[swift-evolution] [Pitch] Property reflection

Ricardo Parada rparada at mac.com
Fri May 27 13:53:06 CDT 2016

> On May 26, 2016, at 9:25 PM, Austin Zheng via swift-evolution <swift-evolution at swift.org> wrote:
> ```
> myPerson.typedReadWriteProperty<Int>("age")?.set(30)
> try myPerson.allNamedProperties["age"]?.set(30)
> ```

Can you elaborate on what this API would be used for?  KVC? For instance, I played with Mirror the other day and my code to get a value given the property name looked more like this:

let age = myPerson.value(forKey:”age”) as! Int

And this is what I did:

// KVC stands for key-value-coding… but I only know how to get values.  I don’t know how to set values

protocol KVC {
   func value(forKey key: String) -> Any!

// Default implementation
extension KVC {
   func value(forKey key: String) -> Any! {
       let aMirror = Mirror(reflecting:self)
       for case let (label?, value) in aMirror.children {
           if label == key {
               return value
       return nil

public struct Person : KVC {
   let firstName: String
   let lastName: String
   let age: Int

   func fullName() -> String {
       return "\(firstName) \(lastName)"

let aPerson = Person(firstName:"John", lastName:"Doe", age:48)

// It works for stored properties
let lastName = aPerson.value(forKey:"lastName") as! String
print("Last name is \(lastName)")

// It does not work for instance methods, i.e. fullName is not a stored property
let fullName = aPerson.value(forKey:"fullName")
if fullName != nil {
   print("Full name is \(fullName)")
} else {
   print("Unable to get fullName via KVC")

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