swift - 奇怪吗？作为 AnyObject 行为

Question

我很难理解以下代码的行为:

let a: Any? = nil
let b: AnyObject? = a as AnyObject

if let c: AnyObject = b {
    print(c)
    print("That's not right, is it?")
} else {
    print("I'd expect this to be printed")
}

在 playground 中运行时，虽然 a 为 nil，但第一个闭包会执行并打印以下内容:

<null>
That's not right, is it?

问:这怎么可能，这是预期的行为吗？

Answer 1

a as AnyObject 会将 a 转换为 NSNull 以便 b 不为 nil

你可以用type(of:)检查它

let a: Any? = nil
let b: AnyObject? = a as AnyObject

if let c: AnyObject = b {
    print(c)
    print(type(of: c)) // will print "NSNull"
    print("That's not right, is it?")
} else {
    print("I'd expect this to be printed")
}