CMotionManager crash on instantiation - ios

Question

EDIT: This appears to be resolved on iOS 13. Leaving all the original details below.
I'm getting a crash when instantiating my CMMotionManager object for Core Motion. This is on an iPhone Xs running iOS 12.0.1.
I can reliably reproduce this with a single-view app, with the following view controller.
import UIKit
import CoreMotion
class ViewController: UIViewController {
var motion: CMMotionManager?
override func viewDidLoad() {
super.viewDidLoad()
// This causes a crash on iPhone Xs, iOS 12.0.1
self.motion = CMMotionManager()
}
}
Full sample project is at https://github.com/doctorcolinsmith/motiontestcrash/tree/master
When running the above, I am getting a fault on a thread with the following output in the debugger.
=================================================================
Main Thread Checker: UI API called on a background thread: -[UIApplication applicationState]
PID: 3634, TID: 630341, Thread name: com.apple.CoreMotion.MotionThread, Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4 libobjc.A.dylib 0x000000019b0d3894 <redacted> + 56
5 CoreMotion 0x00000001a19337a4 CoreMotion + 305060
6 CoreMotion 0x00000001a1933cd8 CoreMotion + 306392
7 CoreMotion 0x00000001a1933be8 CoreMotion + 306152
8 CoreMotion 0x00000001a19653cc CoreMotion + 508876
9 CoreMotion 0x00000001a196542c CoreMotion + 508972
10 CoreFoundation 0x000000019be6c888 <redacted> + 28
11 CoreFoundation 0x000000019be6c16c <redacted> + 276
12 CoreFoundation 0x000000019be67470 <redacted> + 2324
13 CoreFoundation 0x000000019be66844 CFRunLoopRunSpecific + 452
14 CoreFoundation 0x000000019be675a8 CFRunLoopRun + 84
15 CoreMotion 0x00000001a1964d64 CoreMotion + 507236
16 libsystem_pthread.dylib 0x000000019bae1a04 <redacted> + 132
17 libsystem_pthread.dylib 0x000000019bae1960 _pthread_start + 52
18 libsystem_pthread.dylib 0x000000019bae9df4 thread_start + 4
2018-10-24 16:19:31.423680-0700 motiontest[3634:630341] [reports] Main Thread Checker: UI API called on a background thread: -[UIApplication applicationState]
PID: 3634, TID: 630341, Thread name: com.apple.CoreMotion.MotionThread, Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4 libobjc.A.dylib 0x000000019b0d3894 <redacted> + 56
5 CoreMotion 0x00000001a19337a4 CoreMotion + 305060
6 CoreMotion 0x00000001a1933cd8 CoreMotion + 306392
7 CoreMotion 0x00000001a1933be8 CoreMotion + 306152
8 CoreMotion 0x00000001a19653cc CoreMotion + 508876
9 CoreMotion 0x00000001a196542c CoreMotion + 508972
10 CoreFoundation 0x000000019be6c888 <redacted> + 28
11 CoreFoundation 0x000000019be6c16c <redacted> + 276
12 CoreFoundation 0x000000019be67470 <redacted> + 2324
13 CoreFoundation 0x000000019be66844 CFRunLoopRunSpecific + 452
14 CoreFoundation 0x000000019be675a8 CFRunLoopRun + 84
15 CoreMotion 0x00000001a1964d64 CoreMotion + 507236
16 libsystem_pthread.dylib 0x000000019bae1a04 <redacted> + 132
17 libsystem_pthread.dylib 0x000000019bae1960 _pthread_start + 52
18 libsystem_pthread.dylib 0x000000019bae9df4 thread_start + 4
(lldb)
Has anyone encountered this before or have an idea on how to solve the crash?

Answer 1

Well, I've tested your application both physical iPhone 6S and iPhone 7 - obviously with iOS 12.0.1. I've also checked for a couple of things in your project and couldn't reproduce your problem at all - not even in simulators - and I don't think it's possible to download a simulator with specific fixes (12.0.x), only with minor version updates - so 12.1 being the next one. This looks really specific on iPhone XS hardware, and I would suggest you to fill an issue with Apple support.
EDIT: Well, I saw that you've already filled a bug report on openradar. This guy even referenced the bug report status on openradar-mirror.

Answer 2

I don't know much about CoreMotion, but the error message tells you you're trying to call a UI action on a background thread. Try putting your code into:
DispatchQueue.main.async {
//your code here
}
It also seems you're trying to do UI related actions in viewDidLoad(). Technically that's fine, but remember that some views probably aren't layed out at that point in time.