WeakMethod.swift

// Closures cannot be weak, but we can wrap an object and its static factory method that returns an 
// instance to create a pseudo weak closure
//

struct Method {

  static func weak<T: AnyObject, A>(_ base: T, factory: @escaping (T) -> ((A) -> Void)) -> (A) -> Void {
    { [weak base] in base.map(factory)?($0) }
  }

  static func weak<T: AnyObject, A, B>(_ base: T, factory: @escaping (T) -> ((A, B) -> Void)) -> (A, B) -> Void {
    { [weak base] in base.map(factory)?($0, $1) }
  }

  static func weak<T: AnyObject, A, R>(_ base: T, factory: @escaping (T) -> ((A) -> R)) -> (A) -> R? {
    { [weak base] in base.map(factory)?($0) }
  }

  static func weak<T: AnyObject, A, B, R>(_ base: T, factory: @escaping (T) -> ((A, B) -> R)) -> (A, B) -> R? {
    { [weak base] in base.map(factory)?($0, $1) }
  }
}

var text: NSString = "The quick brown fox jumps over the lazy dog"

// func capitalized(with locale: Locale?) -> String
let weakMethod = Method.weak(text, factory: NSString.capitalized)

print(weakMethod(NSLocale.current))
// Optional("The Quick Brown Fox Jumps Over The Lazy Dog")

text = "Hola"
print(weakMethod(NSLocale.current))
// nil