How to save and redisplay InstanceState

Question

I have developed an app where on one screen the user scans some barcodes in sets - maybe 2 in one set, maybe 3 or 4 in another. These barcodes are quite a long way apart, eg. the first one in a set may be at the front of a building and then others round the back of the building, etc. Sometimes they get called away to another site before finishing a building, and have to move to another screen in the same app and do something there. When they come back to the half-scanned building and redisplay the original screen, they want the scans already done in that set to be redisplayed so they only have to finish the set. I have looked at 'protected override void OnSaveInstanceState(Bundle savedInstanceState)' so when they change screens this code runs:

        protected override void OnSaveInstanceState(Bundle savedInstanceState)
    {
        savedInstanceState.PutString("gstrThisSiteID", gstrThisSiteID);
        savedInstanceState.PutString("gstrThisSiteName", gstrThisSiteName);
        savedInstanceState.PutString("gstrThisSiteNameFromUnfinishedSiteScan", gstrThisSiteNameFromUnfinishedSiteScan);
        savedInstanceState.PutString("gstrListOfScansDoneForThisSiteAdd", gstrListOfScansDoneForThisSiteAdd);
        savedInstanceState.PutString("gstrListOfScansForThisSiteRemove", gstrListOfScansForThisSiteRemove);
        savedInstanceState.PutString("gstrThisTagIDOrBarCodeValue", gstrThisTagIDOrBarCodeValue);

        //Always call the base implementation
        base.OnSaveInstanceState(savedInstanceState);

        Toast.MakeText(this, "OnSaveInstanceState(Bundle savedInstanceState)", ToastLength.Long).Show();

    }

This seems to run OK - the toast pops up as expected. However, when the user returns to the bar code scanning screen, these values don't reappear. Here is my code:

            if (savedInstanceState != null)
        {
            //_counter = bundle.GetInt("click_count", 0);

            Toast.MakeText(this, "savedInstanceState != null", ToastLength.Long).Show();

            gstrThisSiteID = savedInstanceState.GetString("gstrThisSiteID", "");
            gstrThisSiteName = savedInstanceState.GetString("gstrThisSiteName", "");
            gstrThisSiteNameFromUnfinishedSiteScan = savedInstanceState.GetString("gstrThisSiteNameFromUnfinishedSiteScan", "");
            gstrListOfScansDoneForThisSiteAdd = savedInstanceState.GetString("gstrListOfScansDoneForThisSiteAdd", "");
            gstrListOfScansForThisSiteRemove = savedInstanceState.GetString("gstrListOfScansForThisSiteRemove", "");
            gstrThisTagIDOrBarCodeValue = savedInstanceState.GetString("gstrThisTagIDOrBarCodeValue", "");

            CPsWaitingTextView = FindViewById<TextView>(Resource.Id.CPsWaitingTextView_id);
            CPsWaitingTextView.SetTextSize(Android.Util.ComplexUnitType.Dip, (float)20);
            CPsWaitingTextView.SetTextColor(Android.Graphics.Color.White);
            CPsWaitingTextView.Text = "strCheckpointsForThisSite";
            CPsWaitingTextView.MovementMethod = new Android.Text.Method.ScrollingMovementMethod();

            CPsScannedTextView = FindViewById<TextView>(Resource.Id.CPsScannedTextView_id);
            CPsScannedTextView.Text = gstrThisSiteName;

        }
        else
        {
            Toast.MakeText(this, "savedInstanceState IS null", ToastLength.Long).Show();

        }

While I am developing the app, when I return to the bar code screen the toast says 'savedInstanceState IS null' and the two TextViews do not display the expected values. I have put the 'if (savedInstanceState != null)' block at the foot of the OnCreate void:

        protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.TagAndBarcodeLayout);

        //Many lines of code setting up the screen etc, etc.
        //.............

        if (savedInstanceState != null)
        {
            //_counter = bundle.GetInt("click_count", 0);

            Toast.MakeText(this, "savedInstanceState != null", ToastLength.Long).Show();

            gstrThisSiteID = savedInstanceState.GetString("gstrThisSiteID", "");
            gstrThisSiteName = savedInstanceState.GetString("gstrThisSiteName", "");
            gstrThisSiteNameFromUnfinishedSiteScan = savedInstanceState.GetString("gstrThisSiteNameFromUnfinishedSiteScan", "");
            gstrListOfScansDoneForThisSiteAdd = savedInstanceState.GetString("gstrListOfScansDoneForThisSiteAdd", "");
            gstrListOfScansForThisSiteRemove = savedInstanceState.GetString("gstrListOfScansForThisSiteRemove", "");
            gstrThisTagIDOrBarCodeValue = savedInstanceState.GetString("gstrThisTagIDOrBarCodeValue", "");

            CPsWaitingTextView = FindViewById<TextView>(Resource.Id.CPsWaitingTextView_id);
            CPsWaitingTextView.SetTextSize(Android.Util.ComplexUnitType.Dip, (float)20);
            CPsWaitingTextView.SetTextColor(Android.Graphics.Color.White);
            CPsWaitingTextView.Text = "strCheckpointsForThisSite";
            CPsWaitingTextView.MovementMethod = new Android.Text.Method.ScrollingMovementMethod();

            CPsScannedTextView = FindViewById<TextView>(Resource.Id.CPsScannedTextView_id);
            CPsScannedTextView.Text = gstrThisSiteName;

        }
        else
        {
            Toast.MakeText(this, "savedInstanceState IS null", ToastLength.Long).Show();

        }

Have I put the 'if (savedInstanceState != null)' block in the wrong place? Where should it go if so? Have I missed anything else?

Thank you in advance.

Answer 1

I created a project and tested the InstanceState. It shows the same situation happened to you.

I searched relevant cases about it. You can refer to this: How can I save an activity state using the save instance state?.

If it's ok, Intent may be an alternative.