Understanding modifedField() Table method in fDynamics 365 Finance Operations

We use modifiedField() method to perform any actions after the field is modifed for example creating new table with 2 fields ItemId and ItemNameDisplay from Extended Data Types node in AOT then Override modifiedField() in Table's method node, something likes:

public void modifiedField(FieldId _fieldId)
{
    InventTable inventTable;
    super(_fieldId);
    switch (_fieldId)
    {
        case fieldNum(IBD_Invent,itemid):
            this.ItemNameDisplay = inventTable::find(this.itemid).NameAlias;
            break;
    }
}

The modifiedField() method is located on tables and it is called by a form (or a dataset) when any field is changed. It has field ID as a parameter and therefore it is very easy to write code reacting to a change of a particular field. It is so easy that the modifiedField() method is often used even in such cases, when it really shouldn’t be used.

If you want to get previous field value in the modifiedField() for comparative purpose, you can use this.orig() method:

public void modifiedField(fieldId _fieldId)
{
    super(_fieldId);
    info(strfmt("Field number %1 changed from %2 to %3",_fieldId,this.orig().(_fieldId),this.(_fieldId)));
}