How do iOS chat apps keep running in the background?

Question

I have always coded for Android, and now I'm looking to expand my knowledge to iOS development; so I'm really new at this, please be patient.

I understand that only a small group of apps are allowed to run indefinitely in the background. Those are VoIP, Music players and location tracking apps.

I want to write a chat app using the XMPP framework. Everything is fine until the user puts the app in the background, in which case, the app will stay connected for about ten minutes to then be killed by the system and therefore the user won't be able to receive new messages.

I am aware of hacks to keep the app alive. Hacks such as defining it as a music playing app in the info.plist file and then just play some empty sound indefinitely. But I'm also aware that Apple will reject the app when it's time to publish to the App Store.

So, normally, how do other apps do it? How can other chat apps stay alive in the background to receive new messages from the servers? Apps like Google Hangouts, IM+ and such?

Answer 1

The iOS operating system allows for the existence of something called a PUSH NOTIFICATION

There exists hundreds of tutorials online which teach you how to implement the notification code and how to respond accordingly when you receive such a message! http://www.raywenderlich.com/32960/apple-push-notification-services-in-ios-6-tutorial-part-1

Check this link out for an in-depth tutorial on push notifications! http://maniacdev.com/2011/05/tutorial-ios-push-notification-services-for-beginners

Answer 2

Ideally, they aren't really running in the background, but use push notifications, as others have mentioned.

But some chat clients seem to do something else: I've verified (by sniffing the traffic of an idle iOS device) that at least Google Hangouts, Facebook and Skype all keep a persistent socket opened in the background, and regularly send traffic to keep it alive.

I'm suspecting that they are using the VoIP exceptions to Apple's otherwise strict background execution policies. iOS allows "VoIP apps" to run in the background and keep one socket open to be notified about incoming calls and messages.

Maybe they are also using the new "background fetch" feature of iOS 7, but as far as I know, that doesn't allow persistent socket connections.

Answer 3

I think most of these apps use push notifications and just load the last messages from the server as soon as the app is being opened.

Answer 4

While there are some hacks, and your app can ask for more time when it goes in background (up to a point, and with no guarantees), this is a perfect application for push notifications.

The server tells the phone there's a message, and iOS wakes your app up to process it.

https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Introduction.html

Answer 5

As of iOS 7 there is a new background-execution mode - 'fetch' for apps that need to periodically fetch new data. It sounds like your case would meet that definition.

You can find the information in the iOS App Programming Guide -

Fetching Small Amounts of Content Regularly

In iOS 7 and later, an app that retrieves content regularly from the network can ask the system for background execution time to check for new content. You enable support for background fetches from the Background modes section of the Capabilities tab in your Xcode project. (You can also enable this support by including the UIBackgroundModes key with the fetch value in your app’s Info.plist file.) At appropriate times, the system gives background execution time to the apps that support this background mode, launching the app directly into the background if needed. The app object calls the application:performFetchWithCompletionHandler: method of its app delegate to let you know when execution time is available.

You can also use push notifications, but that requires some server infrastructure

Answer 6

An app running in the background has limited capability. Read App States and Multitasking thoroughly to decide how best to design your app. Chat is not listed as one of the specific exceptions that can operate with a more relaxed policy. You will never be able to "keep [your] app live in background forever." You might be able to leverage an iOS 7 feature also described in this guide, Fetching Small Amounts of Content Regularly.

iOS App Programming Guide: App States and Multitasking https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOS ProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html