1.利用GameplayKit来为随机数组中的内容
countries = GKRandomSource.sharedRandom().arrayByShufflingObjects(in: countries) as! [String]
2.利用GameplayKit产生随机数
correctAnswer = GKRandomSource.sharedRandom().nextInt(upperBound: 3)
3.利用layer给控件加边框
button3.layer.borderWidth = 3
button1.layer.borderColor = UIColor.lightGray.cgColor