public Validator getTodoValidator(){
		return new AbstractValidator() {
			
			public void validate(ValidationContext ctx) {
				//get the form that will be applied to todo
				Form fx = (Form)ctx.getProperty().getValue();
				//get filed subject of the form
				String subject = (String)fx.getField("subject");
				
				if(Strings.isBlank(subject)){
					Clients.showNotification("Subject is blank, nothing to do ?");
					//mark the validation is invalid, so the data will not update to bean
					//and the further command will be skipped.
					ctx.setInvalid();
				}
			}
		};
	}