Pixata.AspNetCore.Pdf.Telerik 1.0.2

Pixata.AspNetCore.Pdf.Telerik

A PDF converter for the DocumentTemplateHelper in Pixata.AspNetCore, using Telerik Document Processing. It turns the HTML rendered from your Blazor template into a PDF, without a browser or a native PDF engine.

A Nuget package is available for this project.

Version 1.0.2 fixes the image at the top of this readme. No code changes.

Version 1.0.1 references a patched version of System.Security.Cryptography.Pkcs, as the one pulled in transitively had known vulnerabilities (GHSA-846f-f764-76mx, GHSA-2fmv-rv2c-59x7). No code changes.

Setup

This package is a converter for the DocumentTemplateHelper in Pixata.AspNetCore, so you register both. In Program.cs...

builder.Services.AddPixataAspNetCore<Program>();
builder.Services.AddPixataTelerikPdf();

The first line registers DocumentTemplateHelper (along with the HtmlRenderer and IHttpContextAccessor it needs), which renders a Blazor component to HTML, and then hands the HTML to the converter registered by the second line to turn it into a PDF.

The type parameter can be any type in your app. It's only used to find your FluentValidation validators for Pixata.AspNetCore's validation endpoint filter, which is registered at the same time. Program is always there, so it's the easiest choice. If you don't use the validation filter, the non-generic builder.Services.AddPixataAspNetCore() registers everything else. See registering services for how to opt out of the parts you don't want.

The base URL

Templates are given a BaseUrl parameter for linking to images and style sheets on your site, and the converter only loads them from that site (see images and style sheets). You don't need to set it, as the helper takes it from the scheme and host of the current request, so it is right in development, test and live without any changes. This works when you create PDFs from an endpoint, a controller, a Razor page, or a statically rendered or prerendering Blazor component.

There is no current request inside an interactive Blazor Server component (after the first render), or in a hosted or background service. If you create PDFs from either of those, pick up the base URL from the first request the app gets, by adding this to Program.cs after UseHttpsRedirection() (and after UseForwardedHeaders(), if the app is behind a reverse proxy)...

PixataAspNetCoreOptions pixataOptions = app.Services.GetRequiredService<PixataAspNetCoreOptions>();
app.Use(async (HttpContext context, Func<Task> next) => {
  if (pixataOptions.DocumentBaseUrl == "") {
    pixataOptions.DocumentBaseUrl = $"{context.Request.Scheme}://{context.Request.Host}";
  }
  await next();
});

An interactive Blazor Server component always follows a request (the one that loaded its page), so this always works there. A background service that creates a PDF before the site has had any requests gets an InvalidOperationException saying that the base URL can't be worked out.

As this trusts the Host header of the first request, set AllowedHosts in the live site's appsettings.json (eg "AllowedHosts": "www.example.com"), so that a request with a made-up host is rejected rather than becoming the base URL. Your host may already filter requests by host name for you.

Then use DocumentTemplateHelper.CreatePdfFromTemplate as described in generating documents and PDFs.

Telerik Document Processing needs a Telerik licence, in the same way as Telerik UI for Blazor. If you already use the Telerik components, your licence covers it.

Writing templates for this converter

Telerik's HTML import is not a browser. It handles text formatting, tables, lists, images and simple CSS well, but it doesn't support flexbox, grid or positioning. Lay templates out with tables, and keep CSS simple.

Images and style sheets

Images and linked style sheets are loaded over HTTP, but only from the site in PdfOptions.BaseUrl (which DocumentTemplateHelper sets for you from DocumentBaseUrl or the current request). Relative URLs are resolved against it. Anything else (another site, an internal address) is replaced with a blank image. This means that user-supplied content in a template, such as a client's notes on an invoice, can't make your server fetch arbitrary URLs.

Images are decoded with Telerik's ImageUtils, which uses SkiaSharp. The Linux native library is included, so this works on Linux hosts (including Alpine) as well as Windows and macOS. AddPixataTelerikPdf() sets FixedExtensibilityManager.ImagePropertiesResolver and JpegImageConverter, unless your app has already set them.