Making my game object bounce forever? - swift

Question

So I am trying to make a ping pong game and I am able to get the ball moving and bouncing but the ball slows down. Whats the code to keep the balls velocity at a constant? Here's what I got:
var ball = SKSpriteNode()
var enemy = SKSpriteNode()
var main = SKSpriteNode()
override func didMove(to view: SKView) {
ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.size.width)
ball = self.childNode(withName: "ball") as! SKSpriteNode
enemy = self.childNode(withName: "enemy") as! SKSpriteNode
main = self.childNode(withName: "main") as! SKSpriteNode
ball.physicsBody?.applyImpulse(CGVector(dx: 20, dy: 20))
let border = SKPhysicsBody(edgeLoopFrom: self.frame)
border.friction = 0
border.restitution = 1
self.physicsBody = border
}
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
for touch in touches {
let location = touch.location(in: self)
main.run(SKAction.moveTo(x: location.x, duration: 0.2))
}
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
for touch in touches {
let location = touch.location(in: self)
main.run(SKAction.moveTo(x: location.x, duration: 0.2))
}
}
override func update(_ currentTime: TimeInterval) {
// Called before each frame is rendered
enemy.run(SKAction.moveTo(x: ball.position.x, duration: 1.0))
}
}
My physics settings are set to the following:
physics definition
Thanks in advance!