.NET Core开发日志——视图与页面

当一个Action完成它的任务后，通常需要返回一个实现IActionResult的对象，而最常见的就是View或者ViewResult，所谓的视图对象。那么视图与最终所看到的页面之间的联系又是怎样形成的，这便是本文想要探讨的问题。

在ResourceInvoker类之中，可以找到下列的代码。这些代码是对返回结果——IActionResult的进一步处理。

case State.ResultInside:
    {
        ...

        var task = InvokeResultAsync(_result);
        if (task.Status != TaskStatus.RanToCompletion)
        {
            next = State.ResultEnd;
            return task;
        }

        goto case State.ResultEnd;
    }

protected async Task InvokeResultAsync(IActionResult result)
{
    var actionContext = _actionContext;

    _diagnosticSource.BeforeActionResult(actionContext, result);
    _logger.BeforeExecutingActionResult(result);

    try
    {
        await result.ExecuteResultAsync(actionContext);
    }
    finally
    {
        _diagnosticSource.AfterActionResult(actionContext, result);
        _logger.AfterExecutingActionResult(result);
    }
}

IActionResult接口的实现类ViewResult中会调用ViewResultExecutor类的方法。

public override async Task ExecuteResultAsync(ActionContext context)
{
    ...

    var executor = context.HttpContext.RequestServices.GetRequiredService<IActionResultExecutor<ViewResult>>();
    await executor.ExecuteAsync(context, this);
}

ViewResultExecutor类里则需要先通过RazorViewEngine类找到对应的视图。

public async Task ExecuteAsync(ActionContext context, ViewResult result)
{
    ...

    var viewEngineResult = FindView(context, result);
    viewEngineResult.EnsureSuccessful(originalLocations: null);

    var view = viewEngineResult.View;
    using (view as IDisposable)
    {

        await ExecuteAsync(
            context,
            view,
            result.ViewData,
            result.TempData,
            result.ContentType,
            result.StatusCode);
    }

    ...
}

RazorViewEngine类返回的结果是RazorView对象。注意其内部已包含了IRazorPage对象。

public ViewEngineResult GetView(string executingFilePath, string viewPath, bool isMainPage)
{
    ...

    var cacheResult = LocatePageFromPath(executingFilePath, viewPath, isMainPage);
    return CreateViewEngineResult(cacheResult, viewPath);
}

public ViewEngineResult FindView(ActionContext context, string viewName, bool isMainPage)
{
    ...

    var cacheResult = LocatePageFromViewLocations(context, viewName, isMainPage);
    return CreateViewEngineResult(cacheResult, viewName);
}

private ViewEngineResult CreateViewEngineResult(ViewLocationCacheResult result, string viewName)
{
    ...

    var page = result.ViewEntry.PageFactory();

    var viewStarts = new IRazorPage[result.ViewStartEntries.Count];
    for (var i = 0; i < viewStarts.Length; i++)
    {
        var viewStartItem = result.ViewStartEntries[i];
        viewStarts[i] = viewStartItem.PageFactory();
    }

    var view = new RazorView(this, _pageActivator, viewStarts, page, _htmlEncoder, _diagnosticSource);
    return ViewEngineResult.Found(viewName, view);
}

找到视图后，ViewResultExecutor再调用其父类ViewExecutor的ExecuteAsync方法。其内部将调用RazorView类的RenderAsync方法。

protected async Task ExecuteAsync(
    ViewContext viewContext,
    string contentType,
    int? statusCode)
{
    ...

    var response = viewContext.HttpContext.Response;

    ResponseContentTypeHelper.ResolveContentTypeAndEncoding(
        contentType,
        response.ContentType,
        DefaultContentType,
        out var resolvedContentType,
        out var resolvedContentTypeEncoding);

    response.ContentType = resolvedContentType;

    if (statusCode != null)
    {
        response.StatusCode = statusCode.Value;
    }

    using (var writer = WriterFactory.CreateWriter(response.Body, resolvedContentTypeEncoding))
    {
        var view = viewContext.View;

        var oldWriter = viewContext.Writer;
        try
        {
            viewContext.Writer = writer;

            DiagnosticSource.BeforeView(view, viewContext);

            await view.RenderAsync(viewContext);

            DiagnosticSource.AfterView(view, viewContext);
        }
        finally
        {
            viewContext.Writer = oldWriter;
        }

        // Perf: Invoke FlushAsync to ensure any buffered content is asynchronously written to the underlying
        // response asynchronously. In the absence of this line, the buffer gets synchronously written to the
        // response as part of the Dispose which has a perf impact.
        await writer.FlushAsync();
    }
}

RazorView类中可以看到其核心的处理与IRazorPage的ExecuteAsync方法紧密相关。

public virtual async Task RenderAsync(ViewContext context)
{
    ...

    _bufferScope = context.HttpContext.RequestServices.GetRequiredService<IViewBufferScope>();
    var bodyWriter = await RenderPageAsync(RazorPage, context, invokeViewStarts: true);
    await RenderLayoutAsync(context, bodyWriter);
}

private async Task<ViewBufferTextWriter> RenderPageAsync(
    IRazorPage page,
    ViewContext context,
    bool invokeViewStarts)
{
    var writer = context.Writer as ViewBufferTextWriter;
    ...

    // The writer for the body is passed through the ViewContext, allowing things like HtmlHelpers
    // and ViewComponents to reference it.
    var oldWriter = context.Writer;
    var oldFilePath = context.ExecutingFilePath;

    context.Writer = writer;
    context.ExecutingFilePath = page.Path;

    try
    {
        if (invokeViewStarts)
        {
            // Execute view starts using the same context + writer as the page to render.
            await RenderViewStartsAsync(context);
        }

        await RenderPageCoreAsync(page, context);
        return writer;
    }
    finally
    {
        context.Writer = oldWriter;
        context.ExecutingFilePath = oldFilePath;
    }
}

private async Task RenderPageCoreAsync(IRazorPage page, ViewContext context)
{
    page.ViewContext = context;
    _pageActivator.Activate(page, context);

    _diagnosticSource.BeforeViewPage(page, context);

    try
    {
        await page.ExecuteAsync();
    }
    finally
    {
        _diagnosticSource.AfterViewPage(page, context);
    }
}

但当查找IRazorPage接口的实现。从RazorPageBase到RazorPage，再到RazorPage<TModel>，这些都只是抽象类，且都没有对ExecuteAsync方法有具体实现。

源码里找不到进一步的实现类，线索到这里断开了。

这时可以建立一个MVC的应用程序，编译后找到它的bin目录，会看到其中包含一个*.View.dll文件。

使用反编译软件，比如dotPeek，查看里面的内容，会找到一些由cshtml文件生成的类。

以其中Views_Home_Index为例，其实际上为RazorPage<TModel>的一个实现类。

它内部的ExecuteAsync方法正是生成页面内容的关键。

因为是VS模板自动生成的页面，上面的代码十分冗杂。为了更清晰地检查核心的代码，不妨减少下页面的复杂度。

把index.cshtml文件内容改成如下：

@{
    ViewData["Title"] = "Home Page";
    Layout = null;
}

<p>Hello World!</p>

再次编译后，可以看到ExecuteAsync方法的内容变成了下面的样子：

public virtual async Task ExecuteAsync()
{
  ((ViewDataDictionary) this.get_ViewData()).set_Item("Title", (object) "Home Page");
  ((RazorPageBase) this).set_Layout((string) null);
  ((RazorPageBase) this).BeginContext(65, 21, true);
  ((RazorPageBase) this).WriteLiteral("
<p>Hello World!</p>");
  ((RazorPageBase) this).EndContext();
}

不难看出，最终展现的页面内容便是通过RazorPageBase类的WriteLiteral方法生成的。