自定义VirtualPathProvider映射ASP.NET MVC View

假设有多个ASP.NET MVC 应用解决方案，基于Area部署同一个站点。多个站点需要以一致的方式进行布局切换，不同Area映射View的方式不同，它们都以{AreaName}\{Views}的格式存放。相比这种访问方式，将多个Area关联的View更新到一个固定目录更便于管理，比如：App_Themes\{SkinName}\Views\{AreaName}\。自定义ViewEngine可以关联起来，还有一种方式就是自定义VirtualPathProvider。比如当请求一个/Views/Home/Index.cshtml，ASP.NET会通过VirtualPathProvider的FileExists询问路径是否存在，如果存在则会继续调用GetFile获取一个VirtualFile。我们可以将一个View的内容存放到数据库或者添加一个映射配置文件。比如对于A应用的Index.cshtml的虚拟路径：/A/Views/Home/Index.cshtml实际映射到/App_Themes/{SkinName}/Views/A/Index.cshtml，只需要继承于VirtualFile并重写Open函数。下面是一个简单示例：

<?xml version="1.0" encoding="utf-8" ?>
<map>
  <view virtualPath="" mapTo="" />
</map>

/// <summary>
/// 视图映射
/// </summary>
[XmlType(TypeName = "view")]
public class ViewMap
{
    /// <summary>
    /// 虚拟路径
    /// </summary>
    [XmlAttribute(AttributeName = "virtualPath")]
    public String VirtualPath { get; set; }

    /// <summary>
    /// 映射到
    /// </summary>
    [XmlAttribute(AttributeName = "mapTo")]
    public String MapTo { get; set; }
}

/// <summary>
/// 视图映射路径提供器
/// </summary>
public sealed class ViewMapPathProvider : VirtualPathProvider
{
    /// <summary>
    /// 序列化提供器
    /// </summary>
    private static readonly XmlSerializer SerializeProvider;

    /// <summary>
    /// 静态构造函数
    /// </summary>
    static ViewMapPathProvider()
    {
        SerializeProvider = new XmlSerializer(typeof(List<ViewMap>), new XmlRootAttribute("map"));
    }

    private IDictionary<String, ViewMap> m_views;
    private FileSystemWatcher m_fileWatcher;

    /// <summary>
    /// 构造函数
    /// </summary>
    /// <param name="xmlPath">文件路径</param>
    public ViewMapPathProvider(String xmlPath)
    {
        if (String.IsNullOrEmpty(xmlPath))
            throw new ArgumentException("xmlPath不能为空");

        if (!File.Exists(xmlPath))
            throw new FileNotFoundException(String.Format("文件{0}不存在", xmlPath));

        this.XmlPath = xmlPath;

        m_fileWatcher = new FileSystemWatcher(Path.GetDirectoryName(xmlPath));
        m_fileWatcher.Changed += new FileSystemEventHandler(OnFileChanged);
    }

    /// <summary>
    /// 文件路径
    /// </summary>
    public String XmlPath { get; private set; }

    /// <summary>
    /// 视图集合
    /// </summary>
    public IDictionary<String, ViewMap> Views
    {
        get
        {
            if (m_views == null)
            {
                using (TextReader textReader = new StreamReader(this.XmlPath, true))
                {
                    List<ViewMap> views = SerializeProvider.Deserialize(textReader) as List<ViewMap>;

                    m_views = views.ToDictionary(p => p.VirtualPath);
                }
            }

            return m_views;
        }
    }

    /// <summary>
    /// 文件是否存在
    /// </summary>
    /// <param name="virtualPath">虚拟路径</param>
    /// <returns>是否存在</returns>
    public override Boolean FileExists(String virtualPath)
    {
        if (this.Views.ContainsKey(virtualPath))
            return true;

        return base.FileExists(virtualPath);
    }

    /// <summary>
    /// 获取文件
    /// </summary>
    /// <param name="virtualPath">虚拟路径</param>
    /// <returns>虚拟文件</returns>
    public override VirtualFile GetFile(String virtualPath)
    {
        if (this.Views.ContainsKey(virtualPath))
        {
            ViewMap viewMap = this.Views[virtualPath];

            return new ViewMapFile(virtualPath, viewMap);
        }

        return base.GetFile(virtualPath);
    }

    /// <summary>
    /// 文件变化
    /// </summary>
    /// <param name="sender">发送方</param>
    /// <param name="e">事件参数</param>
    private void OnFileChanged(Object sender, FileSystemEventArgs e)
    {
        if (String.Equals(e.FullPath, this.XmlPath, StringComparison.CurrentCultureIgnoreCase))
            m_views = null;
    }
}

/// <summary>
/// 视图映射文件
/// </summary>
public class ViewMapFile : VirtualFile
{
    /// <summary>
    /// 构造函数
    /// </summary>
    /// <param name="virtualPath">虚拟路径</param>
    /// <param name="viewMap">视图映射</param>
    public ViewMapFile(String virtualPath, ViewMap viewMap)
        : base(virtualPath)
    {
        this.ViewMap = viewMap;
    }

    /// <summary>
    /// 视图映射
    /// </summary>
    public ViewMap ViewMap { get; private set; }

    /// <summary>
    /// 打开
    /// </summary>
    /// <returns>流</returns>
    public override Stream Open()
    {
        return new FileStream(this.ViewMap.MapTo, FileMode.Open, FileAccess.Read);
    }
}

/// <summary>
/// 启动
/// </summary>
protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();

    RegisterGlobalFilters(GlobalFilters.Filters);
    RegisterRoutes(RouteTable.Routes);

    String xmlPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, @"App_Data\ViewMap.xml");
    HostingEnvironment.RegisterVirtualPathProvider(new ViewMapPathProvider(xmlPath));
}