Registers a Cocoa application to handle a custom URL scheme (e.g. someapp://action).

URLSchemeHandler.swift

import Foundation

class URLSchemeHandler {

    private let urlHandler: (URL) -> Void

    init(urlHandler: @escaping (URL) -> Void) {
        self.urlHandler = urlHandler

        NSAppleEventManager.shared().setEventHandler(
            self,
            andSelector: #selector(handleURLEvent(_:withReplyEvent:)),
            forEventClass: AEEventClass(kInternetEventClass),
            andEventID: AEEventID(kAEGetURL)
        )
    }

    /// Handles an incoming custom URL.
    @objc
    private func handleURLEvent(_ event: NSAppleEventDescriptor, withReplyEvent: NSAppleEventDescriptor) {
        let keyword = AEKeyword(keyDirectObject)

        guard
            let urlString = event.paramDescriptor(forKeyword: keyword)?.stringValue,
            let url = URL(string: urlString)
        else {
            print("Received invalid custom URL.")
            return
        }

        print("Received URL from custom scheme: \(url)")
        urlHandler(url)
    }
}