Question

I have seen some unusual behavior when my app responds to memory warnings – data getting out of sync primarily.

If my app receives a memory warning, will the warning pass control to the main thread?

If not, I assume I must do some data protection if the memory warning will free data that might be in use on the main thread.

Answer 1

I’m pretty sure that -didReceiveMemoryWarning will only be called on the main thread.

Regardless, this is what you can do to ensure this without (potentially) deadlocking:

void invokeBlockOnMainThread(dispatch_block_t block) {
   if([NSThread isMainThread]) {
      block();
      return;
   }

   dispatch_sync(dispatch_get_main_queue(), block);
}

Call this function within -didReceiveMemoryWarning, passing in a block with everything that you need done and then you are guaranteed to be on the main thread while executing the code in the passed-in block.