Reflexión con Mirror

¿Qué es Mirror?

Crear un Mirror

struct Point { let x: Int; let y: Int }
let m = Mirror(reflecting: Point(x: 1, y: 2))
print(m.children.count)  // 2

Iterar los elementos secundarios

struct User { let name: String; let age: Int }
let m = Mirror(reflecting: User(name: "Ann", age: 30))
for child in m.children {
    print("\(child.label ?? "?") = \(child.value)")
}
// name = Ann
// age = 30

Crear un Dictionary de campos

struct Config { let host: String; let port: Int }
let m = Mirror(reflecting: Config(host: "x", port: 80))
var dict: [String: Any] = [:]
for c in m.children { if let l = c.label { dict[l] = c.value } }
print(dict["port"]!)  // 80

displayStyle

Inspeccionar el estilo de visualización

struct S { let a = 1 }
let m = Mirror(reflecting: S())
if m.displayStyle == .struct {
    print("it is a struct with \(m.children.count) field(s)")
}
// it is a struct with 1 field(s)

Reflejar una tupla

let m = Mirror(reflecting: (10, "hi", true))
for c in m.children { print(c.value) }
// 10
// hi
// true

Reflejar enumeraciones

enum Result2 { case ok(Int); case fail(String) }
let m = Mirror(reflecting: Result2.ok(42))
print(m.children.first?.value ?? "none")  // 42

Un impresor genérico de campos

func dump2(_ value: Any) {
    for c in Mirror(reflecting: value).children {
        print("\(c.label ?? "_"): \(c.value)")
    }
}
struct Car { let make: String; let year: Int }
dump2(Car(make: "Mini", year: 2020))
// make: Mini
// year: 2020

Costes y límites de la reflexión

struct P { let v: Int }
let m = Mirror(reflecting: P(v: 5))
print(m.children.first!.value)  // 5  (read only)

Convertir valores reflejados

struct Box { let count: Int; let label: String }
let m = Mirror(reflecting: Box(count: 3, label: "hi"))
for c in m.children {
    if let n = c.value as? Int { print("int: \(n)") }
    if let s = c.value as? String { print("str: \(s)") }
}
// int: 3
// str: hi

Comprobación rápida

Resumen