printf("retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self))); //打印引用计数
UIViewController的状态栏空出20高度是由于加载了UItableView 状态栏会空出来 y的值是20
self.automaticallyAdjustsScrollViewInsets = NO;关闭后
UItableView.y的值会会变回0
