Get iOS Pressure Updates in Background

Question

I am looking for a way to get barometric pressure data (not altitude) every 10 or so minutes while the app is in the background or while another app is running in the foreground. Is this possible? If so, how?

If not, then how do apps like Pressur work?

Answer 1

Core Motion gives you CMAltimeter. CMAltimeter gives you CMAltitudeData. CMAltitudeData gives you pressure.

(Running in the background is another matter; you cannot run in the background merely in order to use Core Motion, so you'll have to find some other reason to run in the background.)

Answer 2

In order to receive background CoreMotion updates you need to do several things, but it boils down to running CoreLocation in the background. CoreMotion will not run in the background on its own.

• Request authorization via Info.plist Privacy strings ("Location Always and When In Use" and "Location When In Use")
• Add the Background Modes Capability to your project via the second tab on your app's Project pane. Enable Location Updates.
• Create the managers and instances you require:

    let locationManager = CLLocationManager()
    let motionManager = CMMotionManager() // Not required in this case
    let altimeter = CMAltimeter()
  

• Set your location manager's allowsBackgroundLocationUpdates to true
• Register the appropriate callbacks, in this case you want a CMAltimeter instance and something like:

if CMAltimeter.isRelativeAltitudeAvailable() {
    altimeter.startRelativeAltitudeUpdates(to: OperationQueue.main) { (data, error) in
    /* Handle Altitude changes here */
    }
}

Unlike the other sensors (e.g. motionManager.gyroUpdateInterval) you don't appear to have control over how often this is called; it's up to you to average or discard values as appropriate.

I've got an (increasingly less and less) simple demo of background sensor data collection in a repo here.