/// 第二个背景node
if bgNode2.position.y < -bgNode2.size.height {
bgNode2.position.y = bgNode1.position.y + bgNode1.size.height
}

完整的代码如下：

override func update(_ currentTime: TimeInterval) { // 每Frame的时间差 if lastUpdateTimeInterval == 0 { lastUpdateTimeInterval = currentTime } deltaTime = currentTime - lastUpdateTimeInterval lastUpdateTimeInterval = currentTime updateBackground(deltaTime: deltaTime) // endless 无限循环星空背景 } 

/// command + option + <- (箭头) 折叠 || command + option + -> (箭头) 打开 func updateBackground(deltaTime:TimeInterval){ // 下移 bgNode1.position.y -= CGFloat(deltaTime * 300) bgNode2.position.y -= CGFloat(deltaTime * 300) // 第一个背景node if bgNode1.position.y < -bgNode1.size.height { bgNode1.position.y = bgNode2.position.y + bgNode2.size.height } // 第二个背景node if bgNode2.position.y < -bgNode2.size.height { bgNode2.position.y = bgNode1.position.y + bgNode1.size.height } } 

三、SpriteKit物理碰撞

物理碰撞发生在：玩家飞船发射子弹击中外星敌机、发星敌机撞到玩家飞船

SpriteKit SKPhysicsBody类物理体的属性图表：
http://www.ifiero.com/index.php/archives/166

1.Spritekit物理节点categoryBitMask属性

/// 玩家飞船 playerNode.physicsBody = SKPhysicsBody(texture: SKTexture(imageNamed: "Player"), size: SKTexture(imageNamed: "Player").size()) playerNode.physicsBody?.affectedByGravity = false // 不受物理世界的重力影响 playerNode.physicsBody?.isDynamic = true playerNode.physicsBody?.categoryBitMask = PhysicsCategory.SpaceShip playerNode.physicsBody?.contactTestBitMask = PhysicsCategory.Alien ///碰撞时发出通知 playerNode.physicsBody?.collisionBitMask = PhysicsCategory.None 

/// 子弹; bulletNode.physicsBody = SKPhysicsBody(circleOfRadius: bulletNode.size.width / 2) bulletNode.physicsBody?.affectedByGravity = false // 子弹不受重力影响; bulletNode.physicsBody?.categoryBitMask = PhysicsCategory.BulletBlue bulletNode.physicsBody?.contactTestBitMask = PhysicsCategory.Alien bulletNode.physicsBody?.collisionBitMask = PhysicsCategory.None 

/// 外星飞船 // 1.设置物理身体 alien.physicsBody = SKPhysicsBody(circleOfRadius: alien.size.width / 2) // 不受重力影响，自定义飞船移动速度; alien.physicsBody?.affectedByGravity = false // 2.设置唯一属性 alien.physicsBody?.categoryBitMask = PhysicsCategory.Alien // 3.和哪些节点Node发生碰撞后发出通知 alien.physicsBody?.contactTestBitMask = PhysicsCategory.BulletBlue | PhysicsCategory.SpaceShip alien.physicsBody?.collisionBitMask = PhysicsCategory.None 

2.用didBegin来监测碰撞：

 

物理体发生碰撞

didBegin接收playerNode.physicsBody.contactTestMask的碰撞通知：

playerNode.physicsBody?.contactTestBitMask = PhysicsCategory.Alien

//MARK:- 发生碰撞时接收到通知 func didBegin(_ contact: SKPhysicsContact) { let contactMask = contact.bodyA.categoryBitMask | contact.bodyB.categoryBitMask switch contactMask { /// 子弹vs外星人 case PhysicsCategory.Alien | PhysicsCategory.BulletBlue: bulletHitAlien(nodeA: contact.bodyA.node as! SKSpriteNode,nodeB: contact.bodyB.node as! SKSpriteNode) /// 外星人Alien撞击到飞船 case PhysicsCategory.Alien | PhysicsCategory.SpaceShip: alienHitSpaceShip(nodeA: contact.bodyA.node as! SKSpriteNode, nodeB: contact.bodyB.node as! SKSpriteNode) default: break } }