In IOS 11, DeviceMotion in background stopped working

Question

My app reports and records location, altitude, rotation and accelerometer data (DeviceMotion) while in the background. This works fine on ios 10.3.3. On IOS 11, I no longer have access motion data while the device is locked. Altitude data and location data is still streaming to the console, though.

Has something changed in IOS 11 that prevents me from accessing motion data or am I doing trying to access it in a way that Apple now blocks like OperationQueue.main

Here is how I'm starting motion updates. If the phone is unlocked, all works fine. If I locking the phone, no more updates.:

let motionManager = self.motionManager

    if motionManager.isDeviceMotionAvailable {
        motionUpdateInterval = 0.15
        motionManager.deviceMotionUpdateInterval = motionUpdateInterval

        motionManager.startDeviceMotionUpdates(using: .xArbitraryZVertical, to: OperationQueue.main) {deviceMotion, error in
            guard let deviceMotion = deviceMotion else { return }

I can't find anything about Motion background modes changing but it seems there must be a way otherwise RunKeeper, Strava will break. Can someone help me get this working again before IOS11 launch?

Thanks!

Answer 1

Also came across this problem.

Our solution was to ensure we have another background mode enabled and running (in our case location updates + audio) and restart core motion updates when switching background/foreground.

Code sample:

import UIKit
import CoreMotion

final class MotionDetector {
    private let motionManager = CMMotionManager()
    private let opQueue: OperationQueue = {
        let o = OperationQueue()
        o.name = "core-motion-updates"
        return o
    }()

    private var shouldRestartMotionUpdates = false

    init() {
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(appDidEnterBackground),
                                               name: .UIApplicationDidEnterBackground,
                                               object: nil)
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(appDidBecomeActive),
                                               name: .UIApplicationDidBecomeActive,
                                               object: nil)
    }

    deinit {
        NotificationCenter.default.removeObserver(self,
                                                  name: .UIApplicationDidEnterBackground,
                                                  object: nil)
        NotificationCenter.default.removeObserver(self,
                                                  name: .UIApplicationDidBecomeActive,
                                                  object: nil)
    }

    func start() {
        self.shouldRestartMotionUpdates = true
        self.restartMotionUpdates()
    }

    func stop() {
        self.shouldRestartMotionUpdates = false
        self.motionManager.stopDeviceMotionUpdates()
    }

    @objc private func appDidEnterBackground() {
        self.restartMotionUpdates()
    }

    @objc private func appDidBecomeActive() {
        self.restartMotionUpdates()
    }

    private func restartMotionUpdates() {
        guard self.shouldRestartMotionUpdates else { return }

        self.motionManager.stopDeviceMotionUpdates()
        self.motionManager.startDeviceMotionUpdates(using: .xArbitraryZVertical, to: self.opQueue) { deviceMotion, error in
            guard let deviceMotion = deviceMotion else { return }
            print(deviceMotion)
        }
    }
}