How does the web version of Whatsapp work on iOS devices considering the OS shuts apps in 30 seconds?
Just as a side note, Apple introduced the Notification Service extension point in iOS 10, which can be used to achieve this. The following applies only to iOS 9.x or earlier.
No app in iOS can be long alive in background with a keep-alive socket, or guaranteed to wake by remote notifications except those using VoIP background mode (OT: and IIRC Bluetooth background modes).
- An app has only ~5 seconds of runtime on
applicationDidEnterBackground:after being put in background, unless it is registered for any background modes or tasks. The app would be terminated if it runs out of time in this delegate method. - The background task model mentioned by @xoail has a app-specific, system-imposed time limit (up to 30 seconds...?) and cannot be extended. It is for an app to complete its current work, e.g. uploading a media, before being suspended. Background Transfer Service, since iOS 7.0, is an alternative for long running file transfer.
- Silent Remote Notificaiton is observed to be triggered consistently only on charger and Wi-Fi, but always throttled by iOS otherwise. So it is sort of indeterministic - let alone the fact that this can be switched off by flipping the app's Background App Refresh switch.
- VoIP background mode (in iOS 8 and later) guarantees to call the app's handler when a VoIP notification is received from APNs. But App Review Guidelines states clearly that background modes should only be used for their intended purpose.
So either Apple waives WhatsApp the use of VoIP background mode for purpose other than WhatsApp Call, or WhatsApp happens to get away from the "use your phone to sync" architecture and does something new for the iPhones.
As per the docs the app can remain in the background performing finite updates to the App. You can continue extending the background process one after the other. Look into Perform finite-length tasks. I think killing the app from background still executes registered actions by the system.
Whatsapp does some clever web session token + background app token generation to keep session valid.
As mentioned in #32112433 by Steven Darbey this is most likely implemented using the new iOS 8 PushKit Service which includes a VoIP service notification type, allowing applications to resume from background. A misuse of the API for non-VoIP purposes, but Apple apparently putting a blind eye on it.