GameScene
class GameScene: SKScene, SKPhysicsContactDelegate {
  override func didMove(to view: SKView) {
  backgroundColor = .blue
  self.physicsWorld.contactDelegate = self
  self.physicsBody = SKPhysicsBody(edgeLoopFrom: self.frame)
  physicsWorld.gravity = CGVector(dx: 0.0, dy: 0.0)

  let ball = SKShapeNode(circleOfRadius: 40) //circleOfRadiusで円の半径
  ball.position = CGPoint(x:self.frame.midX, y:self.frame.midY+200)
  ball.fillColor = UIColor.red
  ball.lineWidth = 0.0
  ball.strokeColor = UIColor.red
  ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.frame.width/2)
  self.addChild(ball)
  //ボールの飛んでいく方向
  ball.physicsBody?.applyImpulse(CGVector(dx: 40, dy: 40))
  ball.physicsBody?.restitution = 1
  ball.physicsBody?.friction = 0
  ball.physicsBody?.linearDamping = 0
  
  let label = SKLabelNode(fontNamed: "HelveticaNeue-Light")
  label.text = "Hello World!"
  label.position = CGPoint(x: self.frame.midX, y: self.frame.midY)
  label.fontSize = 40
  label.fontColor = SKColor.white
  self.addChild(label)
}
override func touchesBegan(_ touches: Set, with event: UIEvent?) {
 }
}