Apple explains that multitasking on iOS “doesn't slow down the performance of the foreground app or drain battery life too much”—and they’re right. However, it seems as if it’s entered common “knowledge” among iPad, iPhone, and iPod touch users that closing each app after use is required to keep a device operating in its prime. This isn’t true, and here’s a simplified explanation of why it isn’t true.
The App Switcher
The “app switcher,” or the view that appears when you double-click the Home button, shows a list of recently used apps. “Recently used” is a bit of a misnomer—it actually includes any app launched on the device, unless it has been removed from the list.
You can remove a device from the list by swiping up on the app’s preview. Restarting the device does not remove apps from this view.
The app switcher does not show a list of running apps.
An App’s Lifecycle
Tapping an app’s icon launches the app. Unless the app has already been launched, the app goes from not running to being launched and in the foreground. The app is free to run as it pleases, responding to your every touch and swipe.
Now, let’s imagine you hit the home button or the sleep/wake button. The app is put into the “background,” where it’s allowed to run for only five seconds (with a few exceptions listed below). Once it’s done in the background, the app is suspended and is no longer allowed to run. Suspended apps don’t use any battery life.
Apps are allowed to run in the background for very specific tasks. Below are some of the reasons why an app might run in the background.
If the app needs more time to run for a specific task, it’s allowed to ask for more time—up to ten minutes. The system only gives it ten minutes to finish; after ten minutes, the app is suspended.
If the app is playing audio, it’s allowed to stay in the background until you tell it to stop playing. For example, if you’re listening to music and put your device to sleep, the app will enter the background and continue to play music. If you stop playing music, the app will be suspended. If you start playing music again, the app will move back into the background so it can play music.
Some apps use location in the background. For navigation-like apps that require constant updates of your location, the app is moved into the background and allowed to run for as long as it’s keeping track of your location. A location services arrow will appear in the status bar while an app is tracking your location.
Otherwise, apps that just need general notification updates (e.g. you enter or exit specific regions, or your location has changed significantly) will be moved into the background when a noteworthy location change occurs. The app is not running the entire time—the system intelligently moves the app from being suspended to the background as required. The system will display an outlined location arrow in the status bar when an app is monitoring location changes.
There are a couple of other circumstances in which an app might run in the background, but the pattern is fairly clear: when an app has something to do for you, it’ll be moved into the background to run temporarily. Otherwise, the app will be suspended and won’t be running. In general, unless an app is actively doing something on your behalf, it isn’t running, and thus does not need to be manually closed.
A Perfect World
All of the above requires everything to work as planned. Of course, bugs happen, and things do not work perfectly. If your device is getting unusually warm, or your battery is draining quickly, you might want to manually close the apps you’ve used recently (especially if they use the GPS), or simply restart your device. However, this is a last resort and not something that needs to be done on a regular basis.