How to know which Apps are in the background

Question

是否有可能知道后台有哪些应用程序（双击主屏幕按钮时出现的那些应用程序）？

Answer 1

For 3rd party apps, no. Not in a reliable, App Store safe, future proof way. Think of your app as siloed, unaware of what else is running, and you'll be pretty much in the mindset Apple want (and try to technically enforce).

For other apps developed by you, you can use shared containers to write load/unload data each time one of your apps opens/closes, and work out what's currently running.

Answer 2

This is not possible on iOS with public api. There might be private api calls that do this, even though I don't think you will have access to them while running in the sandbox. This however might be possible on jailbroken devices, but you will not be able to submit to the app store with such api usage, unless you know how to hide that from apple.

Also please note that apps shown in the app switcher are not necessarily running. The app switcher shows every app that has been running in the past which the user didn't force-quit. If the user does not force quit an app, every app that has ever been launched is shown in the app switcher. If iOS decides to terminate an app because of memory needs, the app is still shown in the app switcher. This is one of the reasons why everyone should implement state restoration, as the user does not know whether an app is currently running or not!