向下转换时出现 Swift 错误 'Any'

Question

以下代码几乎与Apple Documentation 完全相同并编译无误:

guard let firstItem = (rawItems! as? Array<Dictionary<String, Any>>)?.first else {
    throw AnError()
}

let identityRef = firstItem[kSecImportItemIdentity as String] 
               as! SecIdentity?   // !!!

guard let identity = identityRef else {
    throw AnError()
}

标有!!! 的行包含强制向下转换，将as! 替换为as 很明显会导致编译错误“有吗？”不能转换为“SecIdentity?”... 事实上 SecIdentity 是一个类，而 Any 甚至可能不是一个类。

我真正无法解释的是以下内容。如果我试图通过使用这个来使代码更安全

guard let idenity = firstItem[kSecImportItemIdentity as String] as? SecIdentity
else {
    throw AnError()
}

或者这个

guard let idenityRef = firstItem[kSecImportItemIdentity as String] as? SecIdentity?
else {
    throw AnError()
}

我收到编译错误:条件向下转换为 CoreFoundation 类型“SecIdentity”将始终成功

Answer 1

SecIdentity 是“表示身份的抽象 Core Foundation 类型对象”， Core Foundation 类型的类型可以是使用 CFGetTypeID() 检查。所以你可以先检查类型ID。如果它匹配一个SecIdentity 那么强制转换是安全的:

guard let cfIdentity = firstItem[kSecImportItemIdentity as String] as CFTypeRef?,
    CFGetTypeID(cfIdentity) == SecIdentityGetTypeID() else {
        throw AnError()
}
let identity = cfIdentity as! SecIdentity

另请参阅错误报告 SR-7015 The CoreFoundation conditional downcast diagnostic is not as helpful as it should be :

The diagnostic should be updated with a message that informs the developer to compare CFTypeIds (with a fixit if possible).