Make a Swift dictionary where the key is “Type”?

Unfortunately, it’s currently not possible for metatype types to conform to protocols (see this related question on the matter) – so CellThing.Type does not, and cannot, currently conform to Hashable. This therefore means that it cannot be used directly as the Key of a Dictionary.

However, you can create a wrapper for a metatype, using ObjectIdentifier in order to provide the Hashable implementation. For example:

/// Hashable wrapper for a metatype value.
struct HashableType<T> : Hashable {

  static func == (lhs: HashableType, rhs: HashableType) -> Bool {
    return lhs.base == rhs.base
  }

  let base: T.Type

  init(_ base: T.Type) {
    self.base = base
  }

  func hash(into hasher: inout Hasher) {
    hasher.combine(ObjectIdentifier(base))
  }
  // Pre Swift 4.2:
  // var hashValue: Int { return ObjectIdentifier(base).hashValue }
}

You can then also provide a convenience subscript on Dictionary that takes a metatype and wraps it in a HashableType for you:

extension Dictionary {
  subscript<T>(key: T.Type) -> Value? where Key == HashableType<T> {
    get { return self[HashableType(key)] }
    set { self[HashableType(key)] = newValue }
  }
}

which could then use like so:

class CellThing {}
class A : CellThing {}
class B : CellThing {}

var recycle: [HashableType<CellThing>: [CellThing]] = [:]

recycle[A.self] = [A(), A(), A()]
recycle[B.self] = [B(), B()]

print(recycle[A.self]!) // [A, A, A]
print(recycle[B.self]!) // [B, B]

This should also work fine for generics, you would simply subscript your dictionary with T.self instead.

---

Unfortunately one disadvantage of using a subscript with a get and set here is that you’ll incur a performance hit when working with dictionary values that are copy-on-write types such as Array (such as in your example). I talk about this issue more in this Q&A.

A simple operation like:

recycle[A.self]?.append(A())

will trigger an O(N) copy of the array stored within the dictionary.

This is a problem that is aimed to be solved with generalised accessors, which have been implemented as an unofficial language feature in Swift 5. If you are comfortable using an unofficial language feature that could break in a future version (not really recommended for production code), then you could implement the subscript as:

extension Dictionary {
  subscript<T>(key: T.Type) -> Value? where Key == HashableType<T> {
    get { return self[HashableType(key)] }
    _modify {
      yield &self[HashableType(key)]
    }
  }
}

which solves the performance problem, allowing an array value to be mutated in-place within the dictionary.

Otherwise, a simple alternative is to not define a custom subscript, and instead just add a convenience computed property on your type to let you use it as a key:

class CellThing {
  // Convenience static computed property to get the wrapped metatype value.
  static var hashable: HashableType<CellThing> { return HashableType(self) }
}

class A : CellThing {}
class B : CellThing {}

var recycle: [HashableType<CellThing>: [CellThing]] = [:]

recycle[A.hashable] = [A(), A(), A()]
recycle[B.hashable] = [B(), B()]

print(recycle[A.hashable]!) // [A, A, A]
print(recycle[B.hashable]!) // [B, B]