Swift 向下转换时出错 'Any'

Question

以下代码几乎是 Apple Documentation 的复制品并且编译没有错误：

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()
}

标记为!!!的行包含强制向下转换，而将as!替换为as很明显会导致编译错误'Any?' is not convertible to 'SecIdentity?'...确实SecIdentity是class 而 Any 甚至可能不是 class.

我真的无法解释的是以下内容。如果我试图通过使用 this

使代码更安全

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

或这个

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

我得到一个编译错误：Conditional downcast to CoreFoundation type 'SecIdentity' will always succeed

Answer 1

CoreFoundation 类型的行为与 Foundation 类型略有不同。

不要有条件的贬低身份。如果可选绑定成功，您可以强制解包身份

guard let idenity = firstItem[kSecImportItemIdentity as String] else { throw AnError() }
var privateKey : SecKey?
let status = SecIdentityCopyPrivateKey(identity as! SecIdentity, &privateKey)

旁注：

请不要写as? SecIdentity?.
要么是有条件的向下转换 as? SecIdentity 要么是桥接转换一个可选的 as SecIdentity?

Answer 2

SecIdentity是“代表身份的抽象核心基础类型对象”，核心基础类型的类型可以是用 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).

Swift 向下转换时出错 'Any'

Swift error while downcasting 'Any'

swift

downcast

optional-values