch

Everything in your iOS app happens on “the main thread” by default. It’s like at the hairdresser’s – one thing after another. Since iOS 4 however you can execute things “asynchronously” by using Grand Central Dispatch. With it comes another phenomenon called Blocks.

To make use of this feature you need to

  • define your own dispatch queue (dispatch_queue_t)
  • create your own dispatch queue (giving it a reverse DNS identifier)
  • wrap what you want to execute inside block

The syntax for this looks a bit odd and not like Objective-C, because those are “classic” C statements. As for Blocks: think of them as an anonymous function call. They allow you to specify how the background queue should react after the execution of your method has finished on the other thread.

Let’s see how this works. First define the queue, just under all those import statements will work fine:

dispatch_queue_t specialQueue;

When you’re ready to use it, give it an identifier and add an execution block:

specialQueue = dispatch_queue_create("com.versluis.specialapp", NULL);
dispatch_async(specialQueue, ^{ [self yourMethod]; });

The identifier will show up in log messages and Instruments if applicable. The block is executed at the end of the operation, on your “specialQueue”. It’s a great place to asses success or failure.

Note that UI Elements can only be updated from the main thread, so don’t change any labels from that “specialQueue”. You can however dispatch things specifically from the main queue while in your own queue:

dispatch_async(dispatch_get_main_queue(), ^{ application.networkActivityIndicatorVisible = NO; });

Here we switch off the network indicator while we’re in our own queue, by dispatching this statement on the main queue.

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