Make HTML rendering deferred (IHtmlContent)

[pranavkm]

Currently TemplateRenderer.Render returns a string which gets converted to HtmlString. This causes it to produce a new StringWriter buffer. We could have it return a HelperResult instead - it's consumed in the same way, but would avoid the extra buffer allocation.

[dougbu]

@pranavkm could you clarify the issue? Are you seeing poor performance numbers for a scenario involving templates read from disk?

Also

• TemplateRenderer only creates the StringWriter instance when dealing with templates read from disk; the usual Func<IHtmlHelper, string> doesn't need anything more.
• The Action<TextWriter> delegate would need the IView, ViewData and ViewContext in its closure; this might mess up the IView lifetime.
• HelperResult would need to be moved into Microsoft.AspNet.Mvc.Core.
• The Func<IHtmlHelper, string> default actions would need to be reworked significantly to operate on a TextWriter.

[pranavkm]

I did not profile this in any way, but noticed this when I was looking at https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Razor/RazorTextWriter.cs. The intent of introducing that type was to avoid creating large strings in memory.

• It would be nice if we could simply write to ViewContext.Writer without returning strings, but that would break the API surface.
• I guess some aspects of how the template gets rendered would change, but it might be worthwhile. That said, we certainly need profiling before making that claim.
• We could leave default actions largely untouched, changing

if (defaultActions.TryGetValue(viewName, out defaultAction))
{
    var result = defaultAction(MakeHtmlHelper(_viewContext, _viewData));
    return new HelperResult(writer => writer.Write(result));
}

[yishaigalatzer]

If a template ever renders a large portion of text > 80K there is definitely a major LOH issue that will cause pauses and lots of CPU spent in GC

[yishaigalatzer]

@NTaylorMullen please tie this to the TagHelper Content change when working with @sornaks

[sornaks]

Note to self: Move HelperResult to Razor.Runtime.