gistfile1.m

- (void)applicationDidEnterBackground:(UIApplication *)application
{
  [self update];
  self.backgroundTaskIdentifier =
  [application beginBackgroundTaskWithExpirationHandler:^(void) {
    @autoreleasepool {
      NSLog(@"begin in background after 10 minutes");
      [motionManager stopAccelerometerUpdates];
      motionManager.accelerometerUpdateInterval = 0.2f;
      [self update2];
      [self endBackgroundTask];
    }
  }];
  
}

-(void)update2
{
  
  [motionManager startAccelerometerUpdates];
  NSLog(@"begin back acc");
  while (1) {
    @autoreleasepool {
      //app enter foreground
      if ([[UIApplication sharedApplication] backgroundTimeRemaining] > 600.0f) {
        break;
      }
      ////this make no use for dat.
      //            CMAccelerometerData *dat = motionManager.accelerometerData;
      //            CMAcceleration acceleration=dat.acceleration;
      //            [dat release];
      CMAcceleration acceleration = motionManager.accelerometerData.acceleration;
      NSLog(@"update2: x:%f y:%f z:%f",acceleration.x,acceleration.y,acceleration.z);
      sleep(0.2);
    }
  }
  NSLog(@"%s stoped acc",__func__);
  [motionManager stopAccelerometerUpdates];
}