Form validation, bandbox and DependsOn

// main MVVM class
public class ResourceCategoryListEditVM extends AbstractResourceCategoryVM {
   ...
   ...
   private ResourceCategoryEditDO fEdit = new ResourceCategoryEditDO();
   ...
   ...
}
// subordinate bena hoding all form data properties
public class ResourceCategoryEditDO extends ResourceCategoryDO {
   ...

<groupbox form="@id('fx') @load(vm.edit) @save(vm.edit,before='save')" >

<bandbox id="parentCategoryBB" readonly="true" value="@bind(fx.parentCategoryTitle) @validator(vm.parentCategoryValidator)">
	<bandpopup width="200px" height="250px">
		<tree model="@load(fx.parentCategoryModel)" selectedItem="@bind(fx.selectedParentCategory)" 	checkmark="true" vflex="true" onSelect="parentCategoryBB.close();">
			<treecols>
				<treecol	label="Title" />
			</treecols>
			<template name="model" var="p">
				<treeitem>
					<treerow>
						<treecell 	label="@load(p.data.title)" />
					</treerow>
				</treeitem>
			</template>
		</tree>
	</bandpopup>
</bandbox>

public class ResourceCategoryListEditVM extends AbstractResourceCategoryVM {
   ...
   ...
   private ResourceCategoryEditDO fEdit = new ResourceCategoryEditDO();
   ...
   ...
}

public class ResourceCategoryEditDO extends ResourceCategoryDO {
   ...
   ...
   private String fParentCategoryTitle;
   ...
   ...

   @DependsOn("selectedParentCategory")
   public String getParentCategoryTitle() {
	return fParentCategoryTitle;
   }
   ...
   ...
}