MVC修改视图的默认路径

概述：之前MVC5和之前的版本中，我们要想对View文件的路径进行控制的话，则必须要对IViewEngine接口的FindPartialView或FindView方法进行重写，所有的视图引擎都继承于该IViewEngine接口，比如默认的RazorViewEngine。但新版本MVC6中，对视图文件的路径方式却不太一样了，目前有两种方式，一种是通过RazorViewEngine，另外一种是通过新特性IViewLocationExpander接口。

MVC5和之前的版本:

  public class ViewEngine : RazorViewEngine
    {
         /// <summary>
        /// Initializes a new instance of the <see cref="ViewEngine"/> class.
        /// </summary>
        public ViewEngine()
         {
            var views = new[]
            {
                "~/Views/{1}/{0}.cshtml",
                "~/Views/Shared/{0}.cshtml",
                "~/Views/Base/{0}.cshtml",
                "~/Views/Base/{1}/{0}.cshtml",
            };

            this.PartialViewLocationFormats = views;

            this.ViewLocationFormats = views;
        }

        /// <summary>
        /// 添加视图规则
        /// </summary>
        /// <param name="viewEngineCollection">viewEngineCollection</param>
        internal static void RegisterView(ViewEngineCollection viewEngineCollection)
        {
            viewEngineCollection.Add(new ViewEngine());
        }

在Application_Start()中添加语句：

// 注册视图规则
ViewEngine.RegisterView(ViewEngines.Engines);

MVC6:

通过RazorViewEngine来控制View路径

在新版的RazorViewEngine中，该类提供了两个虚属性（AreaViewLocationFormats和ViewLocationFormats），可以用于重写控制，而不必再对FindPartialView或FindView方法进行重写，示例如下：

public class ThemeViewEngine : RazorViewEngine
{
    public ThemeViewEngine(IRazorPageFactory pageFactory,
        IRazorViewFactory viewFactory,
        IViewLocationExpanderProvider viewLocationExpanderProvider,
        IViewLocationCache viewLocationCache)
        : base(pageFactory,
                viewFactory,
                viewLocationExpanderProvider,
                viewLocationCache)
    {
    }

    public override IEnumerable<string> AreaViewLocationFormats
    {
        get
        {
            var value = new Random().Next(0, 1);
            var theme = value == 0 ? "Theme1" : "Theme2";  // 可通过其它条件，设置皮肤的种类
            return base.AreaViewLocationFormats.Select(f => f.Replace("/Views/", "/Views/" + theme + "/"));
        }
    }

    public override IEnumerable<string> ViewLocationFormats
    {
        get
        {
            var value = new Random().Next(0, 1);
            var theme = value == 0 ? "Theme1" : "Theme2";  // 可通过其它条件，设置皮肤的种类
            return base.ViewLocationFormats.Select(f => f.Replace("/Views/", "/Views/" + theme + "/"));
        }
    }
}

然后，通过修改MVcOptions的实例属性ViewEngines即可完成对视图引擎的替换，代码如下：

services.AddMvc().Configure<MvcOptions>(options =>
{
    options.ViewEngines.Clear();
    options.ViewEngines.Add(typeof(ThemeViewEngine));
});

这样，系统在查找视图文件的时候，就会按照新注册的ThemeViewEngine的逻辑来执行。

通过IViewLocationExpander来控制View路径

在MVC6中，微软还提供了另外一种新的方式来控制View文件的路径，那就是IViewLocationExpander接口，通过实现该接口即可实现自定义逻辑，并且也可以使用相关的上下文对象。示例如下：

public class ThemeViewLocationExpander : IViewLocationExpander
{
    public void PopulateValues(ViewLocationExpanderContext context)
    {
        var value = new Random().Next(0, 1);
        var theme = value == 0 ? "Theme1" : "Theme2";
        context.Values["theme"] = theme;
    }

    public virtual IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context,
                                                            IEnumerable<string> viewLocations)
    {
        return viewLocations.Select(f => f.Replace("/Views/", "/Views/" + context.Values["theme"] + "/"));
    }
}

在上述自定义的IViewLocationExpander中，实现了2个方法分别是PopulateValues和ExpandViewLocations，PopulateValues方法可以让我们想ViewLocationExpanderContext上下文中添加响应的键值对以便后续使用，通过，我们可以利用通过该上下文对象，来查找ActionContext和HttpContext对象，以便利用这些对象做响应的判断操作；而ExpandViewLocations方法，只会在没有View缓存或在View缓存里找不到对应key的View文件时才会调用该方法，在该方法内，我们可以动态返回视图的位置。

最后，我们在Startup.cs里通过修改RazorViewEngineOptions实例对象的ViewLocationExpanders属性，来实现注册目的，代码如下：

services.Configure<RazorViewEngineOptions>(options =>
{
    options.ViewLocationExpanders.Add(typeof(ThemViewLocationExpander));
});