Assuming this is called from the main thread, there isn't really any reason to call CFRunLoopRun, because the default run loop should already be running.

The way you're using NSURLConnection will not block the calling thread. It may spawn additional threads internally, but you don't really have to care about that. initWithRequest:delegate: will return pretty much immediately and your delegate methods will be called later (when a response is received, data is loaded, etc.).

You're doing well. There's not problem to use CoreFoundation when you cannot achieve your goal with Foundation. As CoreFoundation is C, it's easier to mess up with memory management but there is no intrinsic danger in using CFRunLoop rather than NSRunLoop (sometimes it may even be safer: CFRunLoop API are thread-safe whereas NSRunLoop isn't).

If you want to stop your NSRunLoop, you can run it using runMode:beforeDate:. runMode:beforeDate: returns as soon as an input source is processed so you don't need to wait until the timeout date is reached.

 NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
 NSDate *date = [NSDate distantFuture];
 while ( !runLoopIsStopped && [runLoop runMode:NSDefaultRunLoopMode beforeDate:date] );

Then, to stop the run loop, you just need to set runLoopIsStopped to YES.

For a command line interface use this pattern and add a completion handler to your AsyncNetworkingStuff (thanks to Rob for code improvement):

print("start")

let runLoop = CFRunLoopGetCurrent()
startAsyncNetworkingStuff() { result in 
   CFRunLoopStop(runLoop)
}

CFRunLoopRun()
print("end")
exit(EXIT_SUCCESS)

Please don't use ugly while loops.

Update:

In Swift 5.5+ with async/await it has become much more comfortable. There's no need anymore to maintain the run loop.

Rename the file main.swift as something else and use the @main attribute like in a normal application.

@main
struct CLI {

    static func main() async throws {
        let result = await startAsyncNetworkingStuff()
        // do something with result 
    }
}

The name of the struct is arbitrary, the static function main is mandatory and is the entry point.