How to do it...

To create a handler class to add further validation to the state machine, follow these steps:

1. Create a class named ConVMSVehicleTableInspStatusParameters and complete it with the following code:

class ConVMSVehicleTableInspStatusParameters
{
    public ConVMSVehInspStatus fromStatus;
    public ConVMSVehInspStatus toStatus;
    public ConVMSVehicleTable vehicleTable;
}

2. Next, create a class named ConVMSVehicleInspStatusHandler and add the following code to it:

ConVMSVehInspStatus fromStatus;
ConVMSVehInspStatus toStatus;
ConVMSVehicleTable vehicleTable;

public static ConVMSVehicleTableInspStatusHandler 
    NewParameters(ConVMSVehicleTableInspStatusParameters 
     _parameters)
{
    ConVMSVehicleTableInspStatusHandler instance 
        = new ConVMSVehicleTableInspStatusHandler();
    instance.fromStatus = _parameters.fromStatus;
    instance.toStatus = _parameters.toStatus;
    instance.vehicleTable = _parameters.vehicleTable;
    return instance;
}
public boolean Validate()
{
    switch (toStatus)
    {
        case ConVMSVehInspStatus::Complete:
            if (vehicleTable.InspComment == '')
            {
                DictField field = new DictField(
                    tableNum(ConVMSVehicleTable),
                    fieldNum(ConVMSVehicleTable, InspComment));

                //The field %1 must be filled in"
                return checkFailed (strFmt("@SYS110217",
                 field.label()));
            }
            break;
    }
    return true;
}

public void run()
{
    if(toStatus == fromStatus)
    {
        return;
    }
    if(this.Validate())
    {
        switch (toStatus)
        {
            case ConVMSVehInspStatus::Complete:
                Timezone tz = 
                 DateTimeUtil::getClientMachineTimeZone();
                ConWHSVehInspDate inspDate;
                inspDate = DateTimeUtil::getSystemDate(tz);
                vehicleTable.InspDate = inspDate;
                break;
        }
    }
    else
    {
        vehicleTable.InspStatus = fromStatus;
    }
}

There is nothing new about the preceding code, except that we don't (and must not) call update on the record. It is just used to validate the transition; in our case, it will stop the transition if the comment is blank.

3. Create a new class named ConVMSVehicleTableInspStateMachineHandler and add the following method. This ties the transition delegate to our handler class:

[SubscribesTo(classStr(ConVMSVehicleTableInspStateMachine),
    delegateStr(ConVMSVehicleTableInspStateMachine, Transition))]
public static void HandleTransition(ConVMSVehicleTableInspStateMachineTransitionEventArgs _eventArgs)
{
    ConVMSVehicleTableInspStatusParameters parms = new 
     ConVMSVehicleTableInspStatusParameters();
    parms.fromStatus = _eventArgs.ExitState();
    parms.toStatus = _eventArgs.EnterState();
    parms.vehicleTable = _eventArgs.DataEntity();

    ConVMSVehicleTableInspStatusHandler::NewParameters(parms).Run();
}