Background fetch allows apps to download new content in the background without draining the battery because iOS manages the wake-up intervals as efficiently as possible. Since iOS will not allow your application to stay active for a very long time, you must keep in mind that your app might not be able to perform all the tasks you would like it to perform. If this is the case, you're probably implementing background fetch for the wrong reasons. Sometimes, it's a better idea to split work between your background fetch and application launch.
One example of this would be to download a large amount of data and perform some expensive processing with the downloaded data. If your app takes too long to process the data, it won't be able to finish. It may be a good idea to continue processing once the app launches. An even better approach would be to download only the data and perform all of the expensive processing only when the app launches. Background fetch is intended to pull in quick, small updates and not to process large amounts of data.
The time intervals that iOS uses to wake your app up aren't entirely transparent. You can tell iOS how often you would like your app to be woken up to do some background work, but ultimately the OS itself decides whether or not your app is woken up and how much time it has to perform its tasks.
A neat feature of background fetch is that iOS learns how often your app has updated data because you have to report this to the completion callback. In addition to understanding the moments where your app is most likely to retrieve updates, iOS also learns when your users are most likely to use your app. If a user tends to open your app every morning around 10:00 A.M., chances are that your app will be woken up in the background sometime before 10:00 A.M. This feature makes sure that your users will see recent content for your application. Similarly, if iOS notices that your app rarely has new content within a specified time interval, it's likely that your app isn't woken up as often as you'd like.
Leaving this interval in the hands of the OS might seem a bit scary at first; you often want as much control over these things as possible. What if you know that once a month, at a particular time, you will publish a bunch of new content and you want your app to fetch this in the background? How will you know that all users will receive this new content in time? The answer to this is simple: you don't know. Background fetch is a service that attempts to help your users to receive content in the background. There are no guarantees.
You should always be aware that your app might have to update content in the foreground since users can turn off background fetching entirely. One more important aspect of background fetch is that it will only work if your app isn't completely closed. If a user opens the multitasking window and swipes your app upward to close it, it won't be woken up for a background refresh, and your app will have to refresh when the user launches it the next time.
An excellent way to facilitate this in your code is to implement your background fetch logic in a separate method or a helper that you can call. You could invoke a method that's responsible for fetching new content any time your application is launched, or when it's awakened to perform a background fetch. This strategy ensures that your users always receive the latest content when they open your app.
This is all you need to know about background fetching on the surface. It's a feature that periodically wakes up your application so it can perform a task for a short time. You don't have tight control over when and how your app is awakened, which is okay because iOS learns how often your app should be woken up, and more importantly, iOS learns the best time to wake your app up. All you have to do is make sure that iOS knows that your app wants to be woken up.