xaf 设置内置（built-in actions）按钮的可用状态

思路：通过视图控制器 访问到 按钮所属的控制器，给Action设置TargetObjectsCriteria表达式。

以下代码设置框架内置的删除按钮符合一定条件才可用。不符合条件时，按钮处于禁用状态（灰色）。

        protected override void OnActivated()
        {
            base.OnActivated();
            // Perform various tasks depending on the target View.

            Frame.GetController<DeleteObjectsViewController>().DeleteAction.TargetObjectsCriteria =
                CriteriaOperator.Parse("xx = ?", "").LegacyToString();

        }

        protected override void OnDeactivated()
        {
            // Unsubscribe from previously subscribed events and release other references and resources.
            base.OnDeactivated();
            Frame.GetController<DeleteObjectsViewController>().DeleteAction.TargetObjectsCriteria = string.Empty;
        }