遮挡tabbar，navigation的方法

弹小框，但也要整个屏幕大小

UIWindow* window = [UIApplication sharedApplication].keyWindow;
 if (!window) {
  window = [[UIApplication sharedApplication].windows objectAtIndex:0];
 }

[window addSubview:controller.view];

弹整个屏幕大小的视图
[self presentModalViewController:controller]