Cannot load default library in Metal using Swift - swift

Question

For my project (being compiled as a framework) I have a file ops.metal:
kernel void add(device float *lhs [[buffer(0)]],
device float *rhs [[buffer(1)]],
device float *result [[buffer(2)]],
uint id [[ thread_position_in_grid ]])
{
result[id] = lhs[id] + rhs[id];
}
and the following Swift code:
#available(OSX 10.11, *)
public class MTLContext {
var device: MTLDevice!
var commandQueue:MTLCommandQueue!
var library:MTLLibrary!
var commandBuffer:MTLCommandBuffer
var commandEncoder:MTLComputeCommandEncoder
init() {
if let defaultDevice = MTLCreateSystemDefaultDevice() {
device = defaultDevice
print("device created")
} else {
print("Metal is not supported")
}
commandQueue = device.makeCommandQueue()
library = device.newDefaultLibrary()
if let defaultLibrary = device.newDefaultLibrary() {
library = defaultLibrary
} else {
print("could not load default library")
}
commandBuffer = commandQueue.makeCommandBuffer()
commandEncoder = commandBuffer.makeComputeCommandEncoder()
}
deinit {
commandEncoder.endEncoding()
}
}
When I try to create an instance of MTLContext in a unit test, the device is created, but the default library cannot be created ("could not load default library"). I've checked that the compiled framework has a default.metallib in Resources (which is the most common reason given for newDefaultLibrary).
Unfortunately I haven't been able to find any working examples that are creating compute kernels in a Metal shader file (there are a few examples using the performance shaders, but they don't need to make kernels in the shader file).
Any suggestions would be greatly appreciated!

Answer 1

newDefaultLibrary() loads from the main bundle of the currently running application. It doesn't search any embedded frameworks or other locations for libraries.
If you want to use a metallib that was compiled into an embedded framework, the easiest thing to do is to get a reference to its containing Bundle and ask for the default library of that bundle instead:
let frameworkBundle = Bundle(for: SomeClassFromMyShaderFramework.self)
guard let defaultLibrary = try? device.makeDefaultLibrary(bundle: frameworkBundle) else {
fatalError("Could not load default library from specified bundle")
}
This does require that you have at least one publicly-visible class in the framework containing your shaders, but that can be as simple as declaring an empty class strictly for the purpose of doing the bundle look-up:
public class SomeClassFromMyShaderFramework {}