我想在viewDidLoad中设置默认背景:overridefuncviewDidLoad(){super.viewDidLoad()letbkgrnd1=UIImageView(frame:UIScreen.main.bounds)bkgrnd1.image=UIImage(named:"bkgrnd1.png")bkgrnd1.contentMode=UIView.ContentMode.scaleAspectFillself.view.insertSubview(bkgrnd1,at:0)}在那之后，我想要另外2个单独的UIButton来用另一个“bkgrnd2.png”替换和/