Components of XPC service.

main.swift

import Foundation

let delegate = MyServiceDelegate()
let listener = NSXPCListener.service()
listener.delegate = delegate
listener.resume()

MyService.swift

import Foundation

class MyService: NSObject, MyServiceProtocol {
    func upperCaseString(_ string: String, withReply reply: @escaping (String) -> Void) {
        let response = string.uppercased()
        reply(response)
    }
}

MyServiceDelegate.swift

import Foundation

class MyServiceDelegate: NSObject, NSXPCListenerDelegate {
    func listener(_ listener: NSXPCListener, shouldAcceptNewConnection newConnection: NSXPCConnection) -> Bool {
        let exportedObject = MyService()
        newConnection.exportedInterface = NSXPCInterface(with: MyServiceProtocol.self)
        newConnection.exportedObject = exportedObject
        newConnection.resume()
        return true
    }
}

MyServiceProtocol.swift

import Foundation

@objc public protocol MyServiceProtocol {
    func upperCaseString(_ string: String, withReply reply: @escaping (String) -> Void)
}

usage.swift

import MyService

...

let connection = NSXPCConnection(serviceName: "com.matthewminer.MyService")
connection.remoteObjectInterface = NSXPCInterface(with: MyServiceProtocol.self)
connection.resume()

let service = connection.remoteObjectProxyWithErrorHandler { error in
    print("Received error:", error)
} as? MyServiceProtocol

service?.upperCaseString("Hello XPC") { response in
    print("Response from XPC service:", response)
}

I also just ran into this error:

Domain=NSCocoaErrorDomain Code=4099 "The connection to service on pid 0 named <service-name> was invalidated."

It turns out I just misspelled the service name when initializing the NSXPCConnection. Hope this saves someone a few hours of debugging lol.