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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 …