替代 didBeginContact

如果你使用简单类别，每个物理主体只属于一个类别，那么 didBeginContact 的这种替代形式可能更具可读性：

func didBeginContact(contact: SKPhysicsContact) {
    let contactMask = contact.bodyA.categoryBitMask | contact.bodyB.categoryBitMask
    
switch contactMask {

case categoryBitMask.player | categoryBitMask.enemy:
    print("Collision between player and enemy")
    let enemyNode = contact.bodyA.categoryBitMask == categoryBitMask.enemy ? contact.bodyA.node! : contact.bodyB.node!
    enemyNode.explode()
    score += 10

case categoryBitMask.enemy | categoryBitMask.enemy:
    print("Collision between enemy and enemy")
    contact.bodyA.node.explode()
    contact.bodyB.node.explode()

default :
    //Some other contact has occurred
    print("Some other contact")
}
}