Mastering Grand Central Dispatch
Serial and Concurrent Dispatch Queues
You should now have a fundamental understanding of dispatch queues. A dispatch queue is responsible for managing the execution of blocks of work. Grand Central Dispatch determines which thread is used for the execution of a block of work. Developers unfamiliar with Grand Central Dispatch wrongly assume that a dispatch queue is tied to a particular thread. Remember that Grand Central Dispatch doesn't make a guarantee as to which thread is used for the execution of a block of work submitted to a dispatch queue.
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