0001-single-source-of-truth-006.swift

// @propertyWrapper annotation makes this
// struct a property wrapper.
// It will be a generic struct because we will make
// it work with any kind of value type.
@propertyWrapper
struct Sorted<T> {
    // Internally, we will store the actual value of
    // the array in this property
    private var value: [T]

    // As we want to sort the array each time we introduce a new
    // value in it, we need to store the sort function.
    private let sortFunction: (T, T) -> Bool

    // The only requisite for a struct to be a propertyWrapper is this
    // computed property.
    // The wrappedValue property is accesible from the outside.
    // Each time we want to get the property value, the getter will
    // be called, and each time we want to set a value for this
    // property, its setter will be called.
    var wrappedValue: [T] {
        get { return value }
        set { value = newValue.sorted(by: sortFunction) }
    }
    
    // In the init, we have the special argument `wrappedValue`,
    // that represents the initial assigned value
    // and we will also send a sortFunction that will be used for
    // sorting the array each time we add a new value to it.
    init(wrappedValue value: [T], by sortFunction: @escaping (T, T) -> Bool) {
        self.sortFunction = sortFunction
        self.value = value.sorted(by: sortFunction)
    }
}