做一个Swift字典，其中的键是“Type”？ [英] Make a Swift dictionary where the key is "Type"?

问题描述

我正在尝试这样做。

static var recycle: [Type: [CellThing]] = []

但是我不能：）

未声明的类型'Type'

Undeclared type 'Type'

在示例中， CellThing 是我的基类，所以 A：CellThing ， B：CellThing ， C：CellThing 等等。这个想法是我会将各种AAA，BB，CCCC存储在字典数组中。

In the example, CellThing is my base class, so A:CellThing, B:CellThing, C:CellThing and so on. The idea is I would store various A A A, B B, C C C C in the dictionary arrays.

如何使类型（理想情况下，我猜到，限制在CellThing）成为Swift字典中的关键？

How to make a "Type" (ideally I guess, constrained to CellThing) be the key in a Swift dictionary?

我感谢我可以（也许？）使用 String（描述：T.self ），但这会让我失眠。

I appreciate I could (perhaps?) use String(describing: T.self), but that would make me lose sleep.

这是一个用例，设想的代码会看起来像这样...

Here's a use case, envisaged code would look something like this ...

@discardableResult class func make(...)->Self {
  return makeHelper(...)
  }

private class func makeHelper<T: CellThing>(...)->T {
  let c = instantiateViewController(...) as! T
  return c
  }

所以， p>

So then something like ...

static var recycle: [Type: [CellThing]] = []

private class func makeHelper<T: CellThing>(...)->T {
  let c = instantiateViewController(...) as! T

  let t = type whatever of c (so, maybe "A" or "B")
  recycle[t].append( c )

  let k = recycle[t].count
  print wow, you have k of those already!

  return c
  }

推荐答案

不幸的是，目前，metatype类型不符合协议（请参阅此相关问题） - 所以 CellThing.Type 不，并且不能，目前符合 Hashable 。因此，这意味着它不能直接用作字典中的键。

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.

但是，您可以使用 <$ c创建元类型的包装器$ c> ObjectIdentifier ，以便提供 Hashable 实现。例如：

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 Metatype<T> : Hashable {

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

    let base: T.Type

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

    var hashValue: Int {
        return ObjectIdentifier(base).hashValue
    }
}


class CellThing {

    // convenience static computed property to get the wrapped metatype value.
    static var metatype: Metatype<CellThing> {
        return Metatype(self)
    }
}

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

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

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

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

这也应该适用于泛型，您只需将您的字典下标为 T.metatype 。

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

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