Snippet showing how to configure a CFRunLoopObserver.

RunLoopObserving.swift

//
//  RunLoopObserving.swift
//

import Foundation

func name(for thread: Thread) -> String {
    if let name = Thread.current.name, !name.isEmpty {
        return name
    } else {
        return Thread.isMainThread ? "main" : "<unknown>"
    }
}

func addTimer(to runLoop: RunLoop) {
    let timer = Timer(timeInterval: 5, repeats: true) { _ in
        print("[Thread: \(name(for: .current)), Timer] Timer fired")
    }

    runLoop.add(timer, forMode: .default)
}

func observe(_ runLoop: RunLoop) {
    var runLoopContext = CFRunLoopObserverContext(
        version: 0,
        info: nil,
        retain: nil,
        release: nil,
        copyDescription: nil
    )

    guard let runLoopObserver = CFRunLoopObserverCreate(
        kCFAllocatorDefault,
        CFRunLoopActivity.allActivities.rawValue,
        true,
        0,
        { runLoopObserver, runLoopActivity, info in
            switch runLoopActivity {
            case CFRunLoopActivity.entry:
                print("[Thread: \(name(for: .current)), CFRunLoopObserver] Run Loop Event: entry")
            case CFRunLoopActivity.beforeTimers:
                print("[Thread: \(name(for: .current)), CFRunLoopObserver] Run Loop Event: beforeTimers")
            case CFRunLoopActivity.beforeSources:
                print("[Thread: \(name(for: .current)), CFRunLoopObserver] Run Loop Event: beforeSources")
            case CFRunLoopActivity.beforeWaiting:
                print("[Thread: \(name(for: .current)), CFRunLoopObserver] Run Loop Event: beforeWaiting")
            case CFRunLoopActivity.afterWaiting:
                print("[Thread: \(name(for: .current)), CFRunLoopObserver] Run Loop Event: afterWaiting")
            case CFRunLoopActivity.exit:
                print("[Thread: \(name(for: .current)), CFRunLoopObserver] Run Loop Event: exit")
            default:
                assertionFailure("Unknown Run Loop Event")
            }
        },
        &runLoopContext
    ) else {
        return assertionFailure("Expected Observer creation to succeed")
    }

    CFRunLoopAddObserver(
        runLoop.getCFRunLoop(),
        runLoopObserver,
        CFRunLoopMode.defaultMode
    )
}

observe(.current)

addTimer(to: .current)

let thread = Thread {
    Thread.current.name = "worker"

    print("Started Thread: \(name(for: .current))")

    observe(.current)

    addTimer(to: .current)

    RunLoop.current.run()

    print("Exiting Thread: \(name(for: .current))") // Never called
}
thread.start()

RunLoop.current.run()

print("Exiting Thread: \(name(for: .current))") // Never called