title: How do I add a block to the front of a dispatch queue in GCD?
body: <p>I think I read once about a way to add a block to the front of a dispatch queue in Grand Central Dispatch (so it doesn't wait on other blocks that are pending).  But I can't find it in the <a href="https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html" rel="nofollow">GCD Reference</a>. Does anyone know if this is possible, and if so how?</p>