class A: NSObject {
    let value: Int

    init(value: Int) {
        self.value = value
    }
}

class B: NSObject {
    let value: Int

    init(value: Int) {
        self.value = value
    }
}

class Main: NSObject {
    @objc func printValue(_ instanceA: A) {
        print("Value: (instanceA.value)")
        print("instanceA is A? (instanceA is A)")
        print("instanceA is kind of A? (instanceA.isKind(of: A.self))")
    }
}

Main().perform(NSSelectorFromString("printValue:"), with: B(value: 2))

如果我们运行上面的代码，我们可以得到以下结果：

Value: 2
instanceA is A? true
instanceA is kind of A? false

我们可以看到instanceA is A和instanceA.isKind(of: A.self)不同，你知道为什么吗？