Nested Runloops

runloops.swift

import Foundation

func asynchronousFunction(with completion: @escaping () -> Void) {
    DispatchQueue.global().async {
        Thread.sleep(forTimeInterval: 1)
        DispatchQueue.main.async {
            print("running callback on main thread")
            completion()
        }
    }
}

let runloop = CFRunLoopGetCurrent()

asynchronousFunction {
    // Stop the innermost invocation of `runloop`.
    // Supposedly can also be called from threads other than the host
    // thread of `runloop`.
    CFRunLoopStop(runloop)
}

print("Starting nested runloop")
CFRunLoopRun()

print("Nested runloop stopped")