My variant of https://gist.github.com/soffes/10096159b309ca14ddd8dab035f33293

RunLoopSource.swift

final class RunLoopSource {
    private static let eventHandler: @convention(c) (UnsafeMutableRawPointer?) -> Void = { context in
        unsafeBitCast(context, to: RunLoopSource.self).eventHandler()
    }

    private let eventHandler: () -> Void
    private let mode: CFRunLoopMode
    private let runLoop: CFRunLoop
    private var source: CFRunLoopSource!

    init(runLoop: RunLoop, mode: RunLoop.Mode, eventHandler: @escaping () -> Void) {
        self.eventHandler = eventHandler
        self.mode = CFRunLoopMode(mode.rawValue as CFString)
        self.runLoop = runLoop.getCFRunLoop()

        var context = CFRunLoopSourceContext()
        context.info = Unmanaged.passUnretained(self).toOpaque()
        context.perform = RunLoopSource.eventHandler
        self.source = CFRunLoopSourceCreate(nil, 0, &context)!
        
        CFRunLoopAddSource(runLoop, source, mode)
    }

    deinit {
        CFRunLoopRemoveSource(runLoop, source, mode)
    }

    func signal() {
        CFRunLoopSourceSignal(source)
        CFRunLoopWakeUp(runLoop)
    }
}