2

我有一个 WPF 应用程序，其中PageItems是模型对象。

我的主要 ViewModel 有一个PageItemViewModels的 ObservableCollection ，每个都从其匹配的 PageItem 模型对象构建自己。

每个PageItemViewModel 都继承自抽象类BaseViewModel以获得 INotifyPropertyChanged 功能。

每个PageItemViewModel还实现IPageItemViewModel以确保它具有所需的属性。

我最终将有大约 50 页，所以我想消除任何不必要的代码：

  • 已解决（见下文）：有没有办法让 PageItemViewModel 类继承 IdCode 和 Title，所以我不必在每个类中实现它们？我不能把它们放在 BaseViewModel 中，因为其他 ViewModel 继承它不需要这些属性，我不能把它们放在 IPageItemViewModel 中，因为它只是一个接口。我知道我需要C# 不支持的多重继承
  • 已解决（见下文）：有没有办法摆脱switch语句，例如以某种方式使用反射？

下面是一个独立的控制台应用程序，它演示了我在WPF应用程序中的代码：

using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<PageItem> pageItems = PageItems.GetAll();
            List<ViewModelBase> pageItemViewModels = new List<ViewModelBase>();

            foreach (PageItem pageItem in pageItems)
            {
                switch (pageItem.IdCode)
                {
                    case "manageCustomers":
                        pageItemViewModels.Add(new PageItemManageCustomersViewModel(pageItem));
                        break;
                    case "manageEmployees":
                        pageItemViewModels.Add(new PageItemManageEmployeesViewModel(pageItem));
                        break;
                    default:
                        break;
                }
            }
        }
    }

    public class PageItemManageCustomersViewModel : ViewModelBase, IPageItemViewModel
    {
        public string IdCode { get; set; }
        public string Title { get; set; }

        public PageItemManageCustomersViewModel(PageItem pageItem)
        {

        }
    }

    public class PageItemManageEmployeesViewModel : ViewModelBase, IPageItemViewModel
    {
        public string IdCode { get; set; }
        public string Title { get; set; }

        public PageItemManageEmployeesViewModel(PageItem pageItem)
        {

        }
    }

    public interface IPageItemViewModel
    {
        //these are the properties which every PageItemViewModel needs
        string IdCode { get; set; }
        string Title { get; set; }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItems
    {
        public static List<PageItem> GetAll()
        {
            List<PageItem> pageItems = new List<PageItem>();
            pageItems.Add(new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers"});
            pageItems.Add(new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees"});
            return pageItems;
        }
    }

}

重构：接口改为抽象类

using System;
using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<PageItem> pageItems = PageItems.GetAll();
            List<ViewModelPageItemBase> pageItemViewModels = new List<ViewModelPageItemBase>();

            foreach (PageItem pageItem in pageItems)
            {
                switch (pageItem.IdCode)
                {
                    case "manageCustomers":
                        pageItemViewModels.Add(new PageItemManageCustomersViewModel(pageItem));
                        break;
                    case "manageEmployees":
                        pageItemViewModels.Add(new PageItemManageEmployeesViewModel(pageItem));
                        break;
                    default:
                        break;
                }
            }

            foreach (ViewModelPageItemBase pageItemViewModel in pageItemViewModels)
            {
                System.Console.WriteLine("{0}:{1}", pageItemViewModel.IdCode, pageItemViewModel.Title);
            }
            Console.ReadLine();
        }
    }

    public class PageItemManageCustomersViewModel : ViewModelPageItemBase
    {
        public PageItemManageCustomersViewModel(PageItem pageItem)
        {
            IdCode = pageItem.IdCode;
            Title = pageItem.Title;
        }
    }

    public class PageItemManageEmployeesViewModel : ViewModelPageItemBase
    {
        public PageItemManageEmployeesViewModel(PageItem pageItem)
        {
            IdCode = pageItem.IdCode;
            Title = pageItem.Title;
        }
    }

    public abstract class ViewModelPageItemBase : ViewModelBase
    {
        //these are the properties which every PageItemViewModel needs
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItems
    {
        public static List<PageItem> GetAll()
        {
            List<PageItem> pageItems = new List<PageItem>();
            pageItems.Add(new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers"});
            pageItems.Add(new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees"});
            return pageItems;
        }
    }

}

消除 Switch 语句的答案：

谢谢戳：

string assemblyName = System.Reflection.Assembly.GetExecutingAssembly().GetName().Name;
string viewModelName = assemblyName + ".ViewModels.PageItem" + StringHelpers.ForcePascalNotation(pageItem.IdCode) + "ViewModel";
var type = Type.GetType(viewModelName);
var viewModel = Activator.CreateInstance(type, pageItem) as ViewModelBase;
AllPageViewModels.Add(viewModel);
4

5 回答 5

2

您能否创建一个继承自 BaseViewModel 的类来实现这两个属性 - 您的 PageItemViewModel 需要此属性的类可以从该类继承。

于 2009-06-22T14:16:04.410 回答
1

正如 Paddy 所建议的，我刚刚创建了一个额外的抽象类 PageViewModelBase，其中定义了这些自动道具：

using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<PageItem> pageItems = PageItems.GetAll();
            List<ViewModelBase> pageItemViewModels = new List<ViewModelBase>();

            foreach (PageItem pageItem in pageItems)
            {
                switch (pageItem.IdCode)
                {
                    case "manageCustomers":
                        pageItemViewModels.Add(new PageItemManageCustomersViewModel(pageItem));
                        break;
                    case "manageEmployees":
                        pageItemViewModels.Add(new PageItemManageEmployeesViewModel(pageItem));
                        break;
                    default:
                        break;
                }
            }
        }
    }

    public class PageItemManageCustomersViewModel : PageViewModelBase
    {
        public PageItemManageCustomersViewModel(PageItem pageItem)
        {

        }
    }

    public class PageItemManageEmployeesViewModel : PageViewModelBase
    {
        public PageItemManageEmployeesViewModel(PageItem pageItem)
        {


        }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public abstract class PageViewModelBase : ViewModelBase
    {
        //these are the properties which every PageItemViewModel needs
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    public class PageItems
    {
        public static List<PageItem> GetAll()
        {
            List<PageItem> pageItems = new List<PageItem>();
            pageItems.Add(new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers"});
            pageItems.Add(new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees"});
            return pageItems;
        }
    }

}
于 2009-06-22T14:22:07.860 回答
1

为什么不能GetViewModel()在返回适当视图模型的基本 PageItem 类中放置一个虚拟方法？

   foreach (PageItem pageItem in pageItems)
   {
       pageItemViewModels.Add(pageItem.GetViewModel());
   }

立即看起来像代码异味的是“id”属性的使用——这通常可以用多态性代替。因此，您将使用上面的代码替换该switch语句。

编辑：

如果您的 PageItem 类对您的视图模型一无所知，那么它就不能以这种方式实现。基本上，你需要一个你已经拥有的工厂（在某种程度上）。

我通常有一个关系列表（PageItem 到 ViewModel），在你的情况下是Dictionary<String, Type>. 然后您可以在初始化期间填写此列表，并在以后实例化正确的视图模型。

要使用反射来构建此列表，您至少需要以编程方式了解视图模型支持哪个页面项。为此，您可以使用自定义属性来装饰您的类，例如：

public class SupportsPageItemAttribute : Attribute
{
    private readonly string _id;
    public string ID
    {
        get { return _id;}
    }

    public SupportsPageItemAttribute(string id)
    {
        _id = id;
    }
}

然后使用该属性来定义您的模型可以接受的 PageItem ：

[SupportsPageItemAttribute("manageCustomers")
public class PageItemManageCustomersViewModel
{
   // ...
}

然后，您使用反射来获取所有实现 IPageItemViewModel 的类并检查它们的属性以获取 PageItem id 字符串。

例如（没有太多的错误检查）：

Dictionary<String, Type> modelsById = new Dictionary<String, Type>();
String viewModelInterface = typeof(IPageItemViewModel).FullName;

// get the assembly
Assembly assembly = Assembly.GetAssembly(typeof(IPageItemViewModel));

// iterate through all types
foreach (Type viewModel in assembly.GetTypes())
{
    // get classes which implement IPageItemViewModel
    if (viewModel.GetInterface(viewModelInterface) != null)
    {
        // get the attribute we're interested in
        foreach (Attribute att in Attribute.GetCustomAttributes(viewModel))
        {
            if (att is SupportsPageItemAttribute)
            {
                // get the page item id
                String id = (att as SupportsPageItemAttribute).ID;

                // add to dictionary
                modelsById.Add(id, viewModel);
            }
        }
    }
}

另一方面，您可以考虑使用各种控制反转框架，而不是自己进行令人讨厌的反射工作。

于 2009-06-22T14:29:45.317 回答
1

一种不是很漂亮但有效的解决方案是使用约定来摆脱 switch 语句。这假设您可以更改 IdCodes 或至少修改案例以匹配 ViewModel。

    var type = Type.GetType("PageItem" + pageItem.IdCode + "ViewModel");
    var viewModel = Activator.CreateInstance(type) as ViewModelBase;
    pageItemViewModels.Add(viewModel);

请注意，您应该在此处添加错误检查，这里有几个故障点。然而，这比必须维护一个不断增长的 switch 语句要好。

于 2009-06-22T15:14:15.213 回答
1

PageItem一种可能的解决方案是反转代码之间的关系PageItemViewModel。现在，您正在PageItemViewModel基于 a生成 a PageItem，但是如果您PageItemViewModel先创建 s 然后在每个PageItemViewModel的构造函数中创建适当的PageItem怎么办？这消除了对的需求switch并使事情变得更清洁，因为现在您的视图模型负责模型，而不是模型负责视图模型。

基于您当前代码的示例：

using System;
using System.Collections.Generic;

namespace TestInstantiate838
{
    public class Program
    {
        static void Main(string[] args)
        {
            List<ViewModelPageItemBase> pageItemViewModels = PageItemViewModels.GetAll();

            // No switch needed anymore. Each PageItem's view-model contains its PageItem
            // which is exposed as property of the view-model.
            foreach (ViewModelPageItemBase pageItemViewModel in pageItemViewModels)
            {
                System.Console.WriteLine("{0}:{1}", pageItemViewModel.PageItem.IdCode, pageItemViewModel.PageItem.Title);
            }
            Console.ReadLine();
        }
    }

    public class PageItemManageCustomersViewModel : ViewModelPageItemBase
    {
        public PageItemManageCustomersViewModel()
        {
            PageItem = new PageItem { IdCode = "manageCustomers", Title = "ManageCustomers" };
        }
    }

    public class PageItemManageEmployeesViewModel : ViewModelPageItemBase
    {
        public PageItemManageEmployeesViewModel()
        {
            PageItem = new PageItem { IdCode = "manageEmployees", Title = "ManageEmployees" };
        }
    }

    public abstract class ViewModelPageItemBase : ViewModelBase
    {
        //The PageItem associated with this view-model
        public PageItem PageItem { get; protected set; }
    }

    public abstract class ViewModelBase
    {
        protected void OnPropertyChanged(string propertyName)
        {
            //this is the INotifyPropertyChanged method which all ViewModels need
        }
    }

    public class PageItem
    {
        public string IdCode { get; set; }
        public string Title { get; set; }
    }

    // Replaces PageItems class
    public class PageItemViewModels
    {
        // Return a list of PageItemViewModel's instead of PageItem's.
        // Each PageItemViewModel knows how to build it's corresponding PageItem object.
        public static List<PageItemViewModelBase> GetAll()
        {
            List<PageItemViewModelBase> pageItemViewModels = new List<PageItemViewModelBase>();
            pageItemViewModels.Add(new PageItemManageCustomersViewModel());
            pageItemViewModels.Add(new PageItemManageEmployeesViewModel());
            return pageItemViewModels;
        }
    }
}
于 2009-06-22T15:40:34.707 回答