Creating reusable tasks with Operations
You have just learned about DispatchQueues, and how they are used to schedule tasks that need to be performed on a different thread. You saw how this speeds up code and how it avoids blocking the main thread. In this section, you will take this one step further. The first reason for this is because asynchronous work is better organized if it is implemented as an isolated object that can be scheduled for execution instead of having several closures that make your code less readable.
The solution to having closures all over your asynchronous code is to use Operation instead of a closure. And instead of queueing everything in a dispatch queue, Operation instances are queued on OperationQueue. OperationQueue and DispatchQueue are similar, but not quite the same. OperationQueue can schedule Operations on one or more DispatchQueues. This is important because of the way in which Operations work.
When using OperationQueue, you can execute Operations in parallel...