How to customize of the default Shipment workflow via Workflow API and enable field SOShipment.shipVia in status Confirmed?

 So, finally got this to work.

To make Shipment fields enabled in status Confirmed I had to add two overrides:

1. Override in graph RowSelected event
2. Update workflow step

I.e. I guess Acumatica starts from applying logic of the Base graph, then applies logic of the Graph Extension, then logic of the Base workflow, and then logic of the Workflow Extension.

Also found useful page describing how field states are 

https://help.acumatica.com/(W(203))/Help?ScreenId=ShowWiki&pageid=0390a164-104e-492a-b45b-62359e389856
 

Here’s complete code:

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Runtime.Serialization;
using System.Text;
using PX.Data;
using PX.Data.BQL;
using PX.Data.BQL.Fluent;
using PX.Data.WorkflowAPI;
using PX.Common;
using PX.Objects.AR;
using PX.Objects.CM;
using PX.Objects.CR;
using PX.Objects.CS;
using PX.Objects.EP;
using PX.Objects.GL;
using PX.Objects.IN;
using PX.SM;
using PX.Objects.IN.Overrides.INDocumentRelease;
using POLineType = PX.Objects.PO.POLineType;
using POReceiptLine = PX.Objects.PO.POReceiptLine;
using PX.CarrierService;
using PX.Data.DependencyInjection;
using PX.LicensePolicy;
using PX.Objects.SO.Services;
using PX.Objects.PO;
using PX.Objects.AR.MigrationMode;
using PX.Objects.Common;
using PX.Objects.Common.Discount;
using PX.Objects.Common.Extensions;
using PX.Common.Collection;
using PX.Objects.SO.GraphExtensions.CarrierRates;
using PX.Objects.SO.GraphExtensions.SOShipmentEntryExt;
using PX.Api;
using LocationStatus = PX.Objects.IN.Overrides.INDocumentRelease.LocationStatus;
using ShipmentActions = PX.Objects.SO.SOShipmentEntryActionsAttribute;
using PdfSharp.Pdf.IO;
using PX.Objects.IN.Attributes;
using PX.Objects;
using PX.Objects.SO;

namespace PX.Objects.SO
{
  using static BoundedTo<SOShipmentEntry, SOShipment>;
  using State = SOShipmentStatus;

  public class SOShipmentEntry_Extension : PXGraphExtension<PX.Objects.SO.SOShipmentEntry>
  {
    #region Event Handlers

    protected virtual void SOShipment_RowSelected(PXCache cache, PXRowSelectedEventArgs e)
    {

        SOShipment row = e.Row as SOShipment;

        if (row != null)
        {
          if (row.Status == SOShipmentStatus.Confirmed)
          {
            Base.Document.Cache.AllowUpdate = true;

            PXUIFieldAttribute.SetEnabled<SOShipment.shipVia>(cache, row, true);
            PXUIFieldAttribute.SetEnabled<SOShipment.siteID>(cache, row, true);
            PXUIFieldAttribute.SetEnabled<SOShipment.curyFreightCost>(cache, row, true);

          }
        }  
    }
    #endregion Event Handlers
  }

  #region Workflow Customization

  public class SOShipmentEntry_ExtWorkflow : PXGraphExtension<SOShipmentEntry_Workflow, SOShipmentEntry>
  {
    public static bool IsActive() => true;

    public override void Configure(PXScreenConfiguration config)
    {
        ConfigureNewWorkflow(config.GetScreenConfigurationContext<SOShipmentEntry, SOShipment>());
    }

    protected virtual void ConfigureNewWorkflow(WorkflowContext<SOShipmentEntry, SOShipment> context)
    {
        Condition Bql<T>() where T : IBqlUnary, new() => context.Conditions.FromBql<T>();

        var conditions = new
        {
            ShipmentIsInConfirmedStatus = Bql<Where<SOShipment.status, Equal<SOShipmentStatus.confirmed>>>()

        }.AutoNameConditions();

        context.UpdateScreenConfigurationFor(screen =>
        {
            return screen
                .WithActions(actions =>
                {
                    actions.Update(
                    action => action.correctShipmentAction, (ActionDefinition.ConfiguratorAction actionConfiguration) =>
                        actionConfiguration.IsDisabledWhen(conditions.ShipmentIsInConfirmedStatus));
                })
                .UpdateDefaultFlow(flow =>
                {
                    return flow
                        .WithFlowStates(states =>
                            states.Update<SOShipmentStatus.confirmed>(flowState =>
                            flowState
                            .WithFieldStates(containerAdjuster => {
                                containerAdjuster.AddField<SOShipment.shipVia>();
                                containerAdjuster.AddField<SOShipment.siteID>();
                                containerAdjuster.AddField<SOShipment.curyFreightCost>();
                            })
                        ));
                });
         });
     }
   }

  #endregion Workflow Customization            

}