IOS 导航下拉列表，纯代码自动化布局 部分代码UIImageView *bgImage=[[UIImageView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)]; bgImage.image=[UIImage imageNamed:@"bg.png"]; bgImage.translatesAutoresizingMaskIntoConstrain