如何伪装成 `__SwiftValue` 来获取 swift 结构的实际类型

Question

我正在使用 YapDatabase encode/decode 我的 Swift 值类型。解码后，类型信息似乎丢失，即type(of:element)returns__SwiftValue而不是例如Reservation。

如果我在调试器中调用 po element，似乎类型信息仍然保留：

(lldb) po element 
SecureTruckParking.Reservation(reservationId: 12625, accessInformations: [SecureTruckParking.AccessInformation(accessInformationId: 12706, accessTypeId: 1, accessTypeKey: Optional("1"), accessTypeTenantKey: Optional("ROOT"), encodedValue: "XXX", displayedValue: "XXX"), SecureTruckParking.AccessInformation(accessInformationId: 12707, accessTypeId: 51, accessTypeKey: Optional("51"), accessTypeTenantKey: Optional("ROOT"), encodedValue: "918296", displayedValue: "918296")], customerId: 3156, areaId: 552, productId: 1004, state: "PENDING", startTime: 2020-09-10 08:23:00 +0000, endTime: 2020-09-11 08:23:00 +0000, earliestEntryTime: 2020-09-10 08:23:00 +0000, latestExitTime: 2020-09-11 08:23:00 +0000, totalAmount: 2750.0, currency: "€", netPrice: 2311.0, taxPrice: 439.0, invoiceItems: [SecureTruckParking.InvoiceItem(amount: 1, itemText: "Parkplatzreservierung, 10.09.2020 10:23 - 11.09.2020 10:23 \nREWE Logistikzentrum Neu-Isenburg -> REWE Logistikzentrum Neu-Isenburg", netPrice: 2311.0, taxPrice: 439.0, taxRate: 19.0)], productAttributes: [SecureTruckParking.Attribute(key: "early_bird_count", value: Optional("1"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "early_bird_count", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "DETAILS", value: nil, definitionId: nil), SecureTruckParking.Attribute(key: "INFO_DETAILS", value: nil, definitionId: nil), SecureTruckParking.Attribute(key: "early_bird", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "early_bird", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "manualBackofficeCancellationConfirmation", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "manualBackofficeCancellationConfirmation", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "manualBackofficeCancellationConfirmationThreshold", value: Optional("1"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "manualBackofficeCancellationConfirmationThreshold", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "product_icon", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "product_icon", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "product_image", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "product_image", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "product_tariff_group", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "product_tariff_group", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading_dates_fixed", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading_dates_fixed", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading_immediate", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading_immediate", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading_mail", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading_mail", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading_price_surcharge", value: Optional("false"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading_price_surcharge", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "early_bird_duration", value: Optional("1440"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "early_bird_duration", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "recurring_max_trips", value: Optional("1"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "recurring_max_trips", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading_mail_max_lead_time", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading_mail_max_lead_time", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "upgrading_mail_min_lead_time", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "upgrading_mail_min_lead_time", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "bstp_product_type", value: Optional("Reservation"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "bstp_product_type", tenant: "ROOT")))], areaAttributes: [SecureTruckParking.Attribute(key: "product_icon", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "product_icon", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "product_image", value: nil, definitionId: Optional(SecureTruckParking.DefinitionId(key: "product_image", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "Parking_Area_UST", value: Optional("22222222222222"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "Parking_Area_UST", tenant: "MAN_BOSCH"))), SecureTruckParking.Attribute(key: "ipaw_id", value: Optional("4651"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "ipaw_id", tenant: "ROOT"))), SecureTruckParking.Attribute(key: "bstp_area_type", value: Optional("PROFESSIONAL"), definitionId: Optional(SecureTruckParking.DefinitionId(key: "bstp_area_type", tenant: "ROOT")))])

这是什么 __SwiftValue 并且有没有办法获得实际类型（除了解析 String(describing: element 的任何可怕方法）？

Answer 1

首先，您会看到 __SwiftValue，因为 struct 已用 as AnyObject 转换包装。您可以自己轻松验证：

struct TestStruct {}
let ourStruct = TestStruct()
let structSwiftValue = ourStruct as AnyObject
print(type(of: ourStruct))
print(type(of: structSwiftValue))

输出：

TestStruct
__SwiftValue

关于主题的良好讨论：https://forums.swift.org/t/anyobject/35659/9

现在谈谈你问题的本质，不幸的是，我认为目前没有方便的 public API 可以让你从 [=19] 中提取原始元数据 Type =].

这个话题仍然太有趣了，不能为了教育目的而冒险进入私人 API 领域。

让我们从 __SwiftValue 本身开始 https://github.com/apple/swift/blob/master/stdlib/public/runtime/SwiftValue.mm

有希望 API 可通过 obj-c:

// Private methods for debugging purposes.

- (const Metadata *)_swiftTypeMetadata {
  return getSwiftValueTypeMetadata(self);
}
- (id /* NSString */)_swiftTypeName {
  TypeNamePair typeName
    = swift_getTypeName(getSwiftValueTypeMetadata(self), true);
  id str = swift_stdlib_NSStringFromUTF8(typeName.data, typeName.length);
  return [str autorelease];
}
- (const OpaqueValue *)_swiftValue {
  return getValueFromSwiftValue(self).second;
}

让我们从相当简单的事情开始：

print(structSwiftValue.value(forKey: "_swiftTypeName")!)

输出：

ModuleName.TestStruct

只是 String 类型，但对于初学者来说还不错。下一个有前途的候选人似乎是：

- (const Metadata *)_swiftTypeMetadata()

我们可以这样称呼它（可以在 Swift 中找到有关调用选择器的这种特定方式的详细信息 here）

let selector: Selector = NSSelectorFromString("_swiftTypeMetadata")
let methodIMP: IMP! = structSwiftValue.method(for: selector)
let metadataPtr = unsafeBitCast(methodIMP,to:(@convention(c)(Any?,Selector)->OpaquePointer).self)(structSwiftValue,selector)

现在的挑战是如何以 type(of:) 的方式利用 Metadata *（即桥接到 Swift 之后的 OpaquePointer）。

我找到了一种方法来做到这一点，尽管是一个非常 hacky 的方法。
我们从一些虚拟对象或结构的元数据 Type 变量开始（并不重要）。一般的想法是最终我们想用我们从上一阶段获得的元数据指针替换元数据指针。我注意到 type(of:) 的结果产生了一个指向元数据的指针，因此存在额外的间接级别，我们必须适应这一点：

var placeholderTypeVar: Any.Type = type(of: NSObject())
print(placeholderTypeVar)
withUnsafeMutablePointer(to: &placeholderTypeVar) {
    let unsafePtr = UnsafeMutablePointer<OpaquePointer>.allocate(capacity: 1)
    unsafePtr.pointee = metadataPtr
    [=17=].assign(from: UnsafePointer<Any.Type>.init(OpaquePointer(unsafePtr)), count: 1)
}
print(placeholderTypeVar)
print(type(of: structSwiftValue))

输出：

NSObject
TestStruct
__SwiftValue

中间一个是答案。而且它与 type(of: ourStruct) 的结果完全相同，一直到内存级别（内部产生的 Metadata * 等价物是完全相同的地址！）。

阅读更多有关元数据的信息，这是一个非常有趣的话题 Swift:
https://medium.com/ios-os-x-development/types-and-meta-types-in-swift-9cd59ba92295 https://kateinoigakukun.hatenablog.com/entry/2019/03/22/184356 https://medium.com/@weswickwire/creating-a-swift-runtime-library-3cc92fc486cc

如何伪装成 `__SwiftValue` 来获取 swift 结构的实际类型

How to get the actual type of a swift struct in the disguise of a `__SwiftValue`

value-type

any

swift

encoder-decoder