XCode调试多线程遭遇海森伯效应一例

dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
                dispatch_async(dispatch_get_main_queue(), ^{
                    ...
                    [self myMethod];
                   
                });

-(void)myMethod
{
    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。