ARKit Session Paused and Not Resuming

Question

In my ARKit app I am presenting a modal window. When I close the modal and go back to the ARSCNView then I find out that the session is paused due to this code:

 override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)

        // Pause the view's session
        sceneView.session.pause()
    } 

When I close the modal and go back to the ARKit camera view screen this code gets fired:

override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        // Create a session configuration
        let configuration = ARWorldTrackingSessionConfiguration()

        // Run the view's session
        sceneView.session.run(configuration)
    }

But this code never resumes the session. The screen is completely frozen on the last image it read. Any ideas?

I update the viewDidAppear code to be the following. It is still stuck on the camera screen with image frozen.

  override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        // Create a session configuration
        let configuration = ARWorldTrackingSessionConfiguration()

        sceneView.session.delegate = self

        if self.isPaused {
            sceneView.session.run(sceneView.session.configuration!)
        } else {
            // Run the view's session
            sceneView.session.run(configuration)
        }


    }

Answer 1

I don't know if the iOS 11 GM Seed or XCode 9 GM Seed versions fixed this today however I can successfully resume a paused ARSCNview with code as in the original question.

sceneView.session.run(sceneView.session.configuration!)

Answer 2

Not sure why your session isn't resuming, but... this generally isn't a situation you want to be in anyway.

Notice in the readme that ships with Apple's ARKit sample code (attached to the WWDC17 session on ARKit):

Avoid interrupting the AR experience. If the user transitions to another fullscreen UI in your app, the AR view might not be an expected state when coming back.

Use the popover presentation (even on iPhone) for auxiliary view controllers to keep the user in the AR experience while adjusting settings or making a modal selection. In this example, the SettingsViewController and VirtualObjectSelectionViewController classes use popover presentation.

To go into a bit more detail: if you pause the session, it won't be tracking the world while your user is away in a different fullscreen view controller. That means that when you resume, any virtual content placed in the scene won't be in the positions (relative to the camera) where you left it.