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How Nike  GPS on iPhone receives accelerometer updates in the background?
- (void)startAccelerationCollection
{
[self.motionManager startAccelerometerUpdatesToQueue:[[NSOperationQueue alloc] init]
                                         withHandler:^(CMAccelerometerData *data, NSError *error) {
    dispatch_async(dispatch_get_main_queue(), ^{
    [self.accelerometerReadings addObject:  data];
}
];
}