Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Capturing HTML output with a controller action filter

I've got the following filter in place on an action to capture the HTML output, convert it to a string, do some operations to modify the string, and return a ContentResult with the new string. Unfortunately, I keep ending up with an empty string.

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;
        }
    }

I've pinned down that StreamReader(stream).ReadToEnd() returns an empty string, but I can't figure out why.

Any ideas how to fix this?

EDIT: I've changed the OnActionExecuted to OnResultExecuted, and now it is called after the View has been generated, but the stream is still empty!

like image 363
yoozer8 Avatar asked Aug 17 '11 14:08

yoozer8


2 Answers

I solved this by hijacking the HttpWriter, and having it write into a StringBuilder rather than the response, and then doing whatever needs to be done to/with the response before writing it to the output.

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);
    }
}

Above code using the HttpContext to avoid threading errors - see jaminto's comment

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);
    }
}
like image 190
yoozer8 Avatar answered Nov 09 '22 16:11

yoozer8


Try rewinding the stream to the beginning by setting Position = 0; before you read it.

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;
}
like image 22
Jakub Konecki Avatar answered Nov 09 '22 17:11

Jakub Konecki