Is it possible to obtain a Swift type from a strin

问题:

I wonder if it's possible to obtain a Swift type dynamically. For example, say we have the following nested structs:

struct Constants {

  struct BlockA {
    static let kFirstConstantA = "firstConstantA"
    static let kSecondConstantA = "secondConstantA"
  }

 struct BlockB {    
    static let kFirstConstantB = "firstConstantB"
    static let kSecondConstantB = "secondConstantB"
  }

  struct BlockC {
    static let kFirstConstantC = "firstConstantBC"
    static let kSecondConstantC = "secondConstantC"
  }
}

It's possible to get value from kSeconConstantC from a variable). Like:

let variableString = "BlockC"
let constantValue = Constants.variableString.kSecondConstantC

Something akin to NSClassFromString, maybe?

回答1:

No, it's not possible yet (as a language feature, at least).

What you need is your own type registry. Even with a type registry, you wouldn't be able to get static constants unless you had a protocol for that:

var typeRegistry: [String: Any.Type] = [:]

func indexType(type: Any.Type)
{
    typeRegistry[String(type)] = type
}

protocol Foo
{
    static var bar: String { get set }
}

struct X: Foo
{
    static var bar: String = "x-bar"
}

struct Y: Foo
{
    static var bar: String = "y-bar"
}

indexType(X)
indexType(Y)

typeRegistry // ["X": X.Type, "Y": Y.Type]

(typeRegistry["X"] as! Foo.Type).bar // "x-bar"
(typeRegistry["Y"] as! Foo.Type).bar // "y-bar"

A type registry is something that registers your types using a custom Hashable type (say a String or an Int). You can then use this type registry to reference registered types using custom identifiers (a String, in this case).

Since Any.Type by itself isn't all that useful, I constructed an interface Foo through which I could access a static constant bar. Because I know that X.Type and Y.Type both conform to Foo.Type, I forced a cast and read the bar property.