精灵没有消失

Question

这是我的代码。我正在尝试创建一个太空射击游戏，当外星人击中屏幕的底部时，我希望玩家消失。我不确定这段代码有什么问题。

 func gameOver() {
    let borderBody = SKPhysicsBody(edgeLoopFrom: self.frame)
    borderBody.categoryBitMask = borderCategory


    func didBegin(_ contact: SKPhysicsContact) {
        var firstBody:SKPhysicsBody
        var secondBody:SKPhysicsBody

        if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
            firstBody = contact.bodyA
            secondBody = contact.bodyB
        }else {
            firstBody = contact.bodyB
            secondBody = contact.bodyA
        }

        if (firstBody.categoryBitMask & alienCategory) != 0 && (secondBody.categoryBitMask & borderCategory) != 0 {
            player.removeFromParent()
        }