Swift枚举运算符重载

Question

I have define an enum, and I want to use it as a key for a dictionary. When I'm trying to access a value using the enum as a key, i get an error about the enum not being convertible to DictionaryIndex<Constants.PieceValue, Array<String>> where Constants.PieceValue is an enum that looks like this:

public enum PieceValue: Int {
  case    Empty = 0,
  WKing = 16,
  WQueen = 15,
  WRook = 14,
  WBishop = 13,
  WKnight = 12,
  WPawn = 11,
  BKing = 26,
  BQueen = 25,
  BRook = 24,
  BBishop = 23,
  BKnight = 22,
  BPawn = 21
}

I read a few threads but haven't found any clear answers. I also declared the operator overloading function for the enum outside the Constants class.

func == (left:Constants.PieceValue, right:Constants.PieceValue) -> Bool {
        return Int(left) == Int(right)
    }

This is the line that Xcode complains about:

self.label1.text = Constants.pieceMapping[self.pieceValue][0]

Constants.pieceMapping has the following type: Dictionary<PieceValue, Array<String>>

Answer 1

That's the typical optional problem: when you query a dictionary, it returns an optional value, to account for cases when the key is not found. So this:

Constants.pieceMapping[self.pieceValue]

is of Array<String>? type. In order to access to that array, you first have to unwrap from the optional, using either forced unwrapping:

Constants.pieceMapping[Constants.PieceValue.BPawn]![0]

or in a safer way using optional binding:

if let array = Constants.pieceMapping[Constants.PieceValue.BPawn] {
    let elem = array[0]
}