swift – 物理检测多次碰撞

我想我必须错误地检查碰撞：

let ballCategory   : UInt32 = 0x1 << 0 
let cupCategory : UInt32 = 0x1 << 1

在我的didMoveToView中：

//Set physicsBody of scene to a physics body that borders the screen

self.physicsBody = SKPhysicsBody(edgeLoopFromRect: self.frame)
physicsWorld.contactDelegate = self

//Ball
ballSprite.physicsBody = SKPhysicsBody(rectangleOfSize: ballSprite.size)
ballSprite.physicsBody!.categoryBitMask = ballCategory
ballSprite.physicsBody!.contactTestBitMask = cupCategory

//Cup
cupSprite.physicsBody = SKPhysicsBody(edgeLoopFromRect: cupSprite.size) 
cupSprite.physicsBody!.categoryBitMask = cupCategory
cupSprite.physicsBody!.contactTestBitMask = ballCategory

在我的didBeginContact中：

var firstBody: SKPhysicsBody
var secondBody: SKPhysicsBody

//Assign the two physics bodies so that the one with the lower category is always stored in firstBody
if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {

    firstBody = contact.bodyA
    secondBody = contact.bodyB
} 
else {

    firstBody = contact.bodyB
    secondBody = contact.bodyA
}

//contact between ball and cup
if firstBody.categoryBitMask == ballCategory && secondBody.categoryBitMask == cupCategory {

    score++
    println("point scored")
    moveBall() //Move ball back to start position
}

这是因为它正在检测碰撞,但是它发生了多次,而它应该只发生一次.