Launch Trajectory in Swift and SpriteKit

Launch.swift

/// Uses physics equations to calculate position of projectile at given time.
/// This tool is awesome to explain what's happening, and play with sample data: 
/// http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html#tra6
func launchAtPoint(angle:CGFloat, start:CGPoint, end:CGPoint, node:SKNode) -> SKAction {
    
    /// Constants
    let gravity : Double = 9.8
    let pixelsToMeters : CGFloat = 50
    
    /// Calculate everything we can given an angle and an end point
    let launchAngle = angle.degreesToRadians()
    let distance = start.distanceTo(point: end)
    let adjustedRange = Double(distance / pixelsToMeters)
    let launchVelocity = sqrt((adjustedRange * gravity) / Double(sin(launchAngle * 2)))
    let initialVerticalVelocity = launchVelocity * Double(sin(launchAngle))
    let travelTime = CGFloat((2 * initialVerticalVelocity) / gravity)
    
    /// X and Y just go in straight line to point over time, while fake Z value gives the height
    let dx = (end.x - start.x) / travelTime
    let dy = (end.y - start.y) / travelTime
    
    let launch = SKAction.customAction(withDuration: TimeInterval(travelTime)) { node, time in
        
        /// Get the vertical change at this point in time
        let change = Double(0.5 * gravity * Double(time * time))
        let currentZ = CGFloat(initialVerticalVelocity * Double(time)) - CGFloat(change)
        
        /// X just updates along straight line to point
        node.position.x = start.x + (dx * time)
        
        /// Y also goes in straight line to point, but then add in our fake Z value adjust for metters to pixels
        node.position.y = start.y + (dy * time) + CGFloat(currentZ * pixelsToMeters)
    }
    
    return launch
}