GKMinmaxStrategist不会返回任何动作

我的代码中包含以下代码main.swift:

let strategist = GKMinmaxStrategist()
strategist.gameModel = position
strategist.maxLookAheadDepth = 1
strategist.randomSource = nil

let move = strategist.bestMoveForActivePlayer()

... position我的GKGameModel子类的实例在哪里Position.运行此代码后,move是nil.bestMoveForPlayer(position.activePlayer!)也导致nil(但position.activePlayer!导致一个Player对象).

然而,

let moves = position.gameModelUpdatesForPlayer(position.activePlayer!)!

导致非空的可能移动数组.从Apple的文档(关于bestMoveForPlayer(_:)):

如果玩家无效,玩家不是游戏模型的一部分,或玩家没有有效的移动,则返回nil.

据我所知,这种情况都不是这样,但功能仍然存在nil.这可能会发生什么？

如果它可以有任何帮助,这是我的GKGameModel协议的实现:

var players: [GKGameModelPlayer]? = [Player.whitePlayer, Player.blackPlayer]
var activePlayer: GKGameModelPlayer? {
    return playerToMove
}

func setGameModel(gameModel: GKGameModel) {
    let position = gameModel as! Position
    pieces = position.pieces
    ply = position.ply
    reloadLegalMoves()
}

func gameModelUpdatesForPlayer(thePlayer: GKGameModelPlayer) -> [GKGameModelUpdate]? {
    let player = thePlayer as! Player
    let moves = legalMoves(ofPlayer: player)
    return moves.count > 0 ? moves : nil
}

func applyGameModelUpdate(gameModelUpdate: GKGameModelUpdate) {
    let move = gameModelUpdate as! Move
    playMove(move)
}

func unapplyGameModelUpdate(gameModelUpdate: GKGameModelUpdate) {
    let move = gameModelUpdate as! Move
    undoMove(move)
}

func scoreForPlayer(thePlayer: GKGameModelPlayer) -> Int {
    let player = thePlayer as! Player
    var score = 0
    for (_, piece) in pieces {
        score += piece.player == player ? 1 : -1
    }
    return score
}

func isLossForPlayer(thePlayer: GKGameModelPlayer) -> Bool {
    let player = thePlayer as! Player
    return legalMoves(ofPlayer: player).count == 0
}

func isWinForPlayer(thePlayer: GKGameModelPlayer) -> Bool {
    let player = thePlayer as! Player
    return isLossForPlayer(player.opponent)
}

func copyWithZone(zone: NSZone) -> AnyObject {
    let copy = Position(withPieces: pieces.map({ $0.1 }), playerToMove: playerToMove)
    copy.setGameModel(self)
    return copy
}

如果还有我应该展示的其他代码,请告诉我.