Accelerate not getting called when accelerometer initialized within beginBackgroundTaskWithExpirationHandler?

In my app, I want to r开发者_StackOverflow中文版ecord the accelerometer data when the app enters background (expiration time granted by the system will be enough). I have coded this as follows:

- (void)applicationDidEnterBackground:(UIApplication *)application {
    UIDevice* device = [UIDevice currentDevice];
    BOOL backgroundSupported = NO;

    if ([device respondsToSelector:@selector(isMultitaskingSupported)])
        backgroundSupported = device.multitaskingSupported;


    if (backgroundSupported) {
        UIApplication*    app = [UIApplication sharedApplication];

        bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
            [app endBackgroundTask:bgTask];
            bgTask = UIBackgroundTaskInvalid;
        }];

        // Start the long-running task and return immediately.
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
            ^{
            // Do the work associated with the task.
            if ([application backgroundTimeRemaining] > 1.0 ) {
                UIAccelerometer *accel = [UIAccelerometer sharedAccelerometer];
                accel.delegate = self;
                accel.updateInterval = kUpdateInterval;
            }          
        });

In the delegate I mentioned like:

- (void) accelerometer:(UIAccelerometer *)accelerometer 
    didAccelerate:(UIAcceleration *)acceleration {

    if (acceleration.x > kAccelerationThreshold || 
        acceleration.y > kAccelerationThreshold ||
        acceleration.z > kAccelerationThreshold) {

        NSLog(@"Sensed acceleration");
        UILocalNotification *localNotif = [[UILocalNotification alloc] init];

However, didAccelerate doesn't get triggered when the app enters background, rather it gets called after applicationWillEnterForeground. After the app gets relaunched ( from background ), didAccelerate seems to get fired multiple times ( possibly, the number of times accelerometer value changed when in background ). The request seems to get piled up and executed when app comes to foreground.

Is there any way I can record the accelerometer value when the app is in background state ( inside beginBackgroundTaskWithExpirationHandler )

Please help.

Your code is run as-is and then the application exit continues normally. In other words, your accelerometer related code is not synchronious and thus will not block exit process.

Read again the comment:

// Start the long-running task and return immediately.

Your task is not a "long-running" one, unfortunately.