Concurrency

The queue that the code in the dispatch block will run in. A queue is like (but not exactly the same as) a thread; code in different queues can run in parallel. Use dispatch_get_main_queue to get the queue for the main thread To create a new queue, which in turn creates a new thread, use dispatch_queue_create(“QUEUE_NAME”,DISPATCH_QUEUE_CONCURRENT).

The first parameter is the name of the queue, which is displayed in the debugger if you pause while the block is still running. The second parameter doesn’t matter unless you want to use the same queue for multiple dispatch_async or calls. It describes what happens when another block is put on the same queue; will cause both blocks to run at the same time, while DISPATCH_QUEUE_SERIAL will make the second block wait for the first block to finish

Code in this block will run in the queue queue; put code you want to run on the separate queue here. A helpful tip: If you’re writing this in Xcode and the block argument has the blue outline around it, double click on the argument and Xcode will automatically make an empty block (this applies to all block arguments in any function or method) .

Dispatch group — waiting for other threads completed

dispatch_group_t preapreWaitingGroup =…