protected override void OnResume()
        {
            base.OnResume();

            // ARCore requires camera permissions to operate. If we did not yet obtain runtime
            // permission on Android M and above, now is a good time to ask the user for it.
            if (ContextCompat.CheckSelfPermission(this, Android.Manifest.Permission.Camera) == Android.Content.PM.Permission.Granted)
            {
                if (mSession != null)
                {
                    showLoadingMessage();
                    // Note that order matters - see the note in onPause(), the reverse applies here.
                    mSession.Resume();
                }

                // the app may crash here because of a race condition if you've not YET accepted camera
                // permissions. just accept the permissions, and then when the app crashes, restart it,
                // and it should be fine.
                mSurfaceView.OnResume();
                mDisplayRotationHelper.OnResume();
            }
            else
            {
                ActivityCompat.RequestPermissions(this, new string[] { Android.Manifest.Permission.Camera }, 0);
            }
        }
        protected override void OnResume()
        {
            base.OnResume();

            // ARCore requires camera permissions to operate. If we did not yet obtain runtime
            // permission on Android M and above, now is a good time to ask the user for it.
            if (ContextCompat.CheckSelfPermission(this, Android.Manifest.Permission.Camera) == Android.Content.PM.Permission.Granted)
            {
                if (mSession != null)
                {
                    showLoadingMessage();
                    // Note that order matters - see the note in onPause(), the reverse applies here.
                    mSession.Resume();
                }

                mSurfaceView.OnResume();
                mDisplayRotationHelper.OnResume();
            }
            else
            {
                ActivityCompat.RequestPermissions(this, new string[] { Android.Manifest.Permission.Camera }, 0);
            }
        }