如何以`__SwiftValue`的形式获取swift结构的实际类型

Question

我正在使用YapDatabase对Swift值类型进行编码/解码。解码后，类型信息似乎丢失了，即type(of:element)返回__SwiftValue，而不是例如Reservation。

如果我在调试器中调用po element，那么类型信息似乎仍然保留：

(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")))])

这个__SwiftValue是什么？除了解析String(describing: element的任何可怕方法之外，还有什么方法可以获得实际类型吗？

Answer 1

首先，您看到的是__SwiftValue，因为struct已被as AnyObject强制转换封装。你可以很容易地自己验证：

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

输出：

TestStruct
__SwiftValue

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

现在，就你的问题而言，不幸的是，我认为目前还没有方便的公共API可以让你从__SwiftValue中提取原始元数据Type。

尽管如此，这个话题太有趣了，不必冒险进入API私人领地进行教育。

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

有前景的API可通过obb-c:获得

// 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;
}

让我们从一个相当简单的东西开始：

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

输出：

ModuleName.TestStruct

只是String型，但对初学者来说并不坏。下一个有希望探索的候选人似乎是：

- (const Metadata *)_swiftTypeMetadata()

我们可以这样称呼它(关于Swift中调用选择器的这种特殊方式的详细信息可以在这里找到(

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)

现在的挑战是以type(of:)的方式以某种方式使用Metadata *(即桥接到Swift之后的OpaquePointer(。

我找到了一种方法，尽管很难做到
我们从某个伪对象或结构的元数据Type变量开始(其实并不重要(。总的想法是，最终我们希望用上一阶段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，所以有额外的间接级别，我们必须适应它：

var placeholderTypeVar: Any.Type = type(of: NSObject())
print(placeholderTypeVar)
withUnsafeMutablePointer(to: &placeholderTypeVar) {
let unsafePtr = UnsafeMutablePointer<OpaquePointer>.allocate(capacity: 1)
unsafePtr.pointee = metadataPtr
$0.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。此外，它与type(of: ourStruct)的结果在内存级别上是相同的(内部产生的Metadata *等价物是完全相同的地址！(。

更多关于元数据的信息，这是Swift中一个非常有趣的话题：
https://medium.com/ios-os-x-development/types-and-meta-types-in-swift-9cd59ba92295https://kateinoigakukun.hatenablog.com/entry/2019/03/22/184356https://medium.com/@weswickwire/创建-a-swift-运行时间-库-3cc92fc486cc