ComboBox Mediator

Friday I applied the Mediator pattern to a pair of ComboBoxes.

The Mediator, in short, is an interaction between two objects that's been encapsulated into a third object. A go-between. It's almost like a Decorator that works on two objects instead of one.

We have Combo Boxes to choose your country and state. So when a user picks a country, then the list of states has to be filtered for valid selections. We do this all throughout our code for various custom forms (the forms themselves all vary in significant ways, otherwise we could simply re-use one form).

Instead of cloning our code over into yet another class, I wrote a ComboBoxMediator, which wires two ComboBoxes together in a source-sink relationship. The MXML looks like this:


<bgs:ComboBoxMediator
sourceBox="{countryCodeComboBox}"
sourceSelectedValue="{address.countryCode}"
targetBox="{provinceCodeComboBox}"
targetSelectedValue="{address.provinceCode}"
/>


The class itself is short. It sets up filtering in the province ComboBox and adds a change event listener on the country ComboBox.

One important step that I originally didn't anticipate is when the province box is being refreshed: I have to check to see if the filtered data results in no data at all (in other words, in cases where a chosen country doesn't have province data yet assigned to it). In this case, I set the data provider on the province box to an inline array with only one value: "Not Applicable".