dispatch_sync on main queue hangs in unit test

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dispatch_sync runs a block on a given queue and waits for it to complete. In this case, the queue is the main dispatch queue. The main queue runs all its operations on the main thread, in FIFO (first-in-first-out) order. That means that whenever you call dispatch_sync, your new block will be put at the end of the line, and won’t run until everything else before it in the queue is done.

The problem here is that the block you just enqueued is at the end of the line waiting to run on the main thread—while the testSample method is currently running on the main thread. The block at the end of the queue can’t get access to the main thread until the current method (itself) finishes using the main thread. However dispatch_sync means Submits a block object for execution on a dispatch queue and waits until that block completes.

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