Property Wrapper

Property Wrapper

• A property wrapper adds a layer of separation between code that manages how a property is stored and the code that defines a property. 
• For example, if you have properties that provide thread-safety checks or store their underlying data in a database, you have to write that code on every property. When you use a property wrapper, you write the management code once when you define the wrapper, and then reuse that management code by applying it to multiple properties.
• 여러 프로퍼티들에 동일한 로직을 실행해야하는경우 프로퍼티 래퍼를 정의하고 여러 프로퍼티들에 프로퍼티 래퍼를 적용하여 해당 로직을 재사용할수 있다.

 

정의하기

• To define a property wrapper, you make a structure, enumeration, or class that defines a wrappedValue property. 
• The setter ensures that new values are less than or equal to 12, and the getter returns the stored value.

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

 

사용하기

• You apply a wrapper to a property by writing the wrapper’s name before the property as an attribute.
• a structure that stores a rectangle that uses the TwelveOrLess property wrapper to ensure its dimensions are always 12 or less

struct SmallRectangle {
    @TwelveOrLess var height: Int
    @TwelveOrLess var width: Int
}

var rectangle = SmallRectangle()
print(rectangle.height)
// Prints "0"

rectangle.height = 10
print(rectangle.height)
// Prints "10"

rectangle.height = 24
print(rectangle.height)
// Prints "12"

 

초기값 설정

• SmallRectangle에 초기값 설정을 지원하고 싶으면 property wrapper 에 이니셜라이저를 추가해줘야한다

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

struct SmallRectangle {
    @TwelveOrLess var height: Int = 2
    @TwelveOrLess var width: Int
}

var rectangle = SmallRectangle()
print(rectangle.height)
// Prints "2"

print(rectangle.width)
// Prints "0"

 

 

 

 

 

 

 

 

 

 

 

 

 

참고자료

https://eunjin3786.tistory.com/472

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