How to make ball bounce vertically only and not horizontally using SKSpriteKit?

Question

I am making an iOS application in which I only want the ball to bounce up & down, not horizontally or by any other angle. Unfortunately, the way I decided to design the game led to some problems down the road. Here is a GIF that kind of shows what my problem is:

Basically: sometimes when I click to rotate the pentagon, it hits the ball at an angle when I only want it going up and down.

Here is my code for the ball:

func createBallNode(ballColor: String) -> SKSpriteNode {
    let ball = SKSpriteNode(imageNamed: ballColor)
    ball.position = CGPoint(x: CGRectGetMidX(frame), y: CGRectGetMidY(frame)+1)

    ball.physicsBody = SKPhysicsBody(circleOfRadius: 25)
    ball.physicsBody?.affectedByGravity = true
    ball.physicsBody?.restitution = 0.99
    ball.physicsBody?.linearDamping = 0
    ball.physicsBody?.friction = 0

    ball.physicsBody?.categoryBitMask = ColliderType.Ball.rawValue
    ball.physicsBody?.contactTestBitMask = ColliderType.Rect.rawValue
    ball.physicsBody?.collisionBitMask = ColliderType.Rect.rawValue

    return ball
}

Is there anyway I can design the ball to always move in a certain line (vertically)? Does anyone have design suggestions for the ball, or maybe a different way of moving it (SKActions, ex: moveTo rather than using gravity).

Let me know. Thanks!

Answer 1

To restrict a node's x-coordinate to a fixed value, define an SKRange with the minimum and maximum parameters set to the same value.

let centerX = ball.position.x
let range = SKRange(lowerLimit:centerX, upperLimit:centerX)

From the range, define a positionX constraint

let constraint = SKConstraint.positionX(range)

Lastly, add the constraint to node's constraints array

ball.constraints = [constraint]

This should work with your existing code if you add it above the return ball statement.

Answer 2

The ball bounces irregularly because when you are moving the sides of the pentagon, the ball touches a tiny little bit of side and changes its trajectory. That's how gravity works.

If you want to bounce the ball your way, use SKActions like this:

    let up = SKAction.moveBy(CGVector(dx: 0, dy: 40), duration: 0.3)
    let down = SKAction.moveBy(CGVector(dx: 0, dy: -40), duration: 0.3)
    let upAndDown = SKAction.sequence([up, down])
    let foreverUpDown = SKAction.repeatActionForever(upAndDown)

And just call runAction on the ball.