1 dispatch_async(dispatch_get_global_queue(0, 0), ^{ 2          3                 dispatch_async(dispatch_get_main_queue(), ^{ 4                     ... 5                     [self myMethod]; 6                     7                 }); 8  9 -(void)myMethod10 {11     self.view.frame = CGRectMake(0, 20, 320, 460); //在该行加断点，会导致调试过程中该方法执行两遍。