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")
}

当运行在操场上时，虽然a为nil，但执行第一个闭包并打印以下内容：

<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 这样 b 就不是 nil

你可以用type(of:)

查看

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")
}

Answer 2

因为 <null> 不是 nil。 AnyObject 是桥接到 Objective-C space.

的类型

奇怪吗？作为 AnyObject 行为

Strange Any? as AnyObject behaviour

casting

optional

swift

anyobject