以编程方式将 Razor 页面呈现为 HTML 字符串

Question

目标

我想在后端生成一个 HTML 字符串使用 HtmlToPDF 库将其转换为 PDF。
出于 debugging/tweaking 目的，我还希望能够在浏览器中轻松查看生成的 HTML。当 IsDevelopment().
我希望它尽可能简单。

我正在使用 ASP.NET Core 3.1

方法

剃刀页面

我想我会尝试 new Razor Pages，因为它们被宣传为超级简单。

@page
@using MyProject.Pages.Pdf
@model IndexModel

<h2>Test</h2>
<p>
    @Model.Message
</p>

namespace MyProject.Pages.Pdf
{
    public class IndexModel : PageModel
    {
        private readonly MyDbContext _context;

        public IndexModel(MyDbContext context)
        {
            _context = context;
        }

        public string Message { get; private set; } = "PageModel in C#";

        public async Task<IActionResult> OnGetAsync()
        {
            var count = await _context.Foos.CountAsync();

            Message += $" Server time is { DateTime.Now } and the Foo count is { count }";

            return Page();
        }
    }
}

这在浏览器中有效 - 耶！

渲染并获取 HTML 字符串

我发现 Render a Razor Page to string 似乎符合我的要求。

但这就是麻烦的开始:(

问题

首先，我觉得很奇怪，当你通过 _razorViewEngine.FindPage 找到页面时，它不知道如何填充 ViewContext 或 Model。我认为 IndexModel 的工作是填充这些。我希望可以向 ASP.NET 询问 IndexModel 页面，就是这样。

无论如何...下一个问题。为了呈现页面，我必须手动创建一个 ViewContext 并且我必须为其提供一个 Model。但是页面就是模型，因为它是页面，所以它不是简单的 ViewModel。它依赖于 DI，并且期望执行 OnGetAsync() 以填充模型。这几乎是一个陷阱 22。

我还尝试通过 _razorViewEngine.FindView 获取视图而不是页面，但这也需要一个模型，所以我们回到 catch-22。

另一个问题。 debug/tweaking 页面的目的是轻松查看生成的内容。但是，如果我必须在 IndexModel 之外创建一个 Model，那么它不再代表某处服务中实际生成的内容。

所有这些让我想知道我是否走在正确的道路上。还是我遗漏了什么？

Answer 1

请参考以下步骤将部分视图呈现为字符串：

将接口添加到名为 IRazorPartialToStringRenderer.cs 的服务文件夹。

 public interface IRazorPartialToStringRenderer
 {
     Task<string> RenderPartialToStringAsync<TModel>(string partialName, TModel model);
 }

使用以下代码将 C# class 文件添加到名为 RazorPartialToStringRenderer.cs 的服务文件夹中：

 using System;
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.ModelBinding;
 using Microsoft.AspNetCore.Mvc.Razor;
 using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.AspNetCore.Mvc.ViewEngines;
 using Microsoft.AspNetCore.Mvc.ViewFeatures;
 using Microsoft.AspNetCore.Routing;

 namespace RazorPageSample.Services
 {
     public class RazorPartialToStringRenderer : IRazorPartialToStringRenderer
     {
         private IRazorViewEngine _viewEngine;
         private ITempDataProvider _tempDataProvider;
         private IServiceProvider _serviceProvider;
         public RazorPartialToStringRenderer(
             IRazorViewEngine viewEngine,
             ITempDataProvider tempDataProvider,
             IServiceProvider serviceProvider)
         {
             _viewEngine = viewEngine;
             _tempDataProvider = tempDataProvider;
             _serviceProvider = serviceProvider;
         }
         public async Task<string> RenderPartialToStringAsync<TModel>(string partialName, TModel model)
         {
             var actionContext = GetActionContext();
             var partial = FindView(actionContext, partialName);
             using (var output = new StringWriter())
             {
                 var viewContext = new ViewContext(
                     actionContext,
                     partial,
                     new ViewDataDictionary<TModel>(
                         metadataProvider: new EmptyModelMetadataProvider(),
                         modelState: new ModelStateDictionary())
                     {
                         Model = model
                     },
                     new TempDataDictionary(
                         actionContext.HttpContext,
                         _tempDataProvider),
                     output,
                     new HtmlHelperOptions()
                 );
                 await partial.RenderAsync(viewContext);
                 return output.ToString();
             }
         }
         private IView FindView(ActionContext actionContext, string partialName)
         {
             var getPartialResult = _viewEngine.GetView(null, partialName, false);
             if (getPartialResult.Success)
             {
                 return getPartialResult.View;
             }
             var findPartialResult = _viewEngine.FindView(actionContext, partialName, false);
             if (findPartialResult.Success)
             {
                 return findPartialResult.View;
             }
             var searchedLocations = getPartialResult.SearchedLocations.Concat(findPartialResult.SearchedLocations);
             var errorMessage = string.Join(
                 Environment.NewLine,
                 new[] { $"Unable to find partial '{partialName}'. The following locations were searched:" }.Concat(searchedLocations)); ;
             throw new InvalidOperationException(errorMessage);
         }
         private ActionContext GetActionContext()
         {
             var httpContext = new DefaultHttpContext
             {
                 RequestServices = _serviceProvider
             };
             return new ActionContext(httpContext, new RouteData(), new ActionDescriptor());
         }
     }
 }

在ConfigureServices方法中注册服务Startup class:

 public void ConfigureServices(IServiceCollection services)
 {
     services.AddRazorPages(); 
     services.AddTransient<IRazorPartialToStringRenderer, RazorPartialToStringRenderer>();
 }

使用 RenderPartialToStringAsync() 方法将 Razor 页面呈现为 HTML 字符串：

 public class ContactModel : PageModel
 {
     private readonly IRazorPartialToStringRenderer _renderer;
     public ContactModel(IRazorPartialToStringRenderer renderer)
     {
         _renderer = renderer; 
     }
     public void OnGet()
     { 
     }
     [BindProperty]
     public ContactForm ContactForm { get; set; }
     [TempData]
     public string PostResult { get; set; }

     public async Task<IActionResult> OnPostAsync()
     {
         var body = await _renderer.RenderPartialToStringAsync("_ContactEmailPartial", ContactForm);  //transfer model to the partial view, and then render the Partial view to string.
         PostResult = "Check your specified pickup directory";
         return RedirectToPage();
     }
 }
 public class ContactForm
 {
     public string Email { get; set; }
     public string Message { get; set; }
     public string Name { get; set; }
     public string Subject { get; set; }
     public Priority Priority { get; set; }
 }
 public enum Priority
 {
     Low, Medium, High
 }

调试截图如下：

更多详细步骤，请查看此博客Rendering A Partial View To A String。

Answer 2

我成功破解了！毕竟我走错了路……解决方案是使用 ViewComponent。但它仍然很时髦！

感谢

解决方案

将 PageModel 转换为 ViewComponent

namespace MyProject.ViewComponents
{
    public class MyViewComponent : ViewComponent
    {
        private readonly MyDbContext _context;

        public MyViewComponent(MyDbContext context)
        {
            _context = context;
        }

        public async Task<IViewComponentResult> InvokeAsync()
        {
            var count = await _context.Foos.CountAsync();

            var message = $"Server time is { DateTime.Now } and the Foo count is { count }";

            return View<string>(message);
        }
    }
}

并且视图放在Pages/Shared/Components/My/Default.cshtml

@model string

<h2>Test</h2>
<p>
    @Model
</p>

服务

using System;
using System.IO;
using System.Text.Encodings.Web;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewEngines;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Routing;

public class RenderViewComponentService
{
    private readonly IServiceProvider _serviceProvider;
    private readonly ITempDataProvider _tempDataProvider;
    private readonly IViewComponentHelper _viewComponentHelper;

    public RenderViewComponentService(
        IServiceProvider serviceProvider,
        ITempDataProvider tempDataProvider,
        IViewComponentHelper viewComponentHelper
    )
    {
        _serviceProvider = serviceProvider;
        _tempDataProvider = tempDataProvider;
        _viewComponentHelper = viewComponentHelper;
    }

    public async Task<string> RenderViewComponentToStringAsync<TViewComponent>(object args)
        where TViewComponent : ViewComponent
    {
        var viewContext = GetFakeViewContext();
        (_viewComponentHelper as IViewContextAware).Contextualize(viewContext);

        var htmlContent = await _viewComponentHelper.InvokeAsync<TViewComponent>(args);
        using var stringWriter = new StringWriter();
        htmlContent.WriteTo(stringWriter, HtmlEncoder.Default);
        var html = stringWriter.ToString();

        return html;
    }

    private ViewContext GetFakeViewContext(ActionContext actionContext = null, TextWriter writer = null)
    {
        actionContext ??= GetFakeActionContext();
        var viewData = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary());
        var tempData = new TempDataDictionary(actionContext.HttpContext, _tempDataProvider);

        var viewContext = new ViewContext(
            actionContext,
            NullView.Instance,
            viewData,
            tempData,
            writer ?? TextWriter.Null,
            new HtmlHelperOptions());

        return viewContext;
    }

    private ActionContext GetFakeActionContext()
    {
        var httpContext = new DefaultHttpContext
        {
            RequestServices = _serviceProvider,
        };

        var routeData = new RouteData();
        var actionDescriptor = new ActionDescriptor();

        return new ActionContext(httpContext, routeData, actionDescriptor);
    }

    private class NullView : IView
    {
        public static readonly NullView Instance = new NullView();
        public string Path => string.Empty;
        public Task RenderAsync(ViewContext context)
        {
            if (context == null) { throw new ArgumentNullException(nameof(context)); }
            return Task.CompletedTask;
        }
    }
}

用法

来自 Razor 页面（debug/tweaking 页面）

注意文件后面没有代码

@page
@using MyProject.ViewComponents

@await Component.InvokeAsync(typeof(MyViewComponent))

有路由数据

@page "{id}"
@using MyProject.ViewComponents

@await Component.InvokeAsync(typeof(MyViewComponent), RouteData.Values["id"])

来自控制器

[HttpGet]
public async Task<IActionResult> Get()
{
    var html = await _renderViewComponentService
        .RenderViewComponentToStringAsync<MyViewComponent>();

    // do something with the html

    return Ok(new { html });
}

使用 FromRoute

[HttpGet("{id}")]
public async Task<IActionResult> Get([FromRoute] int id)
{
    var html = await _renderViewComponentService
        .RenderViewComponentToStringAsync<MyViewComponent>(id);

    // do something with the html

    return Ok(new { html });
}

奇怪

非常不幸的是，注入的 IViewComponentHelper 不能开箱即用。

所以我们做了这个非常不直观的事情来让它工作。

(_viewComponentHelper as IViewContextAware).Contextualize(viewContext);

这会导致一连串奇怪的事情发生，例如假的 ActionContext 和 ViewContext 需要 TextWriter 但它没有用于任何事情！事实上 ViewContext 这个洞根本就没有用到。它只需要存在:(

另外 NullView... 由于某种原因 Microsoft.AspNetCore.Mvc.ViewFeatures.NullView 是 Internal 所以我们基本上必须 copy/paste 它到我们自己的代码中。

以后可能会改进

无论如何：IMO 这比使用 IRazorViewEngine 更简单，它几乎在每个网络搜索中都会出现 :)

以编程方式将 Razor 页面呈现为 HTML 字符串

Programmatically render Razor Page as HTML string

c#

asp.net-core

razor-pages

asp.net-core-3.1

razorengine

目标

方法