IOS导航下拉列表，纯代码自动化布局 部分代码UIImageView*bgImage=[[UIImageViewalloc]initWithFrame:CGRectMake(0,0,self.view.frame.size.width,self.view.frame.size.height)]; bgImage.image=[UIImageimageNamed:@"bg.png"]; bgImage.translatesAutoresizingMaskIntoConstraints=NO; [self.viewaddSubview:bgImage]