Binding request: CTFontDescriptor.MatchFontDescriptors

[tipa]

Would it be possible to bind/fix the CoreText.CTFontDescriptor.MatchFontDescriptors method?
https://github.com/xamarin/xamarin-macios/blob/6f45caa84db8e45df6866d564603930f1cac9d64/src/CoreText/CTFontDescriptor.cs#L756-L766

My goal is to download fonts dynamically

• https://developer.apple.com/library/archive/samplecode/DownloadFont/Listings/DownloadFont_ViewController_m.html
• https://medium.com/@chunkyguy/downloading-fonts-on-ios-ad97977308b5

Steps to Reproduce

var success = CoreText.CTFontDescriptor.MatchFontDescriptors(new CoreText.CTFontDescriptor[0], null, null);

Expected Behavior

no crash, API works

Actual Behavior

NotImplementedException

Environment

.NET 8 RC2.1

[rolfbjarne]

I've created a PR implementing this API.

It might take a while until we release this though, so I believe you can use something like this in your project:

using System.Runtime.InteropServices;
using ObjCRuntime;
namespace CoreText {
	public static class CTFontDescriptorHelper {
		[DllImport (Constants.CoreTextLibrary)]
		static unsafe extern byte CTFontDescriptorMatchFontDescriptorsWithProgressHandler (IntPtr descriptors, IntPtr mandatoryAttributes, BlockLiteral* progressBlock);

		public delegate bool CTFontDescriptorProgressHandler (CTFontDescriptorMatchingState state, NSDictionary progress);

		[UnmanagedCallersOnly]
		static byte MatchFontDescriptorsHandler (IntPtr block, CTFontDescriptorMatchingState state, IntPtr progress)
		{
			var del = BlockLiteral.GetTarget<CTFontDescriptorProgressHandler> (block);
			if (del is not null) {
				var progressDictionary = Runtime.GetNSObject<NSDictionary> (progress)!;
				var rv = del (state, progressDictionary);
				return rv ? (byte) 1 : (byte) 0;
			}
			return 0;
		}

		public static bool MatchFontDescriptors (CTFontDescriptor [] descriptors, NSSet? mandatoryAttributes, CTFontDescriptorProgressHandler progressHandler)
		{
			if (descriptors is null)
				ObjCRuntime.ThrowHelper.ThrowArgumentNullException (nameof (descriptors));

			if (progressHandler is null)
				ObjCRuntime.ThrowHelper.ThrowArgumentNullException (nameof (progressHandler));

			unsafe {
				delegate* unmanaged<IntPtr, CTFontDescriptorMatchingState, IntPtr, byte> trampoline = &MatchFontDescriptorsHandler;
				using var block = new BlockLiteral (trampoline, progressHandler, typeof (CTFontDescriptorHelper), nameof (MatchFontDescriptorsHandler));
				using var descriptorsArray = NSArray.FromNSObjects (descriptors);
				var rv = CTFontDescriptorMatchFontDescriptorsWithProgressHandler (descriptorsArray.GetHandle (), mandatoryAttributes.GetHandle (), &block);
				return rv != 0;
			}
		}
	}
}

The API in the PR is a bit better because the progress parameter in the callback is strongly typed, but it should be possible to make this work too, there will just be a bit more manual code to get stuff out of the progress NSDictionary.

[tipa]

Wow - thanks so much for the quick response and the code!
Just had to replace typeof (CTFontDescriptor) with typeof(CTFontDescriptorHelper).
You're awesome!

[tipa]

@rolfbjarne Is it possible that this workaround is not compatible with NativeAOT? Is there any way to use it with NativeAOT?

This API is not supported when using NativeAOT. (System.Diagnostics.UnreachableException)
   at ObjCRuntime.BlockLiteral.FindTrampoline(Type, String) + 0x10
   at MyApp.CTFontDescriptorHelper.MatchFontDescriptors(CTFontDescriptor[], NSSet, CTFontDescriptorHelper.CTFontDescriptorProgressHandler) + 0x30
   ...
   at MyApp!<BaseAddress>+0x29e3f4

[rolfbjarne]

@tipa try adding the [BindingImpl (BindingImplOptions.Optimizable)] attribute on the MatchFontDescriptors method to see if that works.

[tipa]

That worked! Thanks so much

[tipa]

Well, it works most of the times. But occasionally, the crash is still happening

[rolfbjarne]

You occasionally get This API is not supported when using NativeAOT.? That's not something that should be randomly happening... do you have a stack trace?