getOnChanged method
Returns a funcation that can be used with the OnChanged event on many StatefulWidgets.
Passes any changes made by the user back to the view model
Implementation
Function getOnChanged(String key, {Object? converterParameter}) {
Binding binding = _getBinding(key);
assert(binding.bindingDirection == BindingDirection.TwoWay);
return (Object newValue) =>
{setValue(key, newValue, converterParameter: converterParameter)};
}