Introducing NativeMarkKit, native Markdown rendering

I’d like to announce the release of one of my side projects: NativeMarkKit. NativeMarkKit is a Swift package implementing the rendering of NativeMark. What’s NativeMark? Well, it’s a flavor of Markdown designed to be rendered by native iOS, macOS, or tvOS apps. i.e. it compiles down to NSAttributedString (mostly) instead of HTML and CSS. It’s basically CommonMark, but without the raw HTML tag support.

I often get mockups from designers who want to style text like they would on web. So they mix in bold, italics, links and other things within the copy. While that can be done natively, it’s a hassle, requiring manually building up NSAttributedStrings and concat-ing them together. NativeMarkKit allows me to have one localized Markdown string, and let the framework figure out how to render that down into a NSAttributedString. As a bonus, NativeMarkKit supports Dynamic Type and Dark Mode. I can also style the Markdown using whatever app-branded NS/UIColors I have in the app.

UIViewRepresentable doesn't respect intrinsicContentSize invalidation

This is a quick write-up of an unexpected SwiftUI limitation I encountered recently. In my own searching, I couldn’t find it documented anywhere so I’m hoping this post will help someone in the future.

Problem summary

You can’t wrap a UIView in SwiftUI’s UIViewRepresentable if that UIView changes its intrinsicContentSize based in its frame width, because UIViewRepresentable will ignore invalidateIntrinsicContentSize(). As a result, the UIView will have its height clipped.

To make it a bit more concrete, imagine you have a UIView that draws multiline text that can line wrap. UILabel would be an example. If you were to adjust the label’s frame width, it would reflow the text and the intrinsic content height would change to reflect that.

During SwiftUI layout, what UIViewRepresentable does is ask the UIView for its intrinsic content size before it has set the UIView‘s frame. It then takes the minimum of the available size from the superview and the UIView‘s intrinsicContentSize, and sets that as the UIView‘s frame. This causes the UIView to reflow the text and invalidate its intrinsic content size. UIViewRepresentable ignores this, and as a result the UIView is clipped too short.

You can find a simplified version of this bug using UILabel on Github.

Investigation

I tried many suggestions from Stack Overflow, and override any size or layout related UIView method that I could. None of the suggestions worked, and almost none of the UIView methods were ever called during layout. I felt I was probably missing something obvious, so I filed a Developer Tech Support incident with Apple to get an official answer. My submission is summarized in the Github repo.

Results

Here’s the official response from Apple:

Hello Andy,

Thank you for contacting Apple Developer Technical Support (DTS). We have reviewed your request and have concluded that there is no supported way to achieve the desired functionality given the currently shipping system configurations.

If you would like for Apple to consider adding support for such features in the future, please submit an enhancement request via Feedback Assistant (https://feedbackassistant.apple.com). For more information on Feedback Assistant, please visit https://developer.apple.com/bug-reporting/.

While a Technical Support Incident (TSI) was initially debited from your Apple Developer Program account for this request, we have assigned a replacement incident back to your account.

Best Regards,

Developer Technical Support
Worldwide Developer Relations
Apple, Inc.

According to the response this is expected behavior, so any change would an enhancement request. I have since filed the enhancement request as feedback FB8499811.

Moment of Zen

I have the exact same view on macOS as an NSView wrapped up in an NSViewRepresentable. It works great. When the NSView invalidates its intrinsic size, NSViewRepresentable re-queries the intrinsic size and updates the frame appropriately.

I guess someone already filed that enhancement request.