Swift 3 device motion gravity - ios

Question

I have an iPhone application with a level in it that is based on the gravityY parameter of a device motion call to motionmanager. I have fixed the level to the pitch of the phone, as I wish to show the user whether the phone is elevated or declined relative to a flat plane (flat to the ground) through its x-axis...side to side or rotated is not relevant. To do that, I have programmed the app to slide an indicator (red when out of level) along the level (a bar)....its maximum travel is each end of the level.
The level works great...and a correct value is displayed, until the user locks the phone and puts it in his or her back pocket. While in this stage, the level indicator shifts to one end of the level (the end of the phone that is elevated in the pocket), and when the phone is pulled out and unlocked, the app does not immediately restore the level - it remains out of level, even if I do a manual function call to restore the level. After about 5 minutes, the level seems to restore itself.
Here is the code:
func getElevation () {
//now get the device orientation - want the gravity value
if self.motionManager.isDeviceMotionAvailable {
self.motionManager.deviceMotionUpdateInterval = 0.05
self.motionManager.startDeviceMotionUpdates(
to: OperationQueue.current!, withHandler: {
deviceMotion, error -> Void in
var gravityValueY:Double = 0
if(error == nil) {
let gravityData = self.motionManager.deviceMotion
let gravityValueYRad = (gravityData!.gravity.y)
gravityValueY = round(180/(.pi) * (gravityValueYRad))
self.Angle.text = "\(String(round(gravityValueY)))"
}
else {
//handle the error
self.Angle.text = "0"
gravityValueY = 0
}
var elevationY = gravityValueY
//limit movement of bubble
if elevationY > 45 {
elevationY = 45
}
else if elevationY < -45 {
elevationY = -45
}
let outofLevel: UIImage? = #imageLiteral(resourceName: "levelBubble-1")
let alignLevel: UIImage? = #imageLiteral(resourceName: "levelBubbleGR-1")
let highElevation:Double = 1.75
let lowElevation:Double = -1.75
if highElevation < elevationY {
self.bubble.image = outofLevel
}
else if elevationY < lowElevation {
self.bubble.image = outofLevel
}
else {
self.bubble.image = alignLevel
}
// Move the bubble on the level
if let bubble = self.bubble {
UIView.animate(withDuration: 1.5, animations: { () -> Void in
bubble.transform = CGAffineTransform(translationX: 0, y: CGFloat(elevationY))
})
}
})
}
}
I would like the level to restore almost immediately (within 2-3 seconds). I have no way to force calibration or an update. This is my first post....help appreciated.
Edit - I have tried setting up a separate application without any animation with the code that follows:
//
import UIKit
import CoreMotion
class ViewController: UIViewController {
let motionManager = CMMotionManager()
#IBOutlet weak var angle: UITextField!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
#IBAction func startLevel(_ sender: Any) {
startLevel()
}
func startLevel() {
//now get the device orientation - want the gravity value
if self.motionManager.isDeviceMotionAvailable {
self.motionManager.deviceMotionUpdateInterval = 0.1
self.motionManager.startDeviceMotionUpdates(
to: OperationQueue.current!, withHandler: {
deviceMotion, error -> Void in
var gravityValueY:Double = 0
if(error == nil) {
let gravityData = self.motionManager.deviceMotion
let gravityValueYRad = (gravityData!.gravity.y)
gravityValueY = round(180/(.pi) * (gravityValueYRad))
}
else {
//handle the error
gravityValueY = 0
}
self.angle.text = "(\(gravityValueY))"
})}
}
}
Still behaves exactly the same way....

Answer 1

OK....so I figured this out through trial and error. First, I built a stopGravity function as follows:
func stopGravity () {
if self.motionManager.isDeviceMotionAvailable {
self.motionManager.stopDeviceMotionUpdates()
}
}
I found that the level was always set properly if I called that function, for example by moving to a new view, then restarting updates when returning to the original view. When locking the device or clicking the home button, I needed to call the same function, then restart the gravity features on reloading or returning to the view.
To do that I inserted the following in the viewDidLoad()...
NotificationCenter.default.addObserver(self, selector: #selector(stopGravity), name: NSNotification.Name.UIApplicationWillResignActive, object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(stopGravity), name: NSNotification.Name.UIApplicationWillTerminate, object: nil)
This notifies the AppDelegate and runs the function. That fixed the issue immediately.