Databinding: A Custom Observable for a Widget

Databinding: A Custom Observable for a Widget

The introduction of the databinding framework in Eclipse 3.3 is with no doubt one of the most useful tools in the hands of the form developer. The ability to transform and validate user input in such a flexible and reusable way is a great enhancement. But where there is light, there is shadow. Sometimes there is just no IObservable available for your target or model object. This blog entry will demonstrate how easy it can be to create a custom IObservable for a DateTime widget.

The DateTime widget represents one value: a java.util.Date. This Date object is the one we want to get and set on the target (target being the UI widget). Therefore we wrap the DateTime in an IObservableValue by extending the AbstractObservableValue class. Essentially an IObservable* offers methods to get and set data, to determine the type of data and to register listeners to be notified of changes. The following code demonstrates a skeleton implementation of an IObservableValue.
[code lang=’java‘]
public class DateTimeObservableValue extends AbstractObservableValue {

private final DateTime dateTime;
Listener listener = new Listener() { … };

public DateTimeObservableValue(final DateTime dateTime) {
this.dateTime = dateTime;
this.dateTime.addSelectionListener(this.listener);
}

@Override
protected Object doGetValue() {
// the utility method creates a Date from the DateTime
return dateTimeToDate();
}

@Override
protected void doSetValue(final Object value) {
if (value instanceof Date) {
// the utility method sets the date on the DateTime
dateToDateTime((Date) value);
}
}

@Override
public Object getValueType() {
return Date.class;
}

@Override
public synchronized void dispose() {
this.dateTime.removeSelectionListener(this.listener);
super.dispose();
}
}
[/code]
The implementation details are not very special. The getValueType() method has to return the type represented by this IObservableValue (which is the type Date). The do methods set and get the Date value. Since the observable has to propagate changes in the DateTime widget as soon as they ocurre, we attach a listener on the DateTime widget to inform any registered IValueChangeListener of the event. The listener implementation looks like the following:
[code lang=’java‘]
Listener listener = new Listener() {

@Override
public void handleEvent(final Event event) {
Date newValue = dateTimeToDate();

if (!newValue.equals(DateTimeObservableValue.this.oldValue)) {
fireValueChange(Diffs.createValueDiff(DateTimeObservableValue.this.oldValue, newValue));
DateTimeObservableValue.this.oldValue = newValue;
}
}
};
[/code]
In the DateTime listener we inform any interested IValueChangeListener of our DateTimeObservableValue. In order to avoid unnecessary propagation of update events in the databinding context, we compare the last set Date in the IObservableValue with the new value. Next we create a ValueDiff from our new date value and fire the the value change event. The advantage of listening to the changes in the DateTime widget, is that we are able to fire events which are either invoked by the user changing the DateTime widget or by programmatic changes of the IObservableValues wrapped Date.

You can download the full listing of the observable class here: DateTimeObservableValue.zip

As we can see, it is quite easy to write a custom observable for any kind of widget or datastructure, represented by a single value…  So, how do you embed your data in custom observables? Any obstacles you had to overcome? Problems you faced? Share them with us. 🙂