ASP.NET MVC：当您的 ViewModel 是 Collection/List/IEnumerable 时如何维护 TextBox 状态

Question

我正在使用 ASP.NET MVC 2 Beta。我可以使用 Steven Sanderson 的技术（在他的《Pro ASP.NET MVC Framework》一书中）创建一个类似向导的工作流程，除了使用 Session 而不是隐藏表单字段来跨请求保留数据。当我的模型不是集合时，我可以在页面之间来回切换并维护文本框中的值，没有任何问题。一个例子是一个简单的 Person 模型：

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Email { get; set; }
}

但是当我传递 IEnumerable 时，我无法让它工作。在我看来，我试图运行模型并为列表中的每个人生成一个用于名称和电子邮件的文本框。我可以很好地生成表单，并且可以使用我的值提交表单并转到步骤 2。但是，当我单击步骤 2 中的“后退”按钮时，它会将我带回步骤 1，并显示一个空表单。我之前填写的字段都不存在。一定有什么东西我错过了。有人可以帮我吗？

这是我的视图：

<% using (Html.BeginForm()) { %>
<% int index = 0;
   foreach (var person in Model) { %>
       <fieldset>
            <%= Html.Hidden("persons.index", index.ToString())%>
            <div>Name: <%= Html.TextBox("persons[" + index.ToString() + "].Name")%></div>
            <div>Email: <%= Html.TextBox("persons[" + index.ToString() + "].Email")%></div>
       </fieldset>
       <% index++;
   } %>  
   <p><input type="submit" name="btnNext" value="Next >>" /></p>
<% } %>

这是我的控制器：

public class PersonListController : Controller
{
    public IEnumerable<Person> persons;

    protected override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        persons = (Session["persons"]
            ?? TempData["persons"]
            ?? new List<Person>()) as List<Person>;
        // I've tried this with and without the prefix.
        TryUpdateModel(persons, "persons"); 
    }

    protected override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        Session["persons"] = persons;

        if (filterContext.Result is RedirectToRouteResult)
            TempData["persons"] = persons;
    }

    public ActionResult Step1(string btnBack, string btnNext)
    {
        if (btnNext != null)
            return RedirectToAction("Step2");

        // Setup some fake data
        var personsList = new List<Person> 
            { 
                new Person { Name = "Jared", Email = "[email protected]", },
                new Person { Name = "John", Email = "[email protected]" } 
            };

        // Populate the model with fake data the first time
        // the action method is called only. This is to simulate
        // pulling some data in from a DB.
        if (persons == null || persons.Count() == 0)
            persons = personsList;

        return View(persons);
    }

    // Step2 is just a page that provides a back button to Step1
    public ActionResult Step2(string btnBack, string btnNext)
    {
        if (btnBack != null)
            return RedirectToAction("Step1");

        return View(persons);
    }
}

Answer 1

据我所知，ASP.NET MVC 2 Beta 不支持此功能，ASP.NET MVC 2 RC 也不支持此功能。我仔细研究了 MVC 源代码，看起来支持字典，但不支持 IEnumerable<> 的模型。 （或包含嵌套的 IEnumerable 对象）及其继承者，例如 IList<>。

问题出在 ViewDataDictionary 类中。特别是，GetPropertyValue 方法仅提供一种从字典属性（通过调用 GetIndexedPropertyValue）或简单属性（通过使用 PropertyDescriptor.GetValue 方法提取值）检索属性值的方法。

为了解决这个问题，我创建了一个 GetCollectionPropertyValue 方法来处理集合模型（甚至包含嵌套集合的模型）。我把代码贴在这里供参考。注意：我没有对优雅做出任何声明 - 事实上所有字符串解析都非常丑陋，但它似乎有效。方法如下：

// Can be used to pull out values from Models with collections and nested collections.
        // E.g. Persons[0].Phones[3].AreaCode
        private static ViewDataInfo GetCollectionPropertyValue(object indexableObject, string key)
        {
            Type enumerableType = TypeHelpers.ExtractGenericInterface(indexableObject.GetType(), typeof(IEnumerable<>));
            if (enumerableType != null)
            {
                IList listOfModelElements = (IList)indexableObject;

                int firstOpenBracketPosition = key.IndexOf('[');
                int firstCloseBracketPosition = key.IndexOf(']');

                string firstIndexString = key.Substring(firstOpenBracketPosition + 1, firstCloseBracketPosition - firstOpenBracketPosition - 1);
                int firstIndex = 0;
                bool canParse = int.TryParse(firstIndexString, out firstIndex);

                object element = null;
                // if the index was numeric we should be able to grab the element from the list
                if (canParse)
                    element = listOfModelElements[firstIndex];

                if (element != null)
                {
                    int firstDotPosition = key.IndexOf('.');
                    int nextOpenBracketPosition = key.IndexOf('[', firstCloseBracketPosition);

                    PropertyDescriptor descriptor = TypeDescriptor.GetProperties(element).Find(key.Substring(firstDotPosition + 1), true);

                    // If the Model has nested collections, we need to keep digging recursively
                    if (nextOpenBracketPosition >= 0)
                    {
                        string nextObjectName = key.Substring(firstDotPosition+1, nextOpenBracketPosition-firstDotPosition-1);
                        string nextKey = key.Substring(firstDotPosition + 1);

                        PropertyInfo property = element.GetType().GetProperty(nextObjectName);
                        object nestedCollection = property.GetValue(element,null);
                        // Recursively pull out the nested value
                        return GetCollectionPropertyValue(nestedCollection, nextKey);
                    }
                    else
                    {
                        return new ViewDataInfo(() => descriptor.GetValue(element))
                        {
                            Container = indexableObject,
                            PropertyDescriptor = descriptor
                        };
                    }
                }
            }

            return null;
        }

下面是修改后的 GetPropertyValue 方法，它调用新方法：

private static ViewDataInfo GetPropertyValue(object container, string propertyName) {
            // This method handles one "segment" of a complex property expression

            // First, we try to evaluate the property based on its indexer
            ViewDataInfo value = GetIndexedPropertyValue(container, propertyName);
            if (value != null) {
                return value;
            }

            // If the indexer didn't return anything useful, continue...

            // If the container is a ViewDataDictionary then treat its Model property
            // as the container instead of the ViewDataDictionary itself.
            ViewDataDictionary vdd = container as ViewDataDictionary;
            if (vdd != null) {
                container = vdd.Model;
            }

            // Second, we try to evaluate the property based on the assumption
            // that it is a collection of some sort (e.g. IList<>, IEnumerable<>)
            value = GetCollectionPropertyValue(container, propertyName);
            if (value != null)
            {
                return value;
            }

            // If the container is null, we're out of options
            if (container == null) {
                return null;
            }

            // Third, we try to use PropertyDescriptors and treat the expression as a property name
            PropertyDescriptor descriptor = TypeDescriptor.GetProperties(container).Find(propertyName, true);


            if (descriptor == null) {
                return null;
            }

            return new ViewDataInfo(() => descriptor.GetValue(container)) {
                Container = container,
                PropertyDescriptor = descriptor
            };
        }

同样，该方法位于 ASP.NET MVC 2 RC 中的 ViewDataDictionary.cs 文件中。我是否应该创建一个新问题来在 MVC Codeplex 网站上跟踪此问题？