Adding error messages into a form AFTER it has passed its own validation?

<hbox>
	<window title="Base case" border="1" apply="org.zkoss.bind.BindComposer" 
		viewModel="@id('vm') @init('org.zkoss.mvvm.examples.confirm_delete.ConfirmDeleteBase')">
		<vbox>
			<listbox model="@bind(vm.items)"
				selectedItem="@bind(vm.selected)" width="200px">
				<template name="model" >
					<listitem>
						<listcell label="@load(each)" />
					</listitem>
				</template>
			</listbox>
		</vbox>
		<label value="@load(vm.message)" />
		<hbox>
			<button label="delete" disabled="@load(empty vm.selected)" onClick="@command('delete')" />
		</hbox>
	</window>
</hbox>

package org.zkoss.mvvm.examples.confirm_delete;

import java.util.ArrayList;
import java.util.List;

import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.NotifyChange;

public class ConfirmDeleteBase {

	List<String> items = new ArrayList<String>();
	
	String selected;
	
	String message;
	
	public ConfirmDeleteBase(){
		items.add("A");
		items.add("B");
		items.add("C");
	}
	
	public List<String> getItems(){
		return items;
	}

	public String getSelected() {
		return selected;
	}

	public void setSelected(String selected) {
		this.selected = selected;
	}
	
	public String getMessage() {
		return message;
	}
	
	@Command @NotifyChange({"message","items","selected"})
	public void delete(){
		if(selected==null){
			message = "You have to select an item";
		}else{
			try{
				if(daoDelete(selected)){
					items.remove(selected);
					message = selected + " was deleted";
					selected = null;
				}else{
					message = selected + " wasn't deleted";
				}
			}catch(Exception x){
				message = x.getMessage();
			}
		}
	}
	
	protected boolean daoDelete(String data){
		if("B".equals(selected)){
			throw new RuntimeException("Oops, I simulate delete "+data+" fial here");
		}
		return true;
	}
}

<hbox>
	<window title="Case 1" border="1" apply="org.zkoss.bind.BindComposer" 
		viewModel="@id('vm') @init('org.zkoss.mvvm.examples.confirm_delete.ConfirmDelete1')">
		<vbox>
			<listbox model="@bind(vm.items)"
				selectedItem="@bind(vm.selected)" width="200px">
				<template name="model" >
					<listitem>
						<listcell label="@load(each)" ></listcell>
					</listitem>
				</template>
			</listbox>
		</vbox>
		<label value="@load(vm.message)" ></label>
		<hbox>
			<button label="delete" disabled="@load(empty vm.selected)" 
				onClick="@command(vm.needToConfirm?'showConfirm':'delete')" ></button>
		</hbox>
		<window title="Confirm " mode="modal" visible="@load(not empty vm.confirmMessage)" border="1" width="300px">
			<vbox hflex="1" pack="center" align="center" style="padding:10px">
				<label value="@load(vm.confirmMessage)"></label>
				<hlayout> 
					<button label="Delete" onClick="@command('delete')" ></button>
					<button label="Cancel" onClick="@command('cancelConfirm')" ></button>
				</hlayout>
			</vbox>
		</window>
	</window>
</hbox>

package org.zkoss.mvvm.examples.confirm_delete;

import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.NotifyChange;

public class ConfirmDelete1 extends ConfirmDeleteBase{

	
	String confirmMessage;
	
	public String getConfirmMessage(){
		return confirmMessage;
	}
	
	@Command @NotifyChange("confirmMessage")
	public void cancelConfirm(){
		confirmMessage = null;
	}
	
	@Command @NotifyChange("confirmMessage")
	public void showConfirm(){
		confirmMessage = "Do you really want to delete "+selected;
	}
	
	@Override
	@Command @NotifyChange({"message","items","selected","confirmMessage"})
	public void delete(){
		super.delete();
		confirmMessage = null;
		//clean confirmMessage and notify
	}
	
	public boolean isNeedToConfirm(){
		//simulate C doesn't need to be confirm
		if("C".equals(selected)){
			return false;
		}
		return true;
	}	
}

<hbox>
	<window title="Case 1" border="1" apply="org.zkoss.bind.BindComposer" 
		viewModel="@id('vm') @init('org.zkoss.mvvm.examples.confirm_delete.ConfirmDelete2')">
		<vbox>
			<listbox model="@bind(vm.items)"
				selectedItem="@bind(vm.selected)" width="200px">
				<template name="model" >
					<listitem>
						<listcell label="@load(each)" ></listcell>
					</listitem>
				</template>
			</listbox>
		</vbox>
		<label value="@load(vm.message)" ></label>
		<hbox>
			<button label="delete" disabled="@load(empty vm.selected)" 
				onClick="@command(vm.needToConfirm?'showConfirm':'delete')" ></button>
		</hbox>
	</window>

</hbox>

package org.zkoss.mvvm.examples.confirm_delete;

import org.zkoss.bind.Binder;
import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.ContextParam;
import org.zkoss.bind.annotation.ContextType;
import org.zkoss.bind.annotation.NotifyChange;
import org.zkoss.zk.ui.event.EventListener;
import org.zkoss.zul.Messagebox;
import org.zkoss.zul.Messagebox.ClickEvent;

public class ConfirmDelete2 extends ConfirmDeleteBase{

	
	@Command
	public void showConfirm(@ContextParam(ContextType.BINDER) final Binder binder){
		Messagebox.show("Do you really want to delete "+selected,"Confirm",
				new Messagebox.Button[] { Messagebox.Button.YES,Messagebox.Button.NO },
				Messagebox.QUESTION,new EventListener<ClickEvent>() {
			public void onEvent(ClickEvent event) throws Exception {
				switch (event.getButton()) {
					case YES:
						//if you call super.delete here, since original zk event is not control by binder
						//the change of viewmodel will not update to the ui.
						//so, I post a delete to trigger to process it in binder controll.
						binder.postCommand("delete", null);
				}
			}
		});
	}
	
	@Override
	protected boolean daoDelete(String data){
		try{
			super.daoDelete(data);
			return true;
		}catch(Exception x){
			Messagebox.show(x.getMessage());
		}
		return false;
	}
	
	public boolean isNeedToConfirm(){
		//simulate C doesn't need to be confirm
		if("C".equals(selected)){
			return false;
		}
		return true;
	}	
}