20191206 Property Wrappers 入門

Transcript

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6. 1SPQFSUZ8SBQQFST
   

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7. final class UserData: ObservableObject {
   @Published var showFavoritesOnly = false
   @Published var landmarks = landmarkData
   }
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   There are property implementation patterns that come
   up repeatedly. Rather than hardcode a fixed set of
   patterns into the compiler (as we have done for lazy
   and @NSCopying), we should provide a general
   "property wrapper" mechanism to allow these patterns
   to be defined as libraries.
   
   

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11. ྫ
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12. struct Object {
    @LateInitialized var text: String
    }
    @propertyWrapper
    struct LateInitialized {
    private var storage: T?
    var wrappedValue: T {
    get {
    guard let value = storage else {
    fatalError()
    }
    return value
    }
    set {
    storage = newValue
    }
    }
    }
    

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13. var obj1 = Object()
    print("\(obj1.text)") // -> fatal error
    var obj2 = Object()
    obj2.text = "hello"
    print("\(obj2.text)") // -> "hello"
    

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14. ྫ
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15. @propertyWrapper
    struct UserDefault {
    let key: String
    let defaultValue: T
    var wrappedValue: T {
    get {
    return
    (UserDefaults.standard.object(forKey: key) as? T)
    ?? defaultValue
    }
    set {
    UserDefaults.standard.set(newValue, forKey: key)
    }
    }
    }
    

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16. @UserDefault(key: "initialized", defaultValue: false)
    var initialized: Bool
    @UserDefault(key: "username", defaultValue: "")
    var username: String
    obj.username = "mobile"
    print("\(UserDefaults.standard.string(forKey: "username")!)")
    // -> "mobile"
    UserDefaults.standard.set("act", forKey: "username")
    print("\(obj.username)") // -> "act"
    

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17. ΋͏গ͠ৄ͘͠
    

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18. ॻ͖׵͑ϧʔϧ
    @propertyWrapper
    struct WrapperA {}
    @propertyWrapper
    struct WrapperB {}
    // ॳظ஋
    @WrapperA var obj: String = "test"
    ɹ↓
    private var _obj: WrapperA(wrappedValue: "test")
    // ࿈݁Մೳ
    @WrapperA @WrapperB var obj: String
    ɹ↓
    private var _obj: WrapperA>
    

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20. ࢀߟࢿྉ
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