In the previous recipe, we looked into using a private serial queue to keep our app responsive by moving long-running operations off the main queue. In this recipe, we will break our operations down into smaller, independent blocks and place them on a concurrent queue.
Getting ready
We are going to build on the app we improved in the last recipe, which is an app that will produce a PDF photo book from a collection of photos. You can get the code for this app at https://github.com/PacktPublishing/Swift-Cookbook-Second-Edition/tree/master/Chapter09 and choose the PhotobookCreator_DispatchGroups folder.
Open the project in XCode and navigate to the PhotoCollectionViewController.swift file.
How to do it...
We saw in the last recipe how dispatch queues operate on a FIFO policy. GCD will execute a block from the top of the queue and remove it from the queue when it has finished executing. The number of blocks that GCD will allow to execute at the same time...
