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我正在使用 YapDatabase 对我的 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。

如果我调用po element调试器，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，有没有办法获得实际类型（除了解析的任何可怕方法String(describing: element）？

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1 回答 1

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首先你看到的是__SwiftValue因为struct已经被as AnyObject演员包裹了。您可以自己轻松地验证这一点：

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

现在回到您问题的本质，不幸的是，我相信暂时没有方便的公共 API 可以让您Type从__SwiftValue.

出于教育目的，这个话题仍然太有趣了，不要冒险进入私有 API 领域。

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

通过 obj-c 可以使用有前途的 API：

// 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

只是一种String类型，但对初学者来说还不错。下一个有希望探索的候选者似乎是：

- (const Metadata *)_swiftTypeMetadata()

我们可以这样称呼它（有关在 Swift 中调用选择器的这种特殊方式的深入细节可以在 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)

现在的挑战是以某种方式利用Metadata *（即OpaquePointer桥接到 Swift 之后）type(of:)。

我找到了一种方法来做到这一点，尽管是一种非常老套的方法。
我们从Type一些虚拟对象或结构的元数据变量开始（并不重要）。一般的想法是最终我们想用我们从前一阶段获得的元数据指针替换元数据指针。我注意到type(of:)产生一个指向元数据的指针的结果，所以有额外的间接级别，我们将不得不适应这一点：

var placeholderTypeVar: Any.Type = type(of: NSObject())
print(placeholderTypeVar)
withUnsafeMutablePointer(to: &placeholderTypeVar) {
    let unsafePtr = UnsafeMutablePointer<OpaquePointer>.allocate(capacity: 1)
    unsafePtr.pointee = metadataPtr
    $0.assign(from: UnsafePointer<Any.Type>.init(OpaquePointer(unsafePtr)), count: 1)
}
print(placeholderTypeVar)
print(type(of: structSwiftValue))

输出：

NSObject
TestStruct
__SwiftValue

中间是答案。而且它与type(of: ourStruct)内存级别的结果相同（Metadata *内部产生的等价物是相同的地址！）。

有关元数据的更多信息，这是 Swift 中一个非常有趣的主题：
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

于 2020-08-22T19:40:31.150 回答