参考文章
下载源码
源码
Optional文档
ImplicitlyUnwrappedOptional文档
import UIKit
class ViewController: UIViewController {
var a1: Int?
var a2: Optional<Int>
var b1: Int!
var b2: ImplicitlyUnwrappedOptional<Int>
var c: Int = 10
let label: UIView = UIView(frame: CGRectZero)
override func viewDidLoad() {
super.viewDidLoad()
a1 = 10
a2 = 10
b1 = 10
b2 = 10
print(a1)
print(a2)
print(b1)
print(b2)
print(c)
// 结论
// var a1: Int? = 10 相当于 var a2: Optional<Int> = Optional<Int>(100)
// !的用法同上类比
// as 和 as? 实现类型转换
let someView = label as? UILabel
if someView is UILabel? {
print(someView)
}
}
}
打印结果
