本篇实践在ASP.NET MVC 4下使用Session来保持表单的状态。
本篇的源码在这里： https://github.com/darrenji/KeepFormStateUsingSession
如上，输入俱乐部名称，点击"添加球员"，输入球员名称。我们希望，点击"到别的地方转转"跳转到另外一个视图页，当再次返回的时候能保持表单的状态。
点击"到别的地方转转"跳转到另外一个视图页如下：
再次返回，表单的状态被保持了：
点击"提交"按钮，显示表单的内容：
关于球员，对应的Model为：
using System.ComponentModel.DataAnnotations;namespace MvcApplication1.Models{public class Player{public int Id { get; set; }[Required(ErrorMessage = "必填")][Display(Name = "球员名称")]public string Name { get; set; }}}
关于俱乐部，对应的Model为：
using System.Collections.Generic;using System.ComponentModel.DataAnnotations;namespace MvcApplication1.Models{public class Club{public Club(){this.Players = new List<Player>();}public int Id { get; set; }[Required(ErrorMessage = "必填")][Display(Name = "俱乐部名称")]public string Name { get; set; }public List<Player> Players { get; set; }}}
在Home/Index.cshtml强类型视图中，
@model MvcApplication1.Models.Club@{ViewBag.Title = "Index";Layout = "~/Views/Shared/_Layout.cshtml";}<h2>Index</h2>@using (Html.BeginForm("Index", "Home", FormMethod.Post, new {id = "myForm"})){@Html.LabelFor(m => m.Name)@Html.TextBoxFor(m => m.Name)@Html.ValidationMessageFor(m => m.Name)<br/><br/><ul id="players" style="list-style-type: none">@if (Model.Players != null){foreach (var item in Model.Players){Html.RenderAction("NewPlayerRow", "Home", new { player = @item });}}</ul><a id="addPlayer" href="javascript:void(0)">添加球员</a><br/><br/><div><a href="javascript:void(0)" id="gotoOther">到别的地方转转</a> <input type="submit" id="up" value="提交" /></div>}@section scripts{<script src="~/Scripts/dynamicvalidation.js"></script><script type="text/javascript">$(function () {//添加关于Player的新行$('#addPlayer').on("click", function() {createPlayerRow();});//到别的页$('#gotoOther').on("click", function() {if ($('#myForm').valid()) {$.ajax({cache: false,url: '@Url.Action("BeforeGoToMustSave", "Home")',type: 'POST',dataType: 'json',data: $('#myForm').serialize(),success: function (data) {if (data.msg) {window.location.href = '@Url.Action("RealGoTo", "Home")';}},error: function (xhr, status) {alert("添加失败，状态码：" + status);}});}});});//添加品牌行function createPlayerRow() {$.ajax({cache: false,url: '@Url.Action("NewPlayerRow", "Home")',type: "GET",data: {},success: function (data) {$('#players').append(data);$.validator.unobtrusive.parseDynamicContent('#players li:last', "#myForm");},error: function (xhr, status) {alert("添加行失败，状态码：" + status);}});}</script>}
以上，
○ 点击"添加球员"，向控制器发出异步请求，把部分视图li动态加载到ul中
○ 点击"到别的地方转转",向控制器发出异步请求，正是在这时候，在控制器的Action中，实施把表单的状态保存到Session中
○ 点击"提交"按钮，把表单信息显示出来
另外，当在页面上点击"添加球员"，为了让动态的部分视图能被验证，需要引入dynamicvalidation.js，调用其$.validator.unobtrusive.parseDynamicContent('#players li:last', "#myForm")方法，dynamicvalidation.js具体如下：
//对动态生成内容客户端验证(function ($) {$.validator.unobtrusive.parseDynamicContent = function (selector, formSelector) {$.validator.unobtrusive.parse(selector);var form = $(formSelector);var unobtrusiveValidation = form.data('unobtrusiveValidation');var validator = form.validate();$.each(unobtrusiveValidation.options.rules, function (elname, elrules) {if (validator.settings.rules[elname] == undefined) {var args = {};$.extend(args, elrules);args.messages = unobtrusiveValidation.options.messages[elname];//edit:use quoted strings for the name selector$("[name='" + elname + "']").rules("add", args);} else {$.each(elrules, function (rulename, data) {if (validator.settings.rules[elname][rulename] == undefined) {var args = {};args[rulename] = data;args.messages = unobtrusiveValidation.options.messages[elname][rulename];//edit:use quoted strings for the name selector$("[name='" + elname + "']").rules("add", args);}});}});};})(jQuery);
具体原理，请参考"Applying unobtrusive jquery validation to dynamic content in ASP.Net MVC"这篇文章。
在HomeController中，
public class HomeController : Controller{private const string sessionKey = "myFormKey";public ActionResult Index(){Club club = null;if (Session[sessionKey] != null){club = (Club) Session[sessionKey];}else{club = new Club();}return View(club);}//提交表单[HttpPost]public ActionResult Index(Club club){if (ModelState.IsValid){StringBuilder sb = new StringBuilder();sb.Append(club.Name);if (club.Players != null && club.Players.Count > 0){foreach (var item in club.Players){sb.AppendFormat("--{0}", item.Name);}}//删除Session//Session.Abandon();//Session.Clear();Session.Remove(sessionKey);return Content(sb.ToString());}else{return View(club);}}//添加新行public ActionResult NewPlayerRow(Player player){return PartialView("_NewPlayer", player ?? new Player());}//跳转之前把表单保存到Session中[HttpPost]public ActionResult BeforeGoToMustSave(Club club){Session[sessionKey] = club;return Json(new { msg = true });}//保存完Club的Session后真正跳转到的页面public ActionResult RealGoTo(){return View();}}
以上，
○ 对于接收[HttpGet]请求的Index方法对应的视图，Session存在就从Session中取出Club实例，否则就创建一个空的club实例
○ 对于接收[HttpPost]请求的Index方法对应的视图，显示表单内容之前把对应的Session删除
○ 添加新行NewPlayerRow方法供显示或添加用，当Player类型参数为null的时候，实际就是点击"添加球员"显示新行
○ BeforeGoToMustSave方法实际是为了在跳转之前保存Session
○ RealGoTo是点击"到别的地方转转"后真正跳转的视图页
另外，所有视图页的公共页Layout.cshtml，必须引用异步验证的js。
<head><meta charset="utf-8" /><meta name="viewport" content="width=device-width" /><title>@ViewBag.Title</title>@Styles.Render("~/Content/css")@Scripts.Render("~/bundles/jquery")@Scripts.Render("~/bundles/jqueryval")</head><body>@RenderBody()@RenderSection("scripts", required: false)</body>
Home/_NewPlayer.cshtml部分视图，是在点击"添加球员"之后动态加载的部分视图。
@using MvcApplication1.Extension@model MvcApplication1.Models.Player<li class="newcarcolorli">@using (Html.BeginCollectionItem("Players")){@Html.HiddenFor(model => model.Id)<div>@Html.LabelFor(m => m.Name)@Html.TextBoxFor(m => m.Name)@Html.ValidationMessageFor(m => m.Name)</div>}</li>
其中，用到了扩展Extension文件夹下CollectionEditingHtmlExtensions类的扩展方法，如下：
using System;using System.Collections.Generic;using System.Web;using System.Web.Mvc;namespace MvcApplication1.Extension{public static class CollectionEditingHtmlExtensions{//目标生成如下格式//<input autocomplete="off" name="FavouriteMovies.Index" type="hidden" value="6d85a95b-1dee-4175-bfae-73fad6a3763b" />//<label>Title</label>//<input class="text-box single-line" name="FavouriteMovies[6d85a95b-1dee-4175-bfae-73fad6a3763b].Title" type="text" value="Movie 1" />//<span class="field-validation-valid"></span>public static IDisposable BeginCollectionItem<TModel>(this HtmlHelper<TModel> html, string collectionName){//构建name="FavouriteMovies.Index"string collectionIndexFieldName = string.Format("{0}.Index", collectionName);//构建Guid字符串string itemIndex = GetCollectionItemIndex(collectionIndexFieldName);//构建带上集合属性+Guid字符串的前缀string collectionItemName = string.Format("{0}[{1}]", collectionName, itemIndex);TagBuilder indexField = new TagBuilder("input");indexField.MergeAttributes(new Dictionary<string, string>(){{"name", string.Format("{0}.Index", collectionName)},{"value", itemIndex},{"type", "hidden"},{"autocomplete", "off"}});html.ViewContext.Writer.WriteLine(indexField.ToString(TagRenderMode.SelfClosing));return new CollectionItemNamePrefixScope(html.ViewData.TemplateInfo, collectionItemName);}private class CollectionItemNamePrefixScope : IDisposable{private readonly TemplateInfo _templateInfo;private readonly string _previousPrfix;//通过构造函数，先把TemplateInfo以及TemplateInfo.HtmlFieldPrefix赋值给私有字段变量，并把集合属性名称赋值给TemplateInfo.HtmlFieldPrefixpublic CollectionItemNamePrefixScope(TemplateInfo templateInfo, string collectionItemName){this._templateInfo = templateInfo;this._previousPrfix = templateInfo.HtmlFieldPrefix;templateInfo.HtmlFieldPrefix = collectionItemName;}public void Dispose(){_templateInfo.HtmlFieldPrefix = _previousPrfix;}}/// <summary>////// </summary>/// <param name="collectionIndexFieldName">比如，FavouriteMovies.Index</param>/// <returns>Guid字符串</returns>private static string GetCollectionItemIndex(string collectionIndexFieldName){Queue<string> previousIndices = (Queue<string>)HttpContext.Current.Items[collectionIndexFieldName];if (previousIndices == null){HttpContext.Current.Items[collectionIndexFieldName] = previousIndices = new Queue<string>();string previousIndicesValues = HttpContext.Current.Request[collectionIndexFieldName];if (!string.IsNullOrWhiteSpace(previousIndicesValues)){foreach (string index in previousIndicesValues.Split(',')){previousIndices.Enqueue(index);}}}return previousIndices.Count > 0 ? previousIndices.Dequeue() : Guid.NewGuid().ToString();}}}
其原理，请参考"MVC批量更新,可验证并解决集合元素不连续控制器接收不完全的问题"这篇文章。
Home/RealGoTo.cshtml视图，是点击"到别的地方转转"后跳转到的页面，仅仅提供了一个跳转到Home/Index视图页的链接。
@{ViewBag.Title = "RealGoTo";Layout = "~/Views/Shared/_Layout.cshtml";}<h2>RealGoTo</h2>@Html.ActionLink("回到表单页","Index","Home")
就这样。