Removed the Mercury, since it cannot be guaranteed to be finished by then. Release of Mac OS Catalina and Xcode 11 will happen during end of September, not sure how far this whole task can be done.

As per the discussion with Pasin, we will be adding a flag in the build script to include option to build XCFramework through `build_framework` script. This part of the task we can try to finish in Mercury.

But on the release builds, builds which are displayed on the downloads page, and sharing the users with XCFramework, which might take more time.

lightandshadow has managed to build one and has shared the directions.

Ah, the help for xcodebuild has the answers:

OVERVIEW:
Utility for packaging multiple build configurations of a given library or framework into a single xcframework.

USAGE:
xcodebuild -create-xcframework -framework <path> [-framework <path>...] -output <path>
xcodebuild -create-xcframework -library <path> [-headers <path>] [-library <path> [-headers <path>]...] -output <path>

OPTIONS:
-framework <path> Adds a framework from the given <path>.
-library <path> Adds a static or dynamic library from the given <path>.
-headers <path> Adds the headers from the given <path>. Only applicable with -library.
-output <path> The <path> to write the xcframework to.
-help Show this help content.

So the steps are

1. Build archive framework for one platform

2. Repeat step 1 for each platform (device, simulator, macOS)

3. Run xcodebuild -create-xcframework appending -framework <path> for each of the frameworks produced by step 1

Jim found the info that, to build for Catalyst (UIKit-on-macOS) you need the compiler flag -target x86_64-apple-ios${MIN_IOS_VERSION}-macabi .

Sigh — The video doesn't really describe how to build an xcframework, and I can't find any Apple documentation, just bits of info scattered around blogs and StackOverflow, like this:

https://stackoverflow.com/questions/56609737/xcodebuild-create-framework-error-unable-to-read-the-file/56702810#56702810

Here are step by step instructions, I think you might be missing step 2:

1) Set Build Library for Distribution in the build settings for the target framework to YES

2) Again in the build settings, set Skip Install to NO otherwise the framework won't show up in the Archive output folder.

3) Archive from the Xcode Product menu after selecting your Generic iOS Device the output will appear in the Organizer. Control-Click on the Archive. Select Show in Finder Drag that to the terminal to get the path to the archive and append the path (yellow part is the dragged path, gray is navigated in subfolders). In this case it looks like this, I used the ~ to avoid showing entire path.

~/Library/Developer/Xcode/Archives/2019-06-22/Output\ 6-22-19,\ 11.50\ AM.xcarchive/Products/Library/Frameworks/MyFramework.framework
4) Then create the XCFramework by inserting the command in front of the above path:

xcodebuild -create-xcframework -output Output.xcframework -framework ~/Library/Developer/Xcode/Archives/2019-06-22/Output\ 6-22-19,\ 11.50\ AM.xcarchive/Products/Library/Frameworks/MyFramework.framework
5) You then should see the output:

xcframework successfully written out to: ~/Project/Output.xcframework
I expect that someday soon Xcode will add a the ability to directly create the XCFramework without the command line.

However, this doesn't say how to combine all the platform binaries (device, simulator, macOS) in one framework...