How to validate inputs and prevent save actions using databinding in eclipse? - eclipse

Question

I want to create input forms which validate user input and prevent the model from being saved with invalid data. I have been using databinding which works up to a point but my implementation is not as intuitive as I would like.
Imagine an input which contains '123' and the value must not be empty. The user deletes the characters one by one until empty. The databinding validator shows an error decoration.
However, if the user saves the form and reloads it, then a '1' is displayed in the field - i.e. the last valid input. The databinding does not transmit the invalid value into the model.
I have a ChangeListener but this is called before the databinding so at that point the invalid state has not been detected.
I would like the error to be displayed in the UI but the model remains valid (this is already so). Also, for as long as the UI contains errors, it should not be possible to save the model.
/**
* Bind a text control to a property in the view model
**/
protected Binding bindText(DataBindingContext ctx, Control control,
Object viewModel, String property, IValidator validator)
{
IObservableValue value = WidgetProperties.text(SWT.Modify).observe(
control);
IObservableValue modelValue = BeanProperties.value(
viewModel.getClass(), property).observe(viewModel);
Binding binding = ctx.bindValue(value, modelValue, getStrategy(validator), null);
binding.getTarget().addChangeListener(listener);
ControlDecorationSupport.create(binding, SWT.TOP | SWT.LEFT);
return binding;
}
private UpdateValueStrategy getStrategy(IValidator validator)
{
if (validator == null)
return null;
UpdateValueStrategy strategy = new UpdateValueStrategy();
strategy.setBeforeSetValidator(validator);
return strategy;
}
private IChangeListener listener = new IChangeListener()
{
#Override
public void handleChange(ChangeEvent event)
{
// notify all form listeners that something has changed
}
};
/**
* Called by form owner to check if the form contains valid data e.g. before saving
**/
public boolean isValid()
{
System.out.println("isValid");
for (Object o : getDataContext().getValidationStatusProviders())
{
ValidationStatusProvider vsp = (ValidationStatusProvider) o;
IStatus status = (IStatus)vsp.getValidationStatus()
.getValue();
if (status.matches(IStatus.ERROR))
return false;
}
return true;
}

Answer 1

Your best bet is to steer clear of ChangeListeners - as you've discovered, their order of execution is either undefined or just not helpful in this case.
Instead, you want to stick with the 'observable' as opposed to 'listener' model for as long as possible. As already mentioned, create an AggregateValidationStatus to listen to the overall state of the DataBindingContext, which has a similar effect to your existing code.
Then you can either listen directly to that (as below) to affect the save ability, or you could even bind it to another bean.
IObservableValue statusValue = new AggregateValidationStatus(dbc, AggregateValidationStatus. MAX_SEVERITY);
statusValue.addListener(new IValueChangeListener() {
handleValueChange(ValueChangeEvent event) {
// change ability to save here...
}
});

Answer 2

You can use AggregateValidationStatus to observe the aggregate validation status:
IObservableValue value = new AggregateValidationStatus(bindContext.getBindings(),
AggregateValidationStatus.MAX_SEVERITY);
You can bind this to something which accepts an IStatus parameter and it will be called each time the validation status changes.