How to: Limit the Amount of Objects Created using the New Action 如何:限制使用"新建"按钮创建的对象数量

This topic describes how to limit the number of objects that an end-user can create using the New Action. Assume you are using the Task business class from the Business Class Library. When creating a new Task using the New Action, the count of existing Task objects will be checked and an end-user will not be allowed to create additional objects if there are already three objects.

本主题介绍如何限制最终用户可以使用"新建操作"创建的对象数。假设您正在使用业务类库中的 Task 业务类。使用"新建操作"创建新任务时，将检查现有 Task 对象的计数，如果已有三个对象，则不允许最终用户创建其他对象。

Note 注意

The approach described in this topic is not supported by the Mobile platform.

移动平台不支持本主题中描述的方法。

 

Tip 提示

A complete sample project is available in the DevExpress Code Examples database at http://www.devexpress.com/example=E239

完整的示例项目可在 DevExpress 代码示例数据库中找到，http://www.devexpress.com/example=E239

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To access the Task List View when the New Action is about to create a new object, handle the NewObjectViewController.ObjectCreating event of the NewObjectViewController, which contains the New Action. To do this, implement a new View Controller and override the OnActivated method in the following manner.

要在"新操作"即将创建新对象时访问任务列表视图，请处理包含"新操作"的 NewObjectViewController.对象创建事件。为此，实现一个新的视图控制器，并重写 OnActivated 方法，以下列方式。

using DevExpress.ExpressApp;
using DevExpress.Persistent.BaseImpl;
using DevExpress.ExpressApp.SystemModule;
//...
public class LimitTaskAmountController : ViewController {
    private NewObjectViewController controller;
    protected override void OnActivated() {
        base.OnActivated();
        controller = Frame.GetController<NewObjectViewController>();
        if (controller != null) {
            controller.ObjectCreating += controller_ObjectCreating;
        }
    }
    void controller_ObjectCreating(object sender, ObjectCreatingEventArgs e) {
        if ((e.ObjectType == typeof(Task)) && 
            (e.ObjectSpace.GetObjectsCount(typeof(Task), null) >= 3)) {
                e.Cancel = true;
                throw new UserFriendlyException(
                    "Cannot create a task. Maximum allowed task count exceeded.");
        }
    }
    protected override void OnDeactivated() {
        if (controller != null) {
            controller.ObjectCreating -= controller_ObjectCreating;
        }
        base.OnDeactivated();
    }
}

Note 注意

You can disable an Action instead of interrupting its execution. See the How to: Disable an Action When the Current View Has Unsaved Changes example.

您可以禁用操作，而不是中断其执行。请参阅如何：在当前视图具有未保存的更改时禁用操作示例。