How can an application perform stuff when its actually closed like google+?
I notice that I got a notification from huddle chat from the google+ app for iOS. But google+ was actually not active, it was closed. Same situation with Whatsapp, I always get push messages both if the app is closed or active.
This kind of behaviour seems for me to be impossible to implement. From other questions I know that we cannot register some kind of background process. How do this apps handle that?
Can I still listen form something when my app is minimized?
When my app is just minimized but not closed I know that a certain method is called. Can I perform a repeating update task, e.g. read geo data?
The apps you mentioned use something called Push Notifications which work regardless of your App being open/in the background/closed. They are notifications sent to the device when an action happens and most of the processing is done on the Server (The notifications are not generated by the App itself. A server pushes the Notification on).
Apple has a fantastic section on Executing Code in the Background when your App is minimised which should provide most of the answers you need. It even has a dedicated section on explaining the Geo Data capture which should help you in this case.