applicationWillTerminate when is it called and when not

Solution 1:

In short, unless you have UIApplicationExitsOnSuspend in your Info.plist set to YES, in iOS4 and above there is no guarantee that applicationWillTerminate: will ever get called.

As the documentation says:

For applications that support background execution, this method is generally not called when the user quits the application because the application simply moves to the background in that case. However, this method may be called in situations where the application is running in the background (not suspended) and the system needs to terminate it for some reason

(Emphasis mine.)

If you need to do something before the app exits you need to do it in applicationDidEnterBackground:. There is no way to catch SIGKILL.

Solution 2:

I see -applicationWillTerminate: getting called with the following test. In a new project (I used the 'Single View Application' template), add the following to the AppDelegate:

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    NSLog(@"%s", __PRETTY_FUNCTION__);

    __block UIBackgroundTaskIdentifier identifier = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        if (identifier != UIBackgroundTaskInvalid) {
            [[UIApplication sharedApplication] endBackgroundTask:identifier];
            identifier = UIBackgroundTaskInvalid;
        }
    }];

    dispatch_async(dispatch_get_main_queue(), ^{
        for (int i=0; i < 20; i++) {
            NSLog(@"%d", i);
            sleep(1);
        }
        if (identifier != UIBackgroundTaskInvalid) {
            [[UIApplication sharedApplication] endBackgroundTask:identifier];
            identifier = UIBackgroundTaskInvalid;
        }
    });
}

- (void)applicationWillTerminate:(UIApplication *)application
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

This example will start a background task when the app enters the background. The task is just a 20s delay (with logging once a second) that keeps the app running in the background (note the difference between running in the background and suspended) long enough to allow it to be killed from the app switcher.

So, to test it, run the app, hit the home button to send the app to the background, then before the 20s delay is up, remove the app from the app switcher. After the end of the 20s, -applicationWillTerminate: is called. You can watch the console in Xcode to verify that this is the case.

I tried this in the iOS Simulator for iOS 5.1 and 6.1 (both iPhone) and saw it happen in both cases. I also tested on iPhone 4S running iOS 6.1.2 and saw the same behavior.

Solution 3:

As I know, there are 3 situations that your application will die.

1. Terminated by the end user, you can do something in -[UIApplication applicationWillEnterBackground:], in which case, -[UIApplication applicationWillTerminate:] will NOT be called.

2. Dropped by the system, such as memory not enough, you can do something in -[UIApplication applicationWillTerminate:], in which case, we do NOT know whether applicationWillEnterBackground: has been called;

3. Crashed, nothing can be done except using some kind of Crash Reporting Tool. (Edited: catching SIGKILL is impossible)