Question

Let’s say I have an object hierarchy like this:

Account > Site > Supply

An Account is an actual company, a Site is a building they have, and a Supply is either an ElecSupply or GasSupply. Supply is never instantiated and could be an abstract class in theory.

I am using Objectify for persistence, and have a page that displays the list of Supplies for each Site, regardless of whether they are an ElecSupply or GasSupply.

Now I am implementing the GWT Editor Framework and have come up against a problem with this polymorphic entity. How do I implement an Editor and set of sub-editors for an object like this?

@Entity
public class Supply implements Serializable
{
    @Id
    protected Long id;
    @Embedded
    protected List<BillingPeriod> billingPeriods = new ArrayList<BillingPeriod>();

    public Supply()
    {

    }
// ...
}

The subclasses: (ElecSupply has 5 unique fields and GasSupply has just one)

@Subclass
public class ElecSupply extends Supply implements Serializable
{
    private String profile;
    private String mtc;
    private String llf;
    private String area;
    private String core;

    public ElecSupply()
    {

    }
}

---

@Subclass
public class GasSupply extends Supply implements Serializable
{
    private String mpr;

    public GasSupply()
    {

    }
// ...
}

So I would like to know if anyone has any experience with this kind of structure? I have tried to make separate editors for ElecSupply and GasSupply, and then show or hide them as part of the edit page.

The other way I was thinking about doing it would be to have a single editor (for Supply), and then load different sub-editors depending on which type of object we are editing.

Any light shed will be gratefully received.

Answer 1

I’ve already been in this case, and I’ve implemented the following solution :

• First create an generic utilitary class named AbstractSubTypeEditor which will activate a specific editor when you edit one of your subclass object :

  import com.google.gwt.editor.client.CompositeEditor;
  import com.google.gwt.editor.client.Editor;
  import com.google.gwt.editor.client.EditorDelegate;
  import com.google.gwt.editor.client.LeafValueEditor;
  
  public abstract class AbstractSubTypeEditor<T, C extends T, E extends Editor<C>> implements CompositeEditor<T, C, E>, LeafValueEditor<T> {
          private EditorChain<C, E> chain;
          private T currentValue;
          private final E subEditor;
  
          /**
           * Construct an AbstractSubTypeEditor backed by the given sub-Editor.
           *
           * @param subEditor the sub-Editor that will be attached to the Editor
           *          hierarchy
           */
          public AbstractSubTypeEditor(E subEditor) {
                  this.subEditor = subEditor;
          }
  
          /**
           * Returns the sub-Editor that the OptionalFieldEditor was constructed
           * with.
           *
           * @return an {@link Editor} of type E
           */
          public E createEditorForTraversal() {
                  return subEditor;
          }
  
          public void flush() {
                  currentValue = chain.getValue(subEditor);
          }
  
          /**
           * Returns an empty string because there is only ever one sub-editor used.
           */
          public String getPathElement(E subEditor) {
                  return "";
          }
  
          public T getValue() {
                  return currentValue;
          }
  
          public void onPropertyChange(String... paths) {
          }
  
          public void setDelegate(EditorDelegate<T> delegate) {
          }
  
          public void setEditorChain(EditorChain<C, E> chain) {
                  this.chain = chain;
          }
  
          public void setValue(T value, boolean instanceOf) {
                  if (currentValue != null && value == null) {
                          chain.detach(subEditor);
                  }
                  currentValue = value;
                  if (value != null && instanceOf) {
                          chain.attach((C)value, subEditor);
                  }
          }
  }
  

• Now you can create an Editor for Supply, containing two sub-editors and two AbstractSubTypeEditor (one for each of your subtypes) :

  public class SupplyEditor extends Composite implements Editor<Supply> {
  
          public class ElecSupplyEditor implements Editor<ElecSupply> {
                  public final TextBox profile = new TextBox();
                  public final TextBox mtc = new TextBox();
                  public final TextBox llf = new TextBox();
                  public final TextBox area = new TextBox();
                  public final TextBox core = new TextBox();
          }
          @Ignore
          final ElecSupplyEditor elecSupplyEditor = new ElecSupplyEditor();
          @Path("")
          final AbstractSubTypeEditor<Supply, ElecSupply, ElecSupplyEditor> elecSupplyEditorWrapper = new AbstractSubTypeEditor<Supply, ElecSupply, SupplyEditor.ElecSupplyEditor>(elecSupplyEditor) {
                  @Override
                  public void setValue(final Supply value) {
                          setValue(value, value instanceof ElecSupply);
                          if (!(value instanceof ElecSupply)) {
                                  elecSupplyEditor.profile.setVisible(false);
                                  elecSupplyEditor.mtc.setVisible(false);
                                  elecSupplyEditor.llf.setVisible(false);
                                  elecSupplyEditor.area.setVisible(false);
                                  elecSupplyEditor.core.setVisible(false);
                          } else {
                                  elecSupplyEditor.profile.setVisible(true);
                                  elecSupplyEditor.mtc.setVisible(true);
                                  elecSupplyEditor.llf.setVisible(true);
                                  elecSupplyEditor.area.setVisible(true);
                                  elecSupplyEditor.core.setVisible(true);
                          }
                  }
          };
  
          public class GasSupplyEditor implements Editor<GasSupply> {
                  public final TextBox mpr = new TextBox();
          }
          @Ignore
          final GasSupplyEditor gasSupplyEditor = new GasSupplyEditor();
          @Path("")
          final AbstractSubTypeEditor<Supply, GasSupply, GasSupplyEditor> gasSupplyEditorWrapper = new AbstractSubTypeEditor<Supply, GasSupply, SupplyEditor.GasSupplyEditor>(gasSupplyEditor) {
                  @Override
                  public void setValue(final Supply value) {
                          setValue(value, value instanceof GasSupply);
                          if (!(value instanceof GasSupply)) {
                                  gasSupplyEditor.mpr.setVisible(false);
                          } else {
                                  gasSupplyEditor.mpr.setVisible(true);
                          }
                  }
          };
  
          public SupplyEditor () {
                  final VerticalPanel page = new VerticalPanel();
                  page.add(elecSupplyEditor.profile);
                  page.add(elecSupplyEditor.mtc);
                  page.add(elecSupplyEditor.llf);
                  page.add(elecSupplyEditor.area);
                  page.add(elecSupplyEditor.code);
                  page.add(gasSupplyEditor.mpr);
                  initWidget(page);
          }
  }
  

This should show/hide your fields according to the subclass you are editing, and bind the properties to the good fields.