The final step in enabling background fetch for MustC is to add the application(_:performFetchWithCompletionHandler:) method to AppDelegate. As explained before, this method is called by iOS whenever the app is woken up in the background, and it allows you to perform an arbitrary amount of work. Once the app is done performing its task, it must call the completion-handler that was passed to this method.
Upon calling the completion-handler, you should inform iOS about the results of the operations you performed. It's important to correctly report this status because background fetch is intended to improve the user experience. If you falsely report to iOS that you have new data all the time, so your app is woken up more often, you're degrading the user experience. You should trust the system to judge when your app is woken up. It's in the best interest of your users, their battery life, and ultimately your app to not abuse background fetch.
To efficiently implement...