ARKit template Xcode project Main Thread Checker log console

Question

I just started a new Xcode ARKit project and ran it on a physical device but in the console I\'m getting this output:

Main Thread Checker: UI API called on a         


        

Answer 1

(Of course you can disable checker, but sometimes it can be helpful. Check https://stackoverflow.com/a/45689250/7183675 for this.)

Error is inside Apple framework, so only way I found do work around it was subclassing UIApplication class and check application state on main thread this way:

1) Add main.swift file (this name is really important!) with these lines

import UIKit

    UIApplicationMain(
        CommandLine.argc,
        CommandLine.unsafeArgv,
        NSStringFromClass(MyApplicationClass.self),
        NSStringFromClass(MyDelegateClass.self)
    )

2) Remove @UIApplicationMain from MyDelegateClass if it was there or project wont compile because of multiple entry points

3) In MyApplicationClass.swift add this:

    import UIKit

    class MyApplicationClass: UIApplication {
    let semaphore = DispatchSemaphore(value: 0)
    override var applicationState: UIApplication.State {
        if Thread.current == Thread.main {
            return super.applicationState
        }

        var toReturn =  UIApplication.State.inactive

        DispatchQueue.main.async { [weak self] in
            guard let self = self else {return}
            toReturn = self.superAppState()
            self.semaphore.signal()
        }
        semaphore.wait()
        return toReturn
    }

    private func superAppState() -> UIApplication.State {
        return super.applicationState
    }
}

Now you are calling UIApplication.applicationState on main thread, so issue won't occur.

Well, there is one more issue. If CMMotionManager is initialized from main thread then if will block main thread and wait for UIApplication.State. That means deadlock. In this case you can't set your semaphore. Only way to return credible state avoiding deadlock would be implementing application state this way:

4)

import UIKit

        class MyApplicationClass: UIApplication {

    private static var configured = false

    private var state = UIApplication.State.inactive

    override var applicationState: UIApplication.State {
        if !MyApplicationClass.configured {
            MyApplicationClass.configured = true
            NotificationCenter.default.addObserver(self, selector: #selector(setStatus(_:)), name: UIApplication.didBecomeActiveNotification, object: nil)
            NotificationCenter.default.addObserver(self, selector: #selector(setStatus(_:)), name: UIApplication.willResignActiveNotification, object: nil)
            NotificationCenter.default.addObserver(self, selector: #selector(setStatus(_:)), name: UIApplication.willEnterForegroundNotification, object: nil)
            NotificationCenter.default.addObserver(self, selector: #selector(setStatus(_:)), name: UIApplication.didEnterBackgroundNotification, object: nil)
        }
        if Thread.current == Thread.main {
            return super.applicationState
        }
        return state
    }

    @objc func setStatus(_ notif: Notification) {
        switch notif.name {
        case UIApplication.didBecomeActiveNotification:
            state = .active
        case UIApplication.willResignActiveNotification:
            state = .inactive
        case UIApplication.willEnterForegroundNotification:
            state = .background
        case UIApplication.didEnterBackgroundNotification:
            state = .background
        default:
            state = .inactive
        }
    }
}

Personally I don't like last solution, but I wasn't able to find another workaround