使用控制器操作过滤器捕获 HTML 输出

Question

我在一个操作上设置了以下过滤器来捕获 HTML 输出，将其转换为字符串，执行一些操作来修改字符串，并返回带有新字符串的 ContentResult。不幸的是，我总是以空字符串结束。

private class UpdateFilter : ActionFilterAttribute
    {
        private Stream stream;

        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            stream = filterContext.HttpContext.Response.Filter;
            stream = new MemoryStream();
            filterContext.HttpContext.Response.Filter = stream;
        }

        public override void OnResultExecuted(ResultExecutedContext filterContext)
        {
            StreamReader responsereader = new StreamReader(filterContext.HttpContext.Response.Filter);  //empty stream? why?
            responsereader.BaseStream.Position = 0;
            string response = responsereader.ReadToEnd();
            ContentResult contres = new ContentResult();
            contres.Content = response;
            filterContext.Result = contres;
        }
    }

我已经确定 StreamReader(stream).ReadToEnd() 返回一个空字符串，但我不明白为什么。

有什么想法如何解决这个问题吗？

编辑：我已将 OnActionExecuted 更改为 OnResultExecuted，现在在生成视图后调用它，但流仍然是空的！

Answer 1

我通过劫持 HttpWriter 并将其写入 StringBuilder 而不是响应中来解决这个问题，然后在将其写入输出之前对响应执行任何需要执行的操作。

private class UpdateFilter : ActionFilterAttribute
{
    private HtmlTextWriter tw;
    private StringWriter sw;
    private StringBuilder sb;
    private HttpWriter output;

    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        sb = new StringBuilder();
        sw = new StringWriter(sb);
        tw = new HtmlTextWriter(sw);
        output = (HttpWriter)filterContext.RequestContext.HttpContext.Response.Output;
        filterContext.RequestContext.HttpContext.Response.Output = tw;
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        string response = sb.ToString();
        //response processing
        output.Write(response);
    }
}

上面的代码使用 HttpContext 来避免线程错误 - 请参阅 jaminto 的评论

private class RenderFilter : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        StringBuilder sb = new StringBuilder();
        StringWriter sw = new StringWriter(sb);
        HtmlTextWriter tw = new HtmlTextWriter(sw);
        HttpWriter output = (HttpWriter)filterContext.RequestContext.HttpContext.Response.Output;
        filterContext.HttpContext.Items["sb"] = sb;
        filterContext.HttpContext.Items["output"] = output;
        filterContext.RequestContext.HttpContext.Response.Output = tw;
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        string response = filterContext.HttpContext.Items["sb"].ToString();
        //response processing
        ((HttpWriter)filterContext.HttpContext.Items["output"]).Write(response);
    }
}

Answer 2

在阅读流之前，尝试通过设置 Position = 0; 将流倒回到开头。

public override void OnActionExecuted(ActionExecutedContext filterContext)
{
    stream.Position = 0;
    string response = new StreamReader(stream).ReadToEnd();
    ContentResult contres = new ContentResult();
    contres.Content = response;
    filterContext.Result = contres;
}

Answer 3

我想我已经开发了一个很好的方法来做到这一点。

• 将响应过滤器替换为自定义过滤器
• 该过滤器接受一个抽象方法的委托，该方法接受一个流
• 这是委托，因此抽象方法在流关闭时被调用，即当所有 HTML 可用时
• 覆盖 OnClose 方法并根据需要播放流。

---

public abstract class ReadOnlyActionFilterAttribute : ActionFilterAttribute
{
    private delegate void ReadOnlyOnClose(Stream stream);

    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        filterContext.HttpContext.Response.Filter = new OnCloseFilter(
            filterContext.HttpContext.Response.Filter, 
            this.OnClose);
        base.OnActionExecuting(filterContext);
    }

    protected abstract void OnClose(Stream stream);

    private class OnCloseFilter : MemoryStream
    {
        private readonly Stream stream;

        private readonly ReadOnlyOnClose onClose;

        public OnCloseFilter(Stream stream, ReadOnlyOnClose onClose)
        {
            this.stream = stream;
            this.onClose = onClose;
        }

        public override void Close()
        {
            this.Position = 0;
            this.onClose(this);
            this.Position = 0;
            this.CopyTo(this.stream);
            base.Close();
        }
    }
}

然后，您可以从此派生到另一个属性来访问流并获取 HTML：

public class MyAttribute : ReadOnlyActionFilterAttribute
{
    protected override void OnClose(Stream stream)
    {
        var html = new HtmlDocument();
        html.Load(stream);
        // play with html
    }
}

Answer 4

您可以验证 OnActionExectulated 方法中的流不为 NULL 吗？我不确定流变量的状态是否通过该过程存储。

为什么不尝试将流从filterContext中取出：

public override void OnActionExecuted(ActionExecutedContext filterContext)
{
    var stream = filterContext.HttpContext.Response.Filter;
    string response = new StreamReader(stream).ReadToEnd();
    ContentResult contres = new ContentResult();
    contres.Content = response;
    filterContext.Result = contres;
}