Concurrent queues and dispatch groups

In the last 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/SwiftProgrammingCookbook/PhotobookCreator and choose the branch start-dispatch-groups, or you can use git:

git clone https://github.com/SwiftProgrammingCookbook/PhotobookCreator.git
git checkout start-dispatch-groups...