条件绑定中的绑定值必须是可选类型

Question

我有一个协议定义:

protocol Usable {
    func use()
}

以及符合该协议的类

class Thing: Usable {
    func use () {
        println ("you use the thing")
    }
}

我想以编程方式测试Thing类是否符合Usable协议.

let thing = Thing()

// Check whether or not a class is useable
if let usableThing = thing as Usable { // error here
    usableThing.use()
}
else {
    println("can't use that")
}

但是我得到了错误

Bound value in a conditional binding must be of Optional Type

如果我试试

let thing:Thing? = Thing()

我收到了错误

Cannot downcast from 'Thing?' to non-@objc protocol type 'Usable'

然后我添加@objc到协议并得到错误

Forced downcast in conditional binding produces non-optional type 'Usable'

在此?之后我添加了as,最终修复了错误.

如何通过使用非@objc协议进行条件绑定来实现此功能,与"高级Swift"2014 WWDC视频中的相同？

Answer 1

您可以通过将演员阵容设为可用来进行编译吗？而不是像我们这样可用:

// Check whether or not a class is useable
if let usableThing = thing as Usable? { // error here
    usableThing.use()
}
else {
    println("can't use that")
}