Swift CFRunLoop
Threetypesofobjectscanbemonitoredbyarunloop:sources,timers,andobservers.Toreceivecallbackswhentheseobjectsneedprocessing,youmustfirstplacetheseobjectsintoarunloopwithCFRunLoopAddSource,CFRunLoopAddTimer,orCFRunLoopAddObserver.Youcanlaterremoveanobjectfrom