Property Wrappers @shtnkgm

Swift5.1ʢXcode11.0ʣ͔Β৽ػೳ Property Wrappers͕࢖͑Δʂ

DocumentΛΈͯΈΔ https://docs.swift.org/swift-book/LanguageGuide/Properties.html#ID617

Property Wrappers A property wrapper adds a layer of separation between code that manages how a property is stored and the code that defines a property.

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Property Wrappersͱ͸ • ϓϩύςΟʹڞ௨ػೳΛ௥ՃͰ͖Δ • ϓϩύςΟ΁ͷΞΫηεํ๏ʹؔ͢ΔAttributeΛͭ͘ΔͨΊͷ Attribute

Attribute?

@͕͍ͭͯΔ͜Μͳ΍ͭ

@autoclosure @escaping @convention @available @discardableResult @objc @nonobjc @objcMembers @GKInspectable @UIApplicationMain @NSApplicationMain @NSCopying @NSKeyedArchiverClassName @NSManaged @testable @IBAction @IBOutlet @IBDesignable @IBInspectable

@ʙͱهड़͢Δ͜ͱͰɺϓϩύςΟ ΍ܕͷએݴʹิ଍৘ใΛઃఆͰ͖Δ ػೳ

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Swift5.1 @functionBuilder @dynamicCallable @dynamicMemberLookup @inlinable @requiresstoredpropertyinits @usableFromInline @warnunqualifiedaccess

@propertyWrapper

ϓϩύςΟͷ஋ʹΞΫηε͢Δํ๏ ΛAttributeͱͯ͠ڞ௨ͰఆٛͰ͖ Δʂ

AttributeΛͭͬͯ͘ΈΔ // 12ҎԼΛอূ͢Δ @propertyWrapper struct TwelveOrLess { private var number = 0 // ඞਢ var wrappedValue: Int { get { return number } set { number = min(newValue, 12) } } }

AttributeΛ͔ͭͬͯΈΔ struct SmallRectangle { @TwelveOrLess var height: Int @TwelveOrLess var width: Int } var rectangle = SmallRectangle() print(rectangle.height) // "0" rectangle.height = 13 print(rectangle.height) // "12"

12Ҏ֎ʹ΋ରԠͰ͖ΔΑ͏ʹ֦ு ͍ͨ͠

Property Wrappersʹॳظ஋Λ༩͑ Δ

@propertyWrapper struct SmallNumber { private var maximum: Int private var number: Int var wrappedValue: Int { get { return number } set { number = min(newValue, maximum) } } init() { maximum = Int.max number = 0 } init(wrappedValue: Int) { maximum = Int.max number = min(wrappedValue, maximum) } init(wrappedValue: Int, maximum: Int) { self.maximum = maximum number = min(wrappedValue, maximum) } }

ॳظ஋Λઃఆ͢Δ struct Rectangle { @SmallNumber var height: Int @SmallNumber(wrappedValue: 0, maximum: 10) var width: Int } var rectangle = Rectangle() rectangle.width = 20 print(rectangle.height, rectangle.width) // 0 10

Projected Value

projected • ܭը͞Εͨ • ݟੵ΋ΒΕͨ • ౤Ө͞Εͨ

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Projected ValueΛఆٛ͢Δ

@propertyWrapper struct SmallNumber { private var number = 0 var projectedValue = false var wrappedValue: Int { get { return number } set { if newValue > 12 { number = 12 projectedValue = true } else { number = newValue projectedValue = false } } } }

Projected ValueΛࢀর͢Δʢ$ϓϩύςΟ ໊ʣ struct SomeStructure { @SmallNumber var someNumber: Int } var someStructure = SomeStructure() someStructure.someNumber = 4 print(someStructure.$someNumber) // Prints "false" someStructure.someNumber = 55 print(someStructure.$someNumber) // Prints "true"

Property Wrapper Composition @A @B var x: Int

Property Wrapperͷ׆༻

CodableͳArrayͷdecode Ұ෦ͷཁૉ͕decodeʹࣦഊ͢Δͱશཁૉࣦഊ͢Δ

struct Response: Codable { var values: [Int] } let json = #"{ "values": [1, 2, null, 4, 5, null] }"#.data(using: .utf8)! let result = try JSONDecoder().decode(Response.self, from: json) // nil

Property WrappersΛ࢖Θͳ͍ํ๏

FailableܕΛఆٛ

public struct Failable: Codable { public let value: Wrapped? public init(from decoder: Decoder) throws { do { let container = try decoder.singleValueContainer() value = try container.decode(Wrapped.self) } catch { value = nil } } public func encode(to encoder: Encoder) throws { var container = encoder.singleValueContainer() if let value = self.value { try container.encode(value) } else { try container.encodeNil() } } }

public struct Response: Codable { public let query: String public let items: [Item] public init(from decoder: Decoder) throws { let container = try decoder.container(keyedBy: CodingKeys.self) self.query = try container.decode(String.self, forKey: .query) self.items = try container.decode([Failable].self, forKey: .suggests).compactMap { $0.value } } public init(query: String, items: [Item]) { self.query = query self.items = items } }

Property WrappersΛ࢖ͬͨํ๏

@LossyArray

@propertyWrapper public struct LossyArray: Codable { private struct AnyDecodableValue: Codable {} private struct LossyDecodableValue: Codable { let value: Value public init(from decoder: Decoder) throws { let container = try decoder.singleValueContainer() value = try container.decode(Value.self) } } public var wrappedValue: [T] public init(wrappedValue: [T]) { self.wrappedValue = wrappedValue } public init(from decoder: Decoder) throws { var container = try decoder.unkeyedContainer() var elements: [T] = [] while !container.isAtEnd { do { let value = try container.decode(LossyDecodableValue.self).value elements.append(value) } catch { _ = try? container.decode(AnyDecodableValue.self) } } self.wrappedValue = elements } public func encode(to encoder: Encoder) throws { try wrappedValue.encode(to: encoder) } }

struct Response: Codable { @LossyArray var values: [Int] } let json = #"{ "values": [1, 2, null, 4, 5, null] }"#.data(using: .utf8)! let result = try JSONDecoder().decode(Response.self, from: json) print(result) // [1, 2, 4, 5]

CodableͰσϑΥϧτ஋Λઃఆ͢Δ ͷ͸ͪΐͬͱखؒ init(from decoder: Decoder)Λॻ͔ͳ͍ͱ͍͚ͳ͍

struct User: Codable { var isAdmin: Bool } let json = #"{ "isAdmin": null }"#.data(using: .utf8)! let user = try? JSONDecoder().decode(User.self, from: json) print(user) // nil

@DefaultFalse

@propertyWrapper public struct DefaultFalse: Codable, Equatable, Hashable { public var wrappedValue: Bool public init(wrappedValue: Bool) { self.wrappedValue = wrappedValue } public init(from decoder: Decoder) throws { let container = try decoder.singleValueContainer() self.wrappedValue = (try? container.decode(Bool.self)) ?? false } public func encode(to encoder: Encoder) throws { try wrappedValue.encode(to: encoder) } }

struct User: Codable { @DefaultFalse var isAdmin: Bool } let json = #"{ "isAdmin": null }"#.data(using: .utf8)! let user = try JSONDecoder().decode(User.self, from: json) print(user) // User(isAdmin: false)

APIϨεϙϯε͕૝ఆͷܕͰͳ͍ or APIʹΑΓܕ͕ҟͳΔ

struct Response: Codable { var price: String var isDiscount: Bool } let json = #"{ "price": 12345, "isDiscount": "true" }"#.data(using: .utf8)! let result = try? JSONDecoder().decode(Response.self, from: json) // nil

@LosslessValue

@propertyWrapper public struct LosslessValue: Codable { private let type: LosslessStringCodable.Type public var wrappedValue: T public init(from decoder: Decoder) throws { do { self.wrappedValue = try T.init(from: decoder) self.type = T.self } catch let error { func decode(_: T.Type) -> (Decoder) -> LosslessStringCodable? { return { try? T.init(from: $0) } } let types: [(Decoder) -> LosslessStringCodable?] = [ decode(String.self),decode(Bool.self),decode(Int.self), decode(Int8.self),decode(Int16.self),decode(Int64.self), decode(UInt.self),decode(UInt8.self),decode(UInt16.self), decode(UInt64.self),decode(Double.self),decode(Float.self), ] guard let rawValue = types.lazy.compactMap({ $0(decoder) }).first, let value = T.init("\(rawValue)") else { throw error } self.wrappedValue = value self.type = Swift.type(of: rawValue) } } public func encode(to encoder: Encoder) throws { let string = String(describing: wrappedValue) guard let original = type.init(string) else { let description = "Unable to encode '\(wrappedValue)' back to source type '\(type)'" throw EncodingError.invalidValue(string, .init(codingPath: [], debugDescription: description)) } try original.encode(to: encoder) } }

struct Response: Codable { @LosslessValue var price: String @LosslessValue var isDiscount: Bool } let json = #"{ "price": 12345, "isDiscount": "true" }"#.data(using: .utf8)! let result = try? JSONDecoder().decode(Response.self, from: json) print(result) // Response(price: "12355", isDiscount: true)

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SwiftUIͰఆٛ͞Εͨ Property Wrappers

struct ContentView : View { @State var value: Bool = false var body: some View { Toggle(isOn: $value) { Text("The value is " + "\($value)") } } }

@Published @ObservedObjectʢچ: @ObjectBindingʣ @Environment @EnvironmentObject @State @FetchRequest

·ͱΊ • @͸Attributeɺ@propertyWrapper͸Attributeͷͻͱͭ • ϑϨʔϜϫʔΫͰఆٛ͞ΕͨAttributeͰͳ͍΋ͷ͸ @propertyWrapperͰఆٛ͞ΕͯΔ͔΋ • $ϓϩύςΟ໊ͰProjected ValueʹΞΫηεͰ͖Δ • Property WrappersͰϓϩύςΟͷڞ௨ػೳΛͭ͘Ζ͏ʂ

ࢀߟ • https://github.com/marksands/BetterCodable • https://www.hackingwithswift.com/quick-start/swiftui/ understanding-property-wrappers-in-swift-and-swiftui • SwiftUIͷProperty Wrappersͱσʔλ΁ͷΞΫηεํ๏

͓ΘΓ