CODEWITHSUNDEEP
iosbackgroundterminate
Ata01
Ata01

Reputation: 303

Detecting iOS app background termination by system

What I'm trying to achieve is detecting when the system terminates an app while it is running in the background (not suspended). According to this article it can be done by process of elimination. One of the steps is deciding that user didn't force quit the app. I assume this is done by checking whether applicationWillTerminate was called. However, according to Apple's documentation this method can also be called by the system, so I'm not sure how it eliminates that option.

For apps that do not support background execution or are linked against iOS 3.x or earlier, this method is always called when the user quits the app. For apps that support background execution, this method is generally not called when the user quits the app because the app simply moves to the background in that case. However, this method may be called in situations where the app is running in the background (not suspended) and the system needs to terminate it for some reason.

Is it possible to detect background system termination, and if so, what am I missing here?

Upvotes: 3

Views: 4668

Answers (1)

Rob
Rob

Reputation: 438487

The App Programming Guide for iOS: The App Life Cycle says:

Suspended apps receive no notification when they are terminated; the system kills the process and reclaims the corresponding memory. If an app is currently running in the background and not suspended, the system calls the applicationWillTerminate: of its app delegate prior to termination. The system does not call this method when the device reboots.

In addition to the system terminating your app, the user can terminate your app explicitly using the multitasking UI. User-initiated termination has the same effect as terminating a suspended app. The app’s process is killed and no notification is sent to the app.

The above is consistent with empirical tests.

In that Reducing FOOMs in the Facebook iOS app article, they mention the force quit situation where the app did "receive a termination call the last time the app was open": That only applies if the app was actively running and the user force quit it. But if the user first returns to the home screen (or otherwise suspends the app) and then force terminates the app, you will not receive that termination.

Bottom line, if the app is suspended at the time it was terminated, I do not believe that you have any reliable way of knowing whether it was jettisoned due to memory pressure or because the user force quit. The force quit scenario contemplated in that FOOM/BOOM article clearly only applies if the app was running when it was force quit.

Upvotes: 2

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