iOS gcd dispatch使用注意

class ViewController: UIViewController {
    var dbQueue:dispatch_queue_t = 0

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        dbQueue = dispatch_queue_create("com.Innocellence.sunflower", nil)
    
        //do some work in dbQueue from main thread
        dispatch_sync(dbQueue, { () -> Void in
  
            //do some work in main thread from dbQueue
            dispatch_sync(dispatch_get_main_queue(), { () -> Void in
                self.test()
            })
            
        })
        
        println("this is some other code")

    }
    
    func test(){
        
        println("this is test")
    
    
    }


}

override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        dbQueue = dispatch_queue_create("com.Innocellence.sunflower", nil)
    
        //do some work in dbQueue from main thread
        dispatch_async(dbQueue, { () -> Void in
  
            //do some work in main thread from dbQueue
            dispatch_sync(dispatch_get_main_queue(), { () -> Void in
                self.test()
            })
            
        })
        
        println("this is some other code")

    }

 override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        dbQueue = dispatch_queue_create("com.Innocellence.sunflower", nil)
    
        //do some work in dbQueue from main thread
        dispatch_sync(dbQueue, { () -> Void in
  
            //do some work in main thread from dbQueue
            dispatch_async(dispatch_get_main_queue(), { () -> Void in
                self.test()
            })
            
        })
        
        println("this is some other code")

    }

输出如下

this is some other code

this is test

第一个为什么无法继续执行了呢，首先代码阻塞了主线程的执行，要求在新线程执行完后才返回，但是新线程也被阻塞了，要求代码在主线程完成后才能返回，这样就形成了循环。这里线程这个词用的不准，其实紧紧是线程中执行的一段顺序代码。还需要仔细研究原因！