Metal file as part of an iOS framework

There are many ways to provide Metal shaders with a static library, all with different tradeoffs. I'll try to enumerate them here.

1) Transform your .metal files into static strings that are baked into your static library.

This is probably the worst option. The idea is that you preprocess your Metal shader code into strings which are included as string literals in your static library. You would then use the newLibraryWithSource:options:error: API (or its asynchronous sibling) to turn the source into an MTLLibrary and retrieve the functions. This requires you to devise a process for doing the .metal-to-string conversion, and you lose the benefit of shader pre-compilation, making the resulting application slower.

2) Ship .metal files alongside your static library and require library users to add them to their app target

All things considered, this is a decent option, though it places more of a burden on your users and exposes your Metal shader source (if that's a concern). Code in your static library can use the "default library" (newDefaultLibrary), since the code will be compiled automatically by Xcode into the app's default.metallib, which is embedded in the app bundle as a resource.

3) Ship a .metallib file alongside your static library

This is a good middle ground between ease-of-use, performance, and security (since it doesn't expose your shader source, only its IR). Basically, you can create a "Metal Library" target in your project, into which you put your shader code. This will produce a .metallib file, which you can ship along with your static library and have your user embed as a resource in their app target. Your static library can load the .metallib at runtime with the newLibraryWithData:error: or newLibraryWithURL:error: API. Since your shaders will be pre-compiled, creating libraries will be faster, and you'll keep the benefit of compile-time diagnostics.

As someone looking to include metal shader functions in a SceneKit / ARKit related framework, the available answers led me in the wrong direction. There is a much simpler solution that uses makeDefaultLibrary(bundle: Bundle) (iOS 10+) to access the functions included in a framework's .metal dependencies. Adding here for people in a similar position.

TL;DR, Access a Framework's MTLLibrary like this:

        //Get the framework bundle by using `Bundle(for: type(of: self))` from inside any framework class.
        //Then use the bundle to define an MTLLibrary.
        let frameworkBundle = Bundle(for: type(of: self))
        let device = MTLCreateSystemDefaultDevice()
        do {
            let bundleLib = try device?.makeDefaultLibrary(bundle: frameworkBundle)
            print(bundleLib.functionNames) //we can access our framework's metal functions! No build tricks/workarounds.
        } catch {
            print("Couldn't locate default library for bundle: \(frameworkBundle)")
            print( error )
        }

Xcode creates a default library of shader functions at build time by compiling .metal dependencies. This is true of both framework targets and app targets, so the real question is, how do I access my framework’s default library?

It’s possible to access a framework’s default library using the using the makeDefaultLibrary(bundle: Bundle) method on MTLDevice. The sample code above shows more detail.

For Scenekit/ARKit with SCNProgram

The bundle library can be set as an SCNProgram’s library property, and then fragment and shader functions can be defined just as if the .metal file was included in the main project:

        //The SCNProgram that will use our framework's metal functions
        var program = SCNProgram()

        //Use the framework's bundle to define an MTLLibrary.
        let frameworkBundle = Bundle(for: type(of: self))
        let device = MTLCreateSystemDefaultDevice()
        do {
            let bundleLib = try device?.makeDefaultLibrary(bundle: frameworkBundle)

            //set the SCNProgram's library, and define functions as usual
            program.library = bundleLib
            program.fragmentFunctionName = "yourCustomFrameworkFragmentFunction"
            program.vertexFunctionName = "yourCustomFrameworkVertexFunction"
        } catch {
            print("Couldn't locate default library for bundle: \(frameworkBundle)")
            print( error )
        }