swift @propertyWrapper的使用

@propertyWrapper
struct Map {
    private var value: String = "123"
    
    var wrappedValue: String {
        get {
            return value
        }
        set {
            value = newValue
        }
    }
    
    var projectedValue: Int? {
        Int(value)
    }
    
   static subscript<EnclosingSelf: AnyObject>(
        _enclosingInstance object: EnclosingSelf,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, String>,
        storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Map>
    ) -> String {
        get {
            object[keyPath: storageKeyPath].value
        }
        set {
            object[keyPath: storageKeyPath].value = newValue
            print(newValue, "------")
        }
        // TODO: Benchmark and explore a possibility to use _modify
    }
}
var text = Text()
test.mothds = "20"
// 20 ------

• projectedValue启用$语法糖
• 函数static subscript<EnclosingSelf: AnyObject>( _enclosingInstance object: EnclosingSelf, wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, String>, storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Map> ) -> String

  1. EnclosingSelf可以不是AnyObject，但如此只能使用get属性，同时wrappedValue也只能是可读属性。
  2. 标签_enclosingInstance、wrapped、storage不可改变，否则无效