Add AttributeTypeHandler to the API, which can be used by multiple classes that need to support user-defined Attributes

Frequently we want to allow implementations to add attributes to different domain objects that are custom to that specific implementation. (This is equivalent to adding a column to a table in custom software.)

For example Person has PersonAttribute (and PersonAttributeType), Visit has VisitAttribute (and VisitAttributeType), etc.

The pattern we used when implementing PersonAttribute and PersonAttributeType (i.e. have a "format" column which specifies the java class of the attribute value) isn't quite right, so we want to provide a better mechanism. Initially this will be used in Visit Attributes, but eventually it will be extended to other attributes as well.

We will add "AttributeTypeHandler", an interface whose implementations:

• control the way that values are converted to and from Strings (for database storage)
• control what widget is displayed in the UI for this attribute type

Finding a way to implement this without mixing up our API and UI layers is going to be tricky...

Approximate design:

interface AttributeHandler<T> { // T is the type returned by this handler, e.g. java.lang.String or java.util.Date.
    String getName(Locale locale);
    String serialize(T);
    T deserialize(String);
}

/** hint to UI technology to display this as a dropdown  */
interface ConstrainedOptionAttributeHandler<T> extends AttributeType<T> {
    Map<String, String> getOptions(Locale locale) // should return an ordered map from attribute value to display label
}

/** hint to UI technology to display this as an autosuggest */
interface SuggestedOptionsAttributeHandler<T> extends AttributeType<T> {
    Map<String, String> getSuggestions(String typedSoFar, Locale locale) // should return an ordered map from attribute value to display label
}

/** explicitly specify what UI widget to use */
interface WidgetDrivenAttributeHandler<T> extends AttributeType<T> {
    String getWidgetName();
    Map<String, Object> getWidgetConfiguration();
}