MainQueue vs MainThread

MainQueueVsMainThread.swift

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?


    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        let k = DispatchSpecificKey<String>()
        DispatchQueue.main.setSpecific(key: k, value: "MainQ")
        
        
        if Thread.isMainThread {
            print("MainThread")
        } else {
            print("Not MainThread")
        }
        if DispatchQueue.getSpecific(key: k) != nil {
            print("MainQ")
        } else {
            print("Not MainQ")
        }
        //prints MainThread, MainQ
        //we are executing on MainThread but not on MainQueue
        
        
        
        
        let newQ = DispatchQueue(label: "NewQ")
        let newQk = DispatchSpecificKey<String>()
        newQ.setSpecific(key: newQk, value: "NewQ")
        newQ.sync {
            if Thread.isMainThread {
                print("MainThread")
            } else {
                print("Not MainThread")
            }
            
            if DispatchQueue.getSpecific(key: k) != nil {
                print("MainQ")
            } else {
                print("Not MainQ")
            }
            
            if DispatchQueue.getSpecific(key: k) != nil {
                print("MainQ")
            } else {
                print("Not MainQ")
            }
            
            if DispatchQueue.getSpecific(key: newQk) != nil {
                print("NewQ")
            } else {
                print("Not NewQ")
            }
            
            //prints MainThread, Not MainQ, NewQ
        }
        
        
        DispatchQueue.global().async {
            if Thread.isMainThread {
                print("MainThread")
            } else {
                print("Not MainThread")
            }
            
            if DispatchQueue.getSpecific(key: k) != nil {
                print("MainQ")
            } else {
                print("Not MainQ")
            }
            //prints Not MainThread, Not MainQ
            
            DispatchQueue.main.sync {
                if Thread.isMainThread {
                    print("MainThread")
                } else {
                    print("Not MainThread")
                }
                
                if DispatchQueue.getSpecific(key: k) != nil {
                    print("MainQ")
                } else {
                    print("Not MainQ")
                }
                
               //prints MainThread, MainQ
            }
        }
        
        return true
    }
}




/*
    - http://blog.benjamin-encz.de/post/main-queue-vs-main-thread/
    - http://blog.krzyzanowskim.com/2016/06/03/queues-are-not-bound-to-any-specific-thread/
    - Threads are reused. Queues reuse threads in general. If so, the Main thread can be reused by another queue (for example by concurrent queue) when Main thread is idle.
    - Main Queue IS bound to Main Thread.
    - Main Thread IS NOT bound to Main Queue.

*/