Custom fonts with AttributedString and Markdown

Beyond using different Markdown inline or block intents (such as emphasized, code, etc.), you can modify your AttributedString via code after decoding from Markdown to apply additional styling while taking into account the environment of the device (for example, light vs dark mode). To do this you can use the .replaceAttributes(_:with:) or .transformingAttributes(_:_:) functions (along with other mutation functions) to modify the attributes of the AttributedString. For example, to add extra styling to bold ranges, you could add a font attribute to ranges of the inline presentation intent attribute where the intent contains .bold. If you'd like even more control over which ranges to style while still supporting localization and changes from the environment, you can define your own custom attribute key that conforms to MarkdownDecodableAttributedStringKey and use this attribute in your Markdown text to indicate ranges that you should style via code before passing the AttributedString to a UI framework.

Ah -- NSPresentationIntent! That looks very useful to help me identify, say, the bold ranges in order to replace that intent with a custom font. I'll try this once FB9148035 is resolved. This looks much better than the attributes used when NSAttributedString is created via HTML.

It would be even better if I could supply rendering style hints to help the rendered know how to apply various intents, though, since right now I assume UIKit must be using a hardcoded mapping of intent attribute -> styling (font etc).

I made a sample project to show how I solved this:

https://github.com/frankrausch/AttributedStringStyledMarkdown