iOS7 的main thread只是UI預設的執行緒，不是code的

為了要檢查get main queue的順序與值的對應關係，我寫了下列程式碼

__block int i = 1;

    NSLog(@"1. normal 1  i=%i",i);

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

        NSLog(@"2. after two seconds  i=%i",i);

        i = 2;

    });

    dispatch_async(dispatch_get_main_queue(), ^{

        NSLog(@"3. main thread 1  i=%i",i);

        i = 3;

    });

    NSLog(@"4. normal 2  i=%i",i);

    i = 4;

    dispatch_async(dispatch_get_main_queue(), ^{

        NSLog(@"5. main thread 2  i=%i",i);

        i = 5;

    });

    NSLog(@"i =%i",i);

得到的結果如下，

2014-05-20 16:45:00.513 ConceptComfirm[25891:60b] viewDidLoad

2014-05-20 16:45:00.513 ConceptComfirm[25891:60b] class : Concepts

2014-05-20 16:45:00.514 ConceptComfirm[25891:60b] 1. normal 1  i=1

2014-05-20 16:45:00.514 ConceptComfirm[25891:60b] 4. normal 2  i=1

2014-05-20 16:45:00.514 ConceptComfirm[25891:60b] i =4

2014-05-20 16:45:00.515 ConceptComfirm[25891:60b] viewWillAppear

2014-05-20 16:45:00.652 ConceptComfirm[25891:60b] viewDidAppear

2014-05-20 16:45:00.652 ConceptComfirm[25891:60b] 3. main thread 1  i=4

2014-05-20 16:45:00.653 ConceptComfirm[25891:60b] 5. main thread 2  i=3

2014-05-20 16:45:02.516 ConceptComfirm[25891:60b] 2. after two seconds  i=5

結論是，iOS7已經不再將code預設執行的main thread

而是把UI用main thread去跑，你想要跑在main thread的code要自行宣告使用。

Note.
//創造一個新的執行緒

dispatch_queue_t _newQueue   = dispatch_queue_create("forCentralManagerOnly", DISPATCH_QUEUE_SERIAL);