theComboBox.setSimpleValue(entity.getEnum());

Step 5: Oh no! My Dropdown list clears itself if I pick something!

This is trivial but important to understand. Since its a ComboBox at heart it is always filtering the list based on what you type. This is easily overridden by setting a trigger on the field that makes this behavior appear to go away and thus always show the list. This is a problem in two scenarios:

A) The user picked a value, thus narrowing the filter all the way to one item and clearing the picklist

B) You are in an update scenario so the pre-selection does (A) for the user (thanks!).

The answer is simple

theComboBox.setTriggerAction(TriggerAction.ALL);

This causes a lot of people confusion but it is what it is. By setting the trigger you’ll force the list to always display all of your enums (or anything else you may put in a ComboBox)

Step 6: FormBinding: Not your friend with an Enum

The next big trip-up comes when you try to bind your SimpleComboBox to the form so that it auto-updates the model and removes all that CRUD you hate so much. At the end of the day, even though we told the SimpleComboBox it contained MyEnumClass its value is just a string. When we try to bind a string to an enumeration in our backing model we get a big fat CLASS CAST EXCEPTION. Boo hiss.

The solution: Don’t bind the field, handle it manually in your create/update event right before you send your model out over RPC for commitment.

myModel.setBudgetperiod(theComboBox.getSimpleValue());

Since the combobox is pretending to hold real-enums it will pass the value of the users selecting to the model without causing that CLASS CAST EXCEPTION. This is not truly creating Enum<->Enum mapping but it is passing a value back that the model Enum *thinks* it understands and thus, making a mockery of data-types, it successfully updates the model which in turn can update the data-store transparently.

Summary and Final Code

There are other recipes for this as well as all sorts of trickery you can do for display purposes etc but in the end this worked well for my requirements. This app is really heavy on the enums and this, with a touch of code duplication and a bit more maintenance overhead for the few updates a year I may do, gave me seamless movement of enums from Windows Desktop->XML-RPC->Database->Browser/GXT->Database->XML-RPC->Windows Desktop.

Take that JSON! (just kidding, I love you also JSON. Sometimes..)

Enum

public enum BudgetPeriod {
     DAILY("Daily"),WEEKLY("Weekly"),MONTHLY("Monthly"),QUARTERLY("Quarterly"),YEARLY("Yearly");
     private String value;
     BudgetPeriod(String value) {
		this.value = value;
     }
     public String toString() {
		return value;
     }
}

SimpleComboBox creation

A few notes here:

• I assign my list of enum values to a variable and then add that variable. You can do that in one fell swoop and avoid the assignment, this code is debugger friendly
• Notice that I am NOT defining theComboBox here, just assigning it. You need to define it at the class level so that the update routine, which is usually anonymous in a GWT/GXT pattern, can access it without it being final.

theComboBox = new SimpleComboBox<BudgetPeriod>();
java.util.List periodList = Arrays.asList(MyEnumClass.values()); 
theComboBox.add(periodList);
theComboBox.setLazyRender(false);
theComboBox.setForceSelection(true);
theComboBox.setTriggerAction(TriggerAction.ALL);
theComboBox.setSimpleValue(project.getBudgetperiod());

Create/Update handler

createButton.addListener(Events.Select, new Listener() {
     public void handleEvent(ButtonEvent e) {
         grid.mask("Saving..");
         myModel.setBudgetperiod(projectBudgetTimeSpan.getSimpleValue()); // THIS IS ALL YOU CARE ABOUT IN THIS CODE CHUNK
         MyRPC.Util.getInstance().createProject(project, ClientSession.getInstance().getSessionID(),
                                                new ProjectCreatedCallback(grid, index, myModelBean));
         }
});