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Swift Type Metadata
@kateinoigakukun
try! Swift 2019
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let typeName = String(describing: Int.self)
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extension UITableView {
func register(nibWithCellClass: Cell.Type) where Cell: UITableViewCell {
let typeName = String(describing: Cell.self)
let nib = UINib(nibName: typeName, bundle: Bundle.main)
register(nib, forCellReuseIdentifier: typeName)
}
}
tableView.register(nibWithCellClass: TweetCell.self)
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Agenda
1. What is type metadata?
2. Explore String(describing: Int.self)
3. How to use metadata in Swift
4. Use cases in OSS
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What is type metadata?
• Type information in Swift runtime
• Used in Swift internal dynamic behavior
• Metatype is pointer to metadata
let metatype: Int.Type = Int.self
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extension String {
public init(describing instance: Subject) { ... }
public init(describing instance: Subject) { ... }
}
let typeName = String(describing: Int.self) // "Int"
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extension Int.Type: CustomStringConvertible { // Cannot extend a metatype 'Int.Type'
var description: String {
return "Int"
}
}
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SwiftCore
• Swift standard library
• Fundamental types and interfaces
SwiftRuntime
• Swift runtime library
• Dynamic behavior
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stdlib/public/core/Mirror.swift
struct String {
public init(describing instance: Subject) {
_print_unlocked(instance, &self)
}
}
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stdlib/public/core/Misc.swift
public func _typeName(_ type: Any.Type, qualified: Bool = true) -> String {
let (stringPtr, count) = _getTypeName(type, qualified: qualified)
return String._fromUTF8Repairing(
UnsafeBufferPointer(start: stringPtr, count: count)).0
}
@_silgen_name("swift_getTypeName")
public func _getTypeName(_ type: Any.Type, qualified: Bool) -> (UnsafePointer, Int)
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stdlib/public/core/Misc.swift
public func _typeName(_ type: Any.Type, qualified: Bool = true) -> String {
let (stringPtr, count) = _getTypeName(type, qualified: qualified)
return String._fromUTF8Repairing(
UnsafeBufferPointer(start: stringPtr, count: count)).0
}
@_silgen_name("swift_getTypeName")
public func _getTypeName(_ type: Any.Type, qualified: Bool) -> (UnsafePointer, Int)
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stdlib/public/core/Misc.swift
public func _typeName(_ type: Any.Type, qualified: Bool = true) -> String {
let (stringPtr, count) = _getTypeName(type, qualified: qualified)
return String._fromUTF8Repairing(
UnsafeBufferPointer(start: stringPtr, count: count)).0
}
@_silgen_name("swift_getTypeName")
public func _getTypeName(_ type: Any.Type, qualified: Bool) -> (UnsafePointer, Int)
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struct StructMetadata {
let kind: Int
let typeDescriptor: UnsafePointer
}
struct StructTypeDescriptor {
let flags: Int32
let parent: Int32
let name: RelativePointer
}
15 — docs/ABI/TypeMetadata.rst
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struct StructMetadata {
let kind: Int
let typeDescriptor: UnsafePointer
}
struct StructTypeDescriptor {
let flags: Int32
let parent: Int32
let name: RelativePointer
}
15 — docs/ABI/TypeMetadata.rst
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16 — include/swift/Basic/RelativePointer.h
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func getTypeName(of type: Subject.Type) -> String {
let metadataPointer = unsafeBitCast(
type, to: UnsafePointer.self
)
let namePointer: UnsafePointer = metadataPointer.pointee
.typeDescriptor.pointee
.name.advancedPointer()
return String(cString: namePointer)
}
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func getTypeName(of type: Subject.Type) -> String {
let metadataPointer = unsafeBitCast(
type, to: UnsafePointer.self
)
let namePointer: UnsafePointer = metadataPointer.pointee
.typeDescriptor.pointee
.name.advancedPointer()
return String(cString: namePointer)
}
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func getTypeName(of type: Subject.Type) -> String {
let metadataPointer = unsafeBitCast(
type, to: UnsafePointer.self
)
let namePointer: UnsafePointer = metadataPointer.pointee
.typeDescriptor.pointee
.name.advancedPointer()
return String(cString: namePointer)
}
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func getTypeName(of type: Subject.Type) -> String {
let metadataPointer = unsafeBitCast(
type, to: UnsafePointer.self
)
let namePointer: UnsafePointer = metadataPointer.pointee
.typeDescriptor.pointee
.name.advancedPointer()
return String(cString: namePointer)
}
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func getTypeName(of type: Subject.Type) -> String {
let metadataPointer = unsafeBitCast(
type, to: UnsafePointer.self
)
let namePointer: UnsafePointer = metadataPointer.pointee
.typeDescriptor.pointee
.name.advancedPointer()
return String(cString: namePointer)
}
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let typeName = getTypeName(of: Int.self) // "Int"
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Use cases inside of Swift
• Allocate instance
• Value Witness Table
• Dynamic method dispatch
• VTable
• Reflection
• Mirror API
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!
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Method swizzling
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Method swizzling
class Animal {
func bar() { print("bar") }
func foo() { print("foo") }
}
struct ClassMetadata {
...
// VTable
var barRef: FunctionRef
var fooRef: FunctionRef
}
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Method swizzling
class Animal {
func bar() { print("bar") }
func foo() { print("foo") }
}
struct ClassMetadata {
...
// VTable
var barRef: FunctionRef
var fooRef: FunctionRef
}
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let metadata = unsafeBitCast(
Animal.self, to: UnsafeMutablePointer.self
)
let bar = withUnsafeMutablePointer(to: &metadata.pointee.barRef) { $0 }
let foo = withUnsafeMutablePointer(to: &metadata.pointee.fooRef) { $0 }
bar.pointee = foo.pointee
let animal = Animal()
animal.bar() // foo
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let metadata = unsafeBitCast(
Animal.self, to: UnsafeMutablePointer.self
)
let bar = withUnsafeMutablePointer(to: &metadata.pointee.barRef) { $0 }
let foo = withUnsafeMutablePointer(to: &metadata.pointee.fooRef) { $0 }
bar.pointee = foo.pointee
let animal = Animal()
animal.bar() // foo
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let metadata = unsafeBitCast(
Animal.self, to: UnsafeMutablePointer.self
)
let bar = withUnsafeMutablePointer(to: &metadata.pointee.barRef) { $0 }
let foo = withUnsafeMutablePointer(to: &metadata.pointee.fooRef) { $0 }
bar.pointee = foo.pointee
let animal = Animal()
animal.bar() // foo
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let metadata = unsafeBitCast(
Animal.self, to: UnsafeMutablePointer.self
)
let bar = withUnsafeMutablePointer(to: &metadata.pointee.barRef) { $0 }
let foo = withUnsafeMutablePointer(to: &metadata.pointee.fooRef) { $0 }
bar.pointee = foo.pointee
let animal = Animal()
animal.bar() // foo
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let metadata = unsafeBitCast(
Animal.self, to: UnsafeMutablePointer.self
)
let bar = withUnsafeMutablePointer(to: &metadata.pointee.barRef) { $0 }
let foo = withUnsafeMutablePointer(to: &metadata.pointee.fooRef) { $0 }
bar.pointee = foo.pointee
let animal = Animal()
animal.bar() // foo
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Use cases
• Zewo/Reflection
• wickwirew/Runtime
• alibaba/HandyJSON
• kateinoigakukun/StubKit
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alibaba/HandyJSON
struct Item: HandyJSON {
var name: String = ""
var price: Double?
var description: String?
}
if let item = Item.deserialize(from: jsonString) {
// ...
}
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Use cases
• Zewo/Reflection
• wickwirew/Runtime
• alibaba/HandyJSON
• kateinoigakukun/StubKit
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kateinoigakukun/StubKit
import StubKit
struct User: Codable {
let name: String
let age: UInt
}
let user = try Stub.make(User.self)
// User(name: "This is stub string", age: 12345)
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kateinoigakukun/StubKit
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kateinoigakukun/StubKit
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kateinoigakukun/StubKit
func leafStub(of type: T.Type) -> T {
guard let stubbable = type as? Stubbable else { return nil }
return type.stub
}
extension Int: Stubbable {
var stub: Int { return 12345 }
}
extension enum: Stubbable { //
!
Can't extend
var stub: Self {
return enumStub()
}
}
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kateinoigakukun/StubKit
func enumStub(of type: T.Type) -> T? {
if isEnum(type: type) {
let rawValue = 0
let rawPointer = withUnsafePointer(to: rawValue) { UnsafeRawPointer($0) }
return rawPointer.assumingMemoryBound(to: T.self).pointee
}
return nil
}
func isEnum(type: T.Type) -> Bool {
let metadata = unsafeBitCast(type, to: UnsafePointer.self).pointee
return metadata.kind == 1 // kind value of enum is 1
}
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kateinoigakukun/StubKit
func enumStub(of type: T.Type) -> T? {
if isEnum(type: type) {
let rawValue = 0
let rawPointer = withUnsafePointer(to: rawValue) { UnsafeRawPointer($0) }
return rawPointer.assumingMemoryBound(to: T.self).pointee
}
return nil
}
func isEnum(type: T.Type) -> Bool {
let metadata = unsafeBitCast(type, to: UnsafePointer.self).pointee
return metadata.kind == 1 // kind value of enum is 1
}
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kateinoigakukun/StubKit
func enumStub(of type: T.Type) -> T? {
if isEnum(type: type) {
let rawValue = 0
let rawPointer = withUnsafePointer(to: rawValue) { UnsafeRawPointer($0) }
return rawPointer.assumingMemoryBound(to: T.self).pointee
}
return nil
}
func isEnum(type: T.Type) -> Bool {
let metadata = unsafeBitCast(type, to: UnsafePointer.self).pointee
return metadata.kind == 1 // kind value of enum is 1
}
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kateinoigakukun/StubKit
func enumStub(of type: T.Type) -> T? {
if isEnum(type: type) {
let rawValue = 0
let rawPointer = withUnsafePointer(to: rawValue) { UnsafeRawPointer($0) }
return rawPointer.assumingMemoryBound(to: T.self).pointee
}
return nil
}
func isEnum(type: T.Type) -> Bool {
let metadata = unsafeBitCast(type, to: UnsafePointer.self).pointee
return metadata.kind == 1 // kind value of enum is 1
}
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Caution
• ABI stability
• Responsibility
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Summary
• Swift uses metadata for dynamic behavior
• We can use metadata in Swift
• Let's write meta programming libraries!
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