How to limit the movement of two anchored lines so they swing continually like a pendulum

Question

I have created two lines anchored to a sprite, which are 30˚ apart. I want both lines to swing left and right like a pendulum, always swinging from end to end (in such a way

Answer 1

A swift 5 extension version for the good mogelbuster code :

extension SKAction {
    class func pendulum(withAngle angle:CGFloat, period:TimeInterval, key:String) -> SKAction {
        let initialRotate = SKAction.rotate(byAngle: angle/2, duration: period/2)
        initialRotate.timingMode = .easeOut
        let rotate = SKAction.rotate(byAngle: angle, duration: period)
        rotate.timingMode = .easeInEaseOut
        let rotateForever = SKAction.repeatForever(SKAction.sequence([rotate.reversed(), rotate]))
        return SKAction.sequence([initialRotate, rotateForever])
    }
}

Usage:

let pendulum = SKAction.pendulum(withAngle: CGFloat.pi/2, period: 0.5, key: "pendulum")
self.mynode.run(pendulum)