First, a quick description of the end-goal:
I'm building a cross-platform, .NET Core-based, printing app. This app will be able to print all sorts of file types with custom page settings, such as headers, footers, and margins. A key feature is it supports multiple-pages-up (e.g. a landscape sheet of paper with two portrait pages rendered side by side...called "2up").
Printing HTML is important not just because of printing HTML, but I want to use all the great HTML-based syntax-highlighting out there for source code (e.g. www.prismjs.com).
The app is basically done but for one major problem: I can't get the HTML to render well enough. So far I've implemented source code printing three ways:
1) As plain text with my own line-numbering and line-wrap engine. This works wonderfully for everything I can throw at it, but it does not support syntax highlighting.
2) Using Html-Renderer (https://github.com/ArthurHub/HTML-Renderer/issues) an OSS .NET-based Html Renderer. This implementation is the weakest because Html-Renderer's CSS support is really weak. It can't handle hardly anything prismjs or highlightjs' generate.
3) Usinglitehtml' (www.litehtml.com) via LiteHtmlSharp. This was very promising and I almost have it working with some major hacks, but litehtml also does not support key, modern, HTML/CSS features.
Neither Html-Renderer or litehtml support the CSS page-break-before feature that, when combined with media print would let me ensure lines are not split between pages.
What I really want to use is the Chromium rendering engine. litehtml provides a fantastic API for this sort of problem: It calls me whenever it needs to render, and I draw (text, table borders, images, etc...) using GDI+. My dream is to find something in Chromium (CEF, Puppeteer, ???) that provides a similar API.
Or, an alternative, an API that will let me pass in a GDI+ Graphics (or HDC) and the renderer will render to that surface.
With Html-Renderer I measure calculate # pages like this.
SizeF size = HtmlRender.MeasureGdiPlus(g, html, containingSheet.GetPageWidth());
int numPages = (int)(size.Height / containingSheet.GetPageHeight());
My page rendering code (e.g. OnPaint) looks like this:
SizeF size = new SizeF(containingSheet.GetPageWidth(), containingSheet.GetPageHeight());
HtmlRender.RenderGdiPlus(g, html, PointF(0, 0), size );
With htmllite the OnPaint code looks like this:
// Set the clip such that any extraLines are clipped off bottom
g.SetClip(new Rectangle(0, 0, (int)Math.Round(PageSize.Width), (int)Math.Round(PageSize.Height - remainingPartialLineHeight)));
LiteHtmlSize size = new LiteHtmlSize(Math.Round(PageSize.Width), Math.Ceiling(PageSize.Height));
litehtml.Document.Draw((int)-0, (int)-yPos, new position {
x = 0,
y = 0,
width = (int)size.Width,
height = (int)size.Height
});
And in this case the call to litehtml.Document.Draw causes a bunch of callbacks into my app that I process using the same Graphics the OnPaint is called with.
Most discussions of CEF and Chromium point to ScreenshotAsync etc... which will not do because I need to be rendering to a PRINTERS HDC (or GDI+) and blitting bitmaps will loose quality.
I have poured over the Chromium source and I cannot find the obvious way to say to CEF/Chromium "render page 1 (defined as Height/Width) to this GDI+ Graphics object" then "render page 2..." etc... The printing support (and how pdfium is integrated come close!).
Chromium issue 311308 indicates I'm hosed until this work gets picked up again.
Note: I have full access to nodejs w/in my app. I have built a dotnet/nodejs bridge, which is how I convert the raw text file of a source code file to richly formatted, line-numbered, syntax-highlighted html via prismjs. This means I could easily use puppeteer/Headless Chrome if I could just figure out the right APIs.
Does anyone have a suggestion that might help? I'm willing to contribute to Chromium if it's not major heart surgery.
